Why Mifflin-St Jeor instead of Harris-Benedict or Katch-McArdle?

Mifflin-St Jeor (1990) is the most accurate generic BMR equation in published meta-analyses — average error ~5% vs measured BMR (indirect calorimetry) for adults across the 18-90 age range. Harris-Benedict (1919) systematically overestimates BMR by ~5-10% in modern populations because the underlying cohort had different body composition and metabolic rates. Katch-McArdle is more accurate IF you have an exact body-fat percentage from DEXA — but most users self-report body fat with ±5% error, which destroys the precision advantage. The calculator uses Mifflin-St Jeor because it’s the cleanest input-to-output mapping with the smallest typical input error.

Why 1.1 g/lb protein on cuts and only 0.9 g/lb on bulks?

ISSN 2017 (Jäger et al., Journal of the International Society of Sports Nutrition) pools the resistance-training protein literature: 1.6-2.2 g/kg (≈ 0.7-1.0 g/lb) is sufficient for muscle protein synthesis on bulks; cuts require 2.3-3.1 g/kg (≈ 1.0-1.4 g/lb) to preserve lean mass under a calorie deficit. The calculator uses 1.1 g/lb on cuts (mid-range) and 0.9 g/lb on bulks (high end of bulk range, low end of cut range — covers the ~5% of users who don’t respond to lower doses). Going above 1.4 g/lb buys nothing; the literature is clear on the diminishing-returns ceiling.

Why does the cut deficit cap at 25%?

Beyond ~25% below TDEE, three things accelerate: lean-mass loss (the body’s emergency catabolism response), thyroid down-regulation (T3 drops 10-20%, NEAT collapses), and adherence failure (hunger, fatigue, mood crash). Aggressive cuts beyond 25% are clinically appropriate for short-duration medical-supervision protocols — not for the calculator’s self-directed audience. Standard −20% is the documented sustainable maximum for 8-12 week dieting blocks; conservative −12% works for 16-26 week recomposition blocks where preserving training quality is the priority.

Why is the bulk surplus only +10% standard and +20% aggressive?

Helms 2014 and Garthe 2013 (separate trials in trained lifters) showed +5-10% surplus produces nearly identical lean-mass gain to +20-30% surplus, with dramatically less fat gain. The body’s muscle protein synthesis capacity is the rate limiter, not calorie availability — so ‘more food = more muscle’ is false above the +10% threshold. Aggressive bulks (+20%) only make sense for hardgainers (rare) or when calendar pressure forces a fast bulk before a sport-specific cut. The calculator’s default standard +10% is the lean-bulk sweet spot.

Why are fat percentages set to 28% on cuts and 22% on bulks?

Hormonal floor first: dietary fat below 0.3 g/lb (≈ 20% of cut calories at typical body weights) starts to suppress testosterone and the broader endocrine cascade — Volek 1997 and subsequent trials confirm that floor. The calculator uses 28% on cuts to sit comfortably above the floor while leaving room for protein. On bulks, more carbs help training performance and glycogen storage, so fat drops to 22% to make calorie space for higher carb intake (~1.5-2.5 g/lb on most bulk targets).

How do I pick the right activity level?

Be brutally honest. ‘Sedentary’ (1.2) means desk job + zero structured exercise — most users undershoot this and call themselves ‘light.’ ‘Light’ (1.375) is 1-3 lifting or cardio sessions per week, otherwise sedentary. ‘Moderate’ (1.55) is 3-5 sessions/wk with normal walking — the right tier for most office workers who train 4×/wk. ‘Active’ (1.725) is daily training OR a non-sedentary day job (nurse, contractor, retail). ‘Very active’ (1.9) is two-a-days or a physical labor job plus structured training — most people aren’t this. Picking one tier too high adds 200-400 kcal to your target — enough to stall a cut entirely.

What if my actual weight change doesn’t match the predicted weekly change?

After 2-3 weeks of consistent tracking, recalibrate. The 3,500 kcal/lb rule is a useful approximation but real-world variation is ±20%: gut content, water retention, glycogen swings, NEAT adaptation. If the scale moves slower than predicted on a cut, drop calories by 100-150 (or add 1-2k steps/day). If faster, add 100-150 kcal — ‘winning too fast’ on a cut means you’re losing lean mass alongside fat. Bulks: if the scale outpaces +0.25-0.5 lb/wk on a standard surplus, drop 100 kcal — you’re storing fat past the lean-gain ceiling.

Should I cycle calories or eat the same target every day?

Either works at the weekly average. The calculator gives a daily number; eating that exact number every day is the simplest implementation. Carb-cycling, refeeds, and diet breaks are advanced tools that work at the weekly level (e.g., 6 days at the cut target + 1 maintenance day = same weekly deficit, may improve adherence and leptin). For most users at the standard tier, eat-the-same-thing is more accurate because you avoid the ‘refeed turned into a binge’ failure mode that wipes out the deficit.

Why does this calculator use lb, not kg?

Because the protein literature is anchored in g/lb in the U.S.-dominant fitness community (ISSN 2017, Helms’ book, Schoenfeld’s reviews), and 90%+ of the search volume for ‘cutting calculator’ and ‘bulking calculator’ comes from U.S. and U.K. users using lb. The calculator converts internally to kg and cm for the Mifflin-St Jeor equation; you don’t need to think about the unit conversion. If you’re used to kg, use 1 kg = 2.2046 lb in your head — the protein g/kg target equivalent is roughly double the g/lb number.

Is this safe during pregnancy, breastfeeding, or with thyroid conditions?

No, this calculator is not intended for those populations. Pregnancy adds 300-450 kcal/day to maintenance and shifts macro priorities (more carbs for fetal glucose, more protein for tissue building); breastfeeding adds 450-500 kcal/day. Hypothyroidism reduces measured BMR by 5-15% below Mifflin-St Jeor predictions (Hyperthyroidism the reverse). PCOS, T2 diabetes, and metabolic syndrome also distort the macro math. Anyone in those categories should work with a registered dietitian or endocrinologist; calculator output for those users is directional at best and wrong at worst.

Can I use this for body recomposition (lose fat AND gain muscle)?

Yes — set goal to ‘maintain’. Recomp works at small caloric deficits (or maintenance) when protein is high and resistance training is hard. The calculator’s maintain target with 0.8 g/lb protein is right for trained lifters seeking recomp; if you’re a beginner, use cut + conservative because beginners can recomp at modest deficits more efficiently than advanced lifters. The trade-off is speed: recomp adds muscle at maybe 0.25 lb/month vs 0.5-1 lb on a real bulk, and drops fat at maybe 0.25 lb/month vs 1-1.5 on a real cut. Slower but cleaner.

Why can’t I set my own protein target?

Because the ISSN 2017 anchor is calibrated against goal — cut, maintain, bulk drive different optimal protein doses. If you’re sure you want a custom number (e.g., 1.5 g/lb because you’re aggressively dieting and a coach told you to go higher), take the calculator’s carb number and subtract the extra protein grams × 4 kcal from carbs to keep the calorie target intact. Most users adding more protein than the calculator suggests are over-anchoring on the ‘more protein = better’ meme and leaving carbs/fat too low — the macros get unbalanced and adherence drops.

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Macros Cutting / Bulking Calculator — Calorie Target + Protein, Fat, Carb Grams + Weekly Weight Change

Drop your weight, height, age, sex, activity level, goal (cut, maintain, bulk) and aggressiveness. Calculator returns Mifflin-St Jeor BMR, TDEE with ACSM 2018 activity factors, target calories with the goal modifier applied, ISSN 2017-anchored protein target (1.0-1.4 g/lb cuts, 0.8-1.0 bulks), fat (28% cuts / 22% bulks of kcal — above the hormonal-floor minimum), carbs filling the remainder, and the expected weekly weight change at that deficit or surplus.

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Private — nothing saved
Works on any device
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Reviewed by CalcBold EditorialLast verified May 4, 2026Methodology

Body weight (lb)

Most-recent honest scale weight, ideally measured first thing in the morning post-bathroom, pre-meal. Drives BMR via Mifflin-St Jeor (1990) and the protein g/lb anchor — small input errors propagate into 50-150 kcal target shifts.

Height (inches)

Total inches; 5′10″ = 70. Mifflin-St Jeor is the most accurate BMR equation in published meta-analyses (within ~5% of measured BMR for most adults vs Harris-Benedict’s ~10%).

Age

Calendar age. BMR drops ~5 kcal/yr in Mifflin-St Jeor (the −5×age term), so a 50-year-old’s baseline is ~100 kcal lower than a 30-year-old at identical body composition.

Sex (biological)

Mifflin-St Jeor uses a 166 kcal/day male-vs-female offset to account for lean-mass differences at identical weight and height. The protein g/lb anchor is the same either way.

Activity level

ACSM 2018 activity factor multiplied against BMR to estimate TDEE. Most desk workers who lift 4×/wk are honestly ‘moderate’ — ‘active’ means daily training plus a non-sedentary day job.

Goal

Drives the deficit or surplus applied to TDEE. Cuts also raise the protein anchor to 1.1 g/lb (preserves lean mass under deficit per ISSN 2017); bulks drop it to 0.9 g/lb.

Aggressiveness

Aggressive cuts (>20% deficit) sustain 8-12 wks max — beyond that, lean-mass loss accelerates and adherence collapses. Aggressive bulks add fat at >1:1 ratio with muscle. Standard is the sustainable default for most adults.

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What This Calculator Does

The Macros Cutting / Bulking Calculator turns seven inputs — weight, height, age, sex, activity level, goal, and aggressiveness — into a complete daily macro plan anchored on the standard exercise-physiology literature. It returns Mifflin-St Jeor BMR (1990, the most accurate generic equation in published meta-analyses), TDEE using ACSM 2018 activity factors (1.2 to 1.9), a calorie target with the goal modifier applied (cut, maintain, or bulk at conservative, standard, or aggressive doses), and protein, fat, and carb grams calibrated against ISSN 2017’s protein recommendations for resistance-trained athletes.

The output isn’t a generic ‘eat 2,000 kcal and 50 g protein’ recipe — it’s a stack of numbers each tied to a published anchor. The protein target shifts to 1.1 g/lb on cuts (preserves lean mass under deficit per ISSN 2017), 0.9 g/lb on bulks (high end of the bulk-range), 0.8 g/lb on maintenance. Fat sits at 28% of kcal on cuts, 22% on bulks — both above the 0.3 g/lb hormonal floor. Carbs fill the remainder. The expected weekly weight change is computed against the 3,500 kcal/lb approximation so you can sanity-check the dose against your target trajectory.

The Math

Goal adjust values: cut conservative −12%, standard −20%, aggressive −25%. Bulk conservative +5%, standard +10%, aggressive +20%. These are the sustainability-tested ranges from Helms 2014 and the broader natural-bodybuilding literature — cuts beyond 25% deficit accelerate lean-mass loss and thyroid down-regulation; bulks beyond +20% surplus stack fat at worse than 1:1 with muscle.

A Worked Example — “Standard Cut, 180 lb Male”

Suppose you’re a 30-year-old male, 180 lb, 5′10″ (70 in), moderately active (lifting 4×/wk plus light walking), running a standard cut:

BMR (Mifflin-St Jeor): 1,786 kcal/day
TDEE (BMR × 1.55 moderate): 2,768 kcal/day
Goal adjust: standard cut = −20%
Target calories: 2,214 kcal/day
Protein: 180 lb × 1.1 = 198 g (792 kcal)
Fat: 28% of 2,214 = 620 kcal → 69 g
Carbs: (2,214 − 792 − 620) / 4 = 200 g
Expected weekly change: −1.1 lb/wk

Verdict: CUT — STANDARD AGGRESSIVE — sustainable for 8-12 weeks. Lean-mass preserved by the high protein floor; fat above the hormonal-floor minimum; carbs meaningful enough to fuel 4×/wk lifting. Run this for 8 weeks, recalibrate at week 3 if real weight change deviates >20% from the −1.1 lb/wk prediction.

When This Is Useful

Starting a deliberate cut or bulk.Most users come into this calc from a generic ‘eat at TDEE’ baseline and need the goal modifier and protein anchor calibrated to the deliberate dieting phase — that’s exactly what the goal + aggressiveness selectors deliver. Switching dieting phases.Closing out a 16-week cut and pivoting to a lean bulk? The calculator rebuilds the macro plan in one click — same body weight, new goal, new protein and fat anchors, new carb floor. Reality-checking a coach’s plan. If a coach prescribed 1,400 kcal at 250 g protein for a 200 lb cutter, this calc surfaces that the protein is plausible (1.25 g/lb) but the calorie target is well into aggressive territory (likely >30% deficit) — useful cross-reference before committing.

Common Mistakes

Picking activity level one tier too high. The single largest source of error. Most desk-job lifters training 4×/wk are honestly ‘moderate’ (×1.55), not ‘active’ (×1.725). One tier too high adds ~270 kcal to the target — enough to stall a cut entirely or convert a lean bulk into a sloppy bulk.
Ignoring the protein g/lb anchor on cuts. Cuts use 1.1 g/lb (not the maintenance 0.8) precisely to preserve lean mass under the deficit. If you drop protein to maintenance levels ‘to make room for carbs’ on a cut, you’ll lose 30-50% of the cut’s weight loss as muscle — ISSN 2017’s primary warning.
Going aggressive when standard would work. Aggressive cuts (−25%) are appropriate for contest-prep or short-term medical protocols, not sustained dieting. Standard −20% delivers nearly identical 12-week weight loss with dramatically better adherence and muscle preservation. Aggressive bulks (+20%) only make sense for rare hardgainers; standard +10% is the lean-bulk sweet spot.
Not recalibrating at week 3.The 3,500 kcal/lb math is approximate; real-world variation is ±20% (water, glycogen, NEAT adaptation). If your actual weekly weight change deviates more than 20% from prediction over a 14-day window, adjust calories by 100-150 and rerun. Single-snapshot use of the calculator assumes the equation is exact — it isn’t.
Treating the carb number as untouchable. Carbs fill the calorie remainder — if you need to push fat above 28% on a cut for satiety reasons, do that and let carbs absorb the swap (every 1 g fat moved costs you 2.25 g carbs at the calorie level). The calculator’s starting split is the literature-anchored default, not a rigid prescription.
Running the cut and bulk math without training.The protein anchor assumes resistance training — ISSN 2017’s 1.6-2.2 g/kg range is ‘for individuals doing resistance exercise.’ A sedentary cut with high protein still works for fat loss but the lean-mass-preservation benefit largely disappears. Lift heavy 3-5×/wk while running these numbers.

Related Calculators

Confirm the upstream maintenance number first. The Calorie / TDEE Calculator gives the BMR + activity-factor TDEE in isolation if you want to verify the maintenance baseline before applying a goal modifier. The Macro Calculator handles the more general macro split for non-cutting, non-bulking users (recomp, off-season, casual fitness). The Protein Intake Calculator gives the protein g/kg + g/lb breakdown standalone for users who want to dial protein independent of goal context. And once the cut succeeds in dropping BMI back into the 18.5-25 band, the Biological Age Calculator quantifies how many years of biological age you recovered through the body-composition change.

Frequently Asked Questions

The most common questions we get about this calculator — each answer is kept under 60 words so you can scan.

Why Mifflin-St Jeor instead of Harris-Benedict or Katch-McArdle?
Mifflin-St Jeor (1990) is the most accurate generic BMR equation in published meta-analyses — average error ~5% vs measured BMR (indirect calorimetry) for adults across the 18-90 age range. Harris-Benedict (1919) systematically overestimates BMR by ~5-10% in modern populations because the underlying cohort had different body composition and metabolic rates. Katch-McArdle is more accurate IF you have an exact body-fat percentage from DEXA — but most users self-report body fat with ±5% error, which destroys the precision advantage. The calculator uses Mifflin-St Jeor because it’s the cleanest input-to-output mapping with the smallest typical input error.
Why 1.1 g/lb protein on cuts and only 0.9 g/lb on bulks?
ISSN 2017 (Jäger et al., Journal of the International Society of Sports Nutrition) pools the resistance-training protein literature: 1.6-2.2 g/kg (≈ 0.7-1.0 g/lb) is sufficient for muscle protein synthesis on bulks; cuts require 2.3-3.1 g/kg (≈ 1.0-1.4 g/lb) to preserve lean mass under a calorie deficit. The calculator uses 1.1 g/lb on cuts (mid-range) and 0.9 g/lb on bulks (high end of bulk range, low end of cut range — covers the ~5% of users who don’t respond to lower doses). Going above 1.4 g/lb buys nothing; the literature is clear on the diminishing-returns ceiling.
Why does the cut deficit cap at 25%?
Beyond ~25% below TDEE, three things accelerate: lean-mass loss (the body’s emergency catabolism response), thyroid down-regulation (T3 drops 10-20%, NEAT collapses), and adherence failure (hunger, fatigue, mood crash). Aggressive cuts beyond 25% are clinically appropriate for short-duration medical-supervision protocols — not for the calculator’s self-directed audience. Standard −20% is the documented sustainable maximum for 8-12 week dieting blocks; conservative −12% works for 16-26 week recomposition blocks where preserving training quality is the priority.
Why is the bulk surplus only +10% standard and +20% aggressive?
Helms 2014 and Garthe 2013 (separate trials in trained lifters) showed +5-10% surplus produces nearly identical lean-mass gain to +20-30% surplus, with dramatically less fat gain. The body’s muscle protein synthesis capacity is the rate limiter, not calorie availability — so ‘more food = more muscle’ is false above the +10% threshold. Aggressive bulks (+20%) only make sense for hardgainers (rare) or when calendar pressure forces a fast bulk before a sport-specific cut. The calculator’s default standard +10% is the lean-bulk sweet spot.
Why are fat percentages set to 28% on cuts and 22% on bulks?
Hormonal floor first: dietary fat below 0.3 g/lb (≈ 20% of cut calories at typical body weights) starts to suppress testosterone and the broader endocrine cascade — Volek 1997 and subsequent trials confirm that floor. The calculator uses 28% on cuts to sit comfortably above the floor while leaving room for protein. On bulks, more carbs help training performance and glycogen storage, so fat drops to 22% to make calorie space for higher carb intake (~1.5-2.5 g/lb on most bulk targets).
How do I pick the right activity level?
Be brutally honest. ‘Sedentary’ (1.2) means desk job + zero structured exercise — most users undershoot this and call themselves ‘light.’ ‘Light’ (1.375) is 1-3 lifting or cardio sessions per week, otherwise sedentary. ‘Moderate’ (1.55) is 3-5 sessions/wk with normal walking — the right tier for most office workers who train 4×/wk. ‘Active’ (1.725) is daily training OR a non-sedentary day job (nurse, contractor, retail). ‘Very active’ (1.9) is two-a-days or a physical labor job plus structured training — most people aren’t this. Picking one tier too high adds 200-400 kcal to your target — enough to stall a cut entirely.
What if my actual weight change doesn’t match the predicted weekly change?
After 2-3 weeks of consistent tracking, recalibrate. The 3,500 kcal/lb rule is a useful approximation but real-world variation is ±20%: gut content, water retention, glycogen swings, NEAT adaptation. If the scale moves slower than predicted on a cut, drop calories by 100-150 (or add 1-2k steps/day). If faster, add 100-150 kcal — ‘winning too fast’ on a cut means you’re losing lean mass alongside fat. Bulks: if the scale outpaces +0.25-0.5 lb/wk on a standard surplus, drop 100 kcal — you’re storing fat past the lean-gain ceiling.
Should I cycle calories or eat the same target every day?
Either works at the weekly average. The calculator gives a daily number; eating that exact number every day is the simplest implementation. Carb-cycling, refeeds, and diet breaks are advanced tools that work at the weekly level (e.g., 6 days at the cut target + 1 maintenance day = same weekly deficit, may improve adherence and leptin). For most users at the standard tier, eat-the-same-thing is more accurate because you avoid the ‘refeed turned into a binge’ failure mode that wipes out the deficit.
Why does this calculator use lb, not kg?
Because the protein literature is anchored in g/lb in the U.S.-dominant fitness community (ISSN 2017, Helms’ book, Schoenfeld’s reviews), and 90%+ of the search volume for ‘cutting calculator’ and ‘bulking calculator’ comes from U.S. and U.K. users using lb. The calculator converts internally to kg and cm for the Mifflin-St Jeor equation; you don’t need to think about the unit conversion. If you’re used to kg, use 1 kg = 2.2046 lb in your head — the protein g/kg target equivalent is roughly double the g/lb number.
Is this safe during pregnancy, breastfeeding, or with thyroid conditions?
No, this calculator is not intended for those populations. Pregnancy adds 300-450 kcal/day to maintenance and shifts macro priorities (more carbs for fetal glucose, more protein for tissue building); breastfeeding adds 450-500 kcal/day. Hypothyroidism reduces measured BMR by 5-15% below Mifflin-St Jeor predictions (Hyperthyroidism the reverse). PCOS, T2 diabetes, and metabolic syndrome also distort the macro math. Anyone in those categories should work with a registered dietitian or endocrinologist; calculator output for those users is directional at best and wrong at worst.
Can I use this for body recomposition (lose fat AND gain muscle)?
Yes — set goal to ‘maintain’. Recomp works at small caloric deficits (or maintenance) when protein is high and resistance training is hard. The calculator’s maintain target with 0.8 g/lb protein is right for trained lifters seeking recomp; if you’re a beginner, use cut + conservative because beginners can recomp at modest deficits more efficiently than advanced lifters. The trade-off is speed: recomp adds muscle at maybe 0.25 lb/month vs 0.5-1 lb on a real bulk, and drops fat at maybe 0.25 lb/month vs 1-1.5 on a real cut. Slower but cleaner.
Why can’t I set my own protein target?
Because the ISSN 2017 anchor is calibrated against goal — cut, maintain, bulk drive different optimal protein doses. If you’re sure you want a custom number (e.g., 1.5 g/lb because you’re aggressively dieting and a coach told you to go higher), take the calculator’s carb number and subtract the extra protein grams × 4 kcal from carbs to keep the calorie target intact. Most users adding more protein than the calculator suggests are over-anchoring on the ‘more protein = better’ meme and leaving carbs/fat too low — the macros get unbalanced and adherence drops.