Free Unit Converter — Length · Weight · Volume · Temperature · Area · Time

What This Calculator Does

A unit converter translates a quantity expressed in one unit of measurement into the equivalent quantity in another. Type 10 kilometers, ask for miles, and the calculator returns 6.21 mialong with the cross-rate (1 km = 0.6214 mi), the inverse rate (1 mi = 1.609 km), and the working steps so you can see exactly how the answer was produced. The same machinery handles every common domain — lengths, weights, volumes, areas, times, and temperatures — with one consistent flow: pick a domain, choose from and to units, type the amount, read the result.

Most converters online silently round to two decimals, hide the formula, and leave you guessing whether the answer is correct. This one returns up to six significant digits, shows the multiplication chain through the base unit, and prints the cross-rate so you can sanity check on a napkin. If you only need the cross-rate, you can ignore the amount — the rate alone is the gear ratio that powers every conversion in that direction.

The Six Measurement Domains

The calculator covers six families of measurement. Inside each family, all units share a single base, and every other unit is defined as a multiplier (or, for temperature, as an offset-and-scale) of that base.

• Length— meters, kilometers, centimeters, millimeters, miles, yards, feet, inches, and nautical miles. Base unit: the meter. The kilometer is 1,000 meters; the inch is exactly 0.0254 meters by international agreement since 1959.
• Weight (mass)— kilograms, grams, milligrams, pounds, ounces, stone, metric tonnes, and US tons. Base unit: the kilogram. One pound is 0.45359237 kg exactly; one US ton is 2,000 lb (different from a UK long ton or a metric tonne).
• Volume— liters, milliliters, cubic meters, US gallons, UK gallons, quarts, pints, cups, fluid ounces, tablespoons, and teaspoons. Base unit: the liter. US and UK gallons differ by about 20% — a critical gotcha covered later.
• Area— square meters, square kilometers, hectares, acres, square feet, square yards, and square miles. Base unit: the square meter. An acre is 4,046.86 m² (43,560 sq ft); a hectare is exactly 10,000 m².
• Time— seconds, milliseconds, minutes, hours, days, weeks, months (averaged at 30.44 days), and years (365.25 days, accounting for leap years). Base unit: the second. Months and years are averages— a real February has 28 or 29 days, not 30.44.
• Temperature— Celsius, Fahrenheit, Kelvin, Rankine. The odd one out: it does not use pure ratios. More on the offset-and-scale arithmetic in the next section.

How Conversion Factors Work (with the temperature exception)

For five of the six domains, conversion is pure multiplication. Each unit has a toBasemultiplier — the number of base units that one of that unit contains. Convert from any unit by multiplying the amount by its toBase; convert toany unit by dividing the base value by the target’s toBase. Internally the calculator does exactly this two-step pivot:

Example for length: 10 km → mi. The toBase for km is 1000 (one km is 1000 m); for mi it is 1609.344. So baseValue = 10 × 1000 = 10,000 m, and result = 10,000 ÷ 1609.344 = 6.2137 mi. Two multiplications, one division. That same pattern is what lets the calculator combine anytwo units in the same domain without coding a separate formula for every pair — n units only need n multipliers, not n × (n − 1) pairwise rates.

Temperature breaks this patternbecause the three common scales do not share a zero-point. Celsius zero is the freezing point of water; Fahrenheit zero is a salt slurry that Daniel Fahrenheit calibrated against in 1724; Kelvin zero is absolute zero. That means a unit of temperature is not a fixed quantity of “temperature stuff” the way a meter is a fixed quantity of length — the scales are translated as well as scaled. The math becomes offset + scale:

Internally the calculator pivots through Celsius the same way other domains pivot through their base unit — convert from-unit to °C first, then from °C to to-unit. The pivot is invisible in the final answer, but it is shown in the working steps so you can verify the offset was applied correctly.

How to Use This Calculator

1. Pick a domain. Length, weight, volume, area, time, or temperature. Cross-domain conversions (e.g., feet to gallons) are physically meaningless and the calculator will not allow them.
2. Pick the from-unit. Use the unit codes:
   Length — m, km, cm, mm, mi, yd, ft, in, nmi
   Weight — kg, g, mg, lb, oz, st, t, ton
   Volume — l, ml, m3, gal_us, gal_uk, qt, pt, cup, fl_oz, tbsp, tsp
   Area — m2, km2, ha, ac, ft2, yd2, mi2
   Time — s, ms, min, hr, day, wk, mo, yr
   Temperature — C, F, K, R (Celsius, Fahrenheit, Kelvin, Rankine)
3. Pick the to-unit. Same code list, same domain.
4. Type the amount.Negatives are accepted (useful for sub-zero temperatures). Decimals are accepted to as many digits as you like — the calculator carries full double-precision arithmetic and only rounds the displayed answer.
5. Read the result, the verdict line, the cross-rate, the inverse, and the two-step working chain. If anything looks off, the working chain shows the exact base-pivot multiplication.

Three Worked Examples

Three scenarios drawn from common conversions — copy any of them into the calculator above to see the full breakdown.

Example 1 — Length: kilometers to miles

You ran a 10 km race; how far is that in miles? Pick length, set from-unit km and to-unit mi, type 10. The calculator returns 6.21 mi.

The math: 10 km × 1000 m/km = 10,000 m (the base-unit pivot). Then 10,000 m ÷ 1609.344 m/mi = 6.2137 mi. The cross-rate displayed is 1 km = 0.6214 mi— multiply any kilometer figure by 0.6214 for a quick miles approximation. The inverse, 1 mi = 1.6093 km, is the same fact rotated. A common mental trick: a kilometer is roughly five-eighths of a mile, so 10 km ≈ 6.25 mi (within 1% of the exact 6.2137).

Example 2 — Weight: kilograms to pounds

Bathroom scale reads 70 kg; what is that in pounds? Pick weight, from kg, to lb, amount 70. Result: 154.32 lbs.

The math: 70 kg × 1 kg/kg = 70 kg base. Then 70 ÷ 0.45359237 = 154.32 lb. The cross-rate is 1 kg = 2.2046 lb. The popular “multiply kg by 2.2” mental shortcut is about 0.2% low — for body weight that is a third of a pound off, never material. Going the other way, 1 lb = 0.4536 kg; doctors and gyms in mixed-system clinics reach for this all day. Note the 70 kg → 154.32 lb conversion uses the international avoirdupois pound (the standard one); troy pounds and apothecary pounds are different animals and not in this calculator.

Example 3 — Temperature: Fahrenheit to Celsius

US weather report says it is 100°F; how hot is that in Celsius? Pick temperature, from F, to C, amount 100. Result: 37.78°C.

The math is not a pure ratio because the scales have different zero-points. (100 − 32) × 5/9 = 68 × 5/9 = 37.78°C. Subtract first to align the zero, then scale by 5/9 because each Fahrenheit degree is smaller than a Celsius degree (a 180°F gap from freezing to boiling versus a 100°C gap, so the F-degree is 100/180 = 5/9 of a C-degree).

Notice that doubling the input does not double the output: 50°F → 10°C, but 100°F → 37.78°C, not 20°C. That is the offset-and-scale arithmetic at work. Forgetting it is the most common temperature-conversion bug, and it is the reason the calculator pivots every temperature through Celsius internally rather than precomputing pairwise factors.

Common Mistakes

• Confusing US and UK gallons.A US gallon is 3.785 L; a UK (imperial) gallon is 4.546 L — about 20% bigger. A car spec listed at “30 mpg” UK is roughly 25 mpg US. The calculator distinguishes them as gal_us and gal_uk, and you should always verify which one a published number means.
• Mixing US ton with metric tonne. A US ton is 907.185 kg (2,000 lb); a metric tonne is 1,000 kg exactly; a UK long ton is 1,016 kg (2,240 lb). The calculator codes them as ton and t; do not assume they are the same. A shipping invoice that says “5 tons” with no qualifier is ambiguous and can mean a 5% pricing error.
• Treating temperature like a pure ratio.A common bug: “if 32°F is 0°C, then 64°F must be 0°C × 2 = 0°C.” Wrong. 64°F is 17.78°C because the offset shifts before the scaling kicks in. Always subtract 32 first when going F → C.
• Square units are squared, not just renamed. 1 m² is not3.281 ft² (the linear factor). It is 3.281² = 10.764 ft². Same for cubic units — 1 m³ = 35.31 ft³, not 3.281. This calculator already squares and cubes correctly, but it is the most common DIY-spreadsheet mistake.
• Using months and years as exact units.A “month” here is 30.44 days (the average across the calendar year). A “year” is 365.25 days (with a quarter-day for leap years). Never use these for legal billing periods, contract deadlines, or interest accrual — for those you need calendar-aware date math, not fixed-multiplier conversion.
• Forgetting that pounds and ounces have two systems. The avoirdupois ounce is 28.35 g (for groceries and bodyweight); the troy ounce is 31.10 g (for precious metals). When the calculator says oz it means avoirdupois. If you are weighing gold, multiply by 31.10/28.35 ≈ 1.097 to correct.

US vs UK vs Metric — Where Things Diverge

Three measurement traditions are in active daily use, and the differences trip up shoppers, engineers, and travelers. Here are the divergences this calculator codifies:

• Volume: 1 US gallon = 3.785 L; 1 UK gallon = 4.546 L. UK pints (568 ml) are bigger than US pints (473 ml). Recipes from The Joy of Cooking and Delia Smithuse different cup sizes — a US cup is 240 ml, a metric cup (Australia/UK) is 250 ml. A small but real 4% drift in any baking recipe.
• Weight:Stone is uniquely UK — 14 lb per stone, used almost exclusively for body weight. A 12-stone person weighs 168 lb (76.2 kg). The US never adopted stone; the metric world never adopted any of these.
• Length:Inches, feet, yards, miles are shared between US and UK customary systems — one of the few measurement areas where they agree. Nautical miles (1,852 m) are aviation/maritime international, distinct from statute miles (1,609.344 m).
• Temperature: The US is essentially alone on Fahrenheit for daily life. Most of the world uses Celsius; scientific work uses Kelvin. Rankine (Fahrenheit-scaled but absolute-zero-anchored) hangs on in some US thermodynamics texts.

When you are reading a spec sheet or a recipe sourced from another country, the safe habit is to verify which gallon, which ton, and which scale is meant before plugging the number into anything that matters.

When This Calculator Decides For You

Unit conversion is rarely abstract — it usually maps to a real-world choice or verification:

1. Cooking and baking. A French recipe in grams, a US oven set in Fahrenheit, a UK measuring cup. Convert all three to one consistent system before you start cooking, not while flour is on your hands. For percentage-based scaling of a recipe, pair this with the percentage calculator.
2. Travel. Speed limits in km/h, fuel gauge in US gallons, weather report in Celsius. The calculator gives you the cross-rate once; commit it to memory or save the math working for the trip.
3. Engineering and construction.A drawing dimensioned in mm, a material spec in inches, a load capacity in lbs. Mismatched units in a single document are the single most common source of construction errors — convert and label everything in the same system before passing the document along.
4. Online shopping.Amazon listings often mix systems — a TV is measured in inches, a pet food bag is in lb, a laundry detergent is in fl oz. Convert to the system you understand to compare two listings honestly.
5. Health and fitness. Bodyweight in lb at the doctor, pool length in meters, running pace in minutes per km versus minutes per mile. A consistent system across your tracking apps is worth ten minutes of conversion up front.

Cross-Domain Combinations

Some everyday measurements combine units from two domains — speed (length over time), density (mass over volume), pressure (force over area). This calculator handles the single-domain conversions; for cross-domain ratios you do two conversions and divide.

Example: miles per hour to kilometers per hour.60 mph means “60 miles in 1 hour.” Convert miles to km: 60 mi × 1.609344 = 96.56 km. The hour stays the same. Result: 96.56 km/h. Notice that when both units of a ratio are in the same domain (here, time on the bottom), only the changed unit needs conversion. The calculator’s cross-rate output (1 mi = 1.6093 km) is the multiplier you reach for here.

Example: liters per 100 km to miles per US gallon.This one is famously nonlinear because it inverts. A 7 L/100km car covers 100 km on 7 L. Convert 100 km to mi (62.137 mi), and 7 L to gal_us (1.849 gal). 62.137 ÷ 1.849 = 33.6 mpg. Worth doing once carefully and writing the answer in your car’s manual.

Example: kg per square meter to lb per square foot(a structural-engineering load figure). 1 kg = 2.2046 lb; 1 m² = 10.7639 ft². So 1 kg/m² = 2.2046 / 10.7639 = 0.2048 lb/ft². Multiply your kg/m² by 0.2048 to get lb/ft². Two single-domain conversions, one division — that is the recipe for any cross-domain rate.

For converting prices when products come in different package sizes (e.g., comparing a 2 L bottle of detergent to a 64 fl oz bottle), pair this calculator with the discount calculator to compute the per-unit savings, and the fraction calculator when the conversion produces awkward fractions you want to simplify.