Free Unit Converter

Convert between common units of length, weight, volume, area, speed, time, and temperature. Imperial ↔ Metric.

Enter your details

length: mm cm m km in ft yd mi · weight: mg g kg oz lb ton · volume: ml l fl-oz cup pint quart gallon · area: cm2 m2 km2 in2 ft2 acre hectare · speed: m/s km/h mph knots · time: sec min hr day week year · temperature: C F K

Result
Enter your details on the left, then press Calculate.

More Everyday Tools tools

See all →

What is this calculator for?

You're following a British baking recipe that calls for 250g flour, but your measuring cups show ounces and tablespoons. Or you're reading a German car spec listing fuel consumption as 6.5 L/100km and need to translate to MPG for comparison shopping. Or you're a runner with a 10K training plan in km but a trail map in miles. The unit converter handles bidirectional conversions across length, weight, volume, temperature, area, speed, and energy — the most common everyday conversions in one tool.

US uses the customary system (inches, pounds, gallons, Fahrenheit) for most everyday measurements. Most of the world uses metric (cm, kg, liters, Celsius). Scientific and medical contexts in the US use metric for precision and international consistency. This calculator handles both directions across the categories people actually need: length (in/ft/yd/mi ↔ mm/cm/m/km), weight (oz/lb/ton ↔ g/kg/metric ton), volume (tsp/tbsp/cup/pt/qt/gal ↔ ml/L), temperature (F ↔ C ↔ K), area (sq ft/acre ↔ sq m/hectare), speed (mph ↔ km/h ↔ m/s), and energy (BTU ↔ kJ ↔ kcal).

Single-unit precision and source data come from NIST (National Institute of Standards and Technology) — the US federal authority on measurement standards.

How to use this calculator

Pick the category (length, weight, volume, temperature, etc.) and the source unit and target unit. Enter the value to convert. The calculator returns the equivalent in the target unit plus typically 2-3 other related units for context (so converting inches to cm also shows mm and m for reference).

For temperature, conversions are non-linear due to different zero points: F to C uses (F − 32) × 5/9 = C. C to F uses C × 9/5 + 32. Kelvin shares Celsius's scale with offset: K = C + 273.15.

For cooking specifically, see the dedicated cooking conversion tool which handles the unusual recipe-specific units (sticks of butter, packages of yeast, dry ounces vs fluid ounces) that don't fit the standard converter.

Understanding your results

The calculator returns the equivalent value in the target unit, plus 2-3 related-unit equivalents for context.

Common conversions worth memorizing for daily life. Length: 1 inch = 2.54 cm exactly. 1 foot = 30.48 cm. 1 mile = 1.609 km. 1 yard = 0.914 m. Weight: 1 ounce = 28.35 g. 1 pound = 453.59 g. 1 kg = 2.205 lb. Volume: 1 fl oz = 29.57 ml. 1 cup = 237 ml. 1 gallon = 3.785 L. Temperature: 0°C = 32°F (water freezes). 100°C = 212°F (water boils at sea level). 37°C = 98.6°F (human body temperature). 20°C = 68°F (room temperature). Speed: 60 mph ≈ 97 km/h. 100 km/h ≈ 62 mph.

The European fuel consumption inversion. L/100km is liters consumed per 100 km driven — LOWER numbers are better. MPG is miles per gallon — HIGHER numbers are better. The two are inversely related: 30 MPG = 7.84 L/100km. 50 MPG = 4.70 L/100km. 25 MPG = 9.41 L/100km. The conversion: L/100km × MPG = 235.215 (a constant). Useful when reading European car reviews or comparing fuel economy across systems.

Recipe scaling for international cooking. A recipe calling for 200g all-purpose flour: in US measurements, that's about 1.6 cups (since 1 cup flour ≈ 125g, depending on packing density). A British "pint" is 568 ml (UK Imperial pint), not 473 ml (US pint) — 20% difference. UK recipes that call for "a pint of milk" mean 568 ml; US recipes mean 473 ml. Get this wrong and recipes fail. Always verify which system a recipe uses (UK vs US) before scaling international recipes.

A worked example

Sofia is following a French pastry blog recipe for tarte tatin. The recipe calls for: 500g apples, 100g sugar, 80g butter, 200°C oven, 25-minute bake time, 25cm round pan.

Conversions she needs:

500g apples: 500 ÷ 453.59 = 1.10 pounds, or about 4 medium apples.

100g sugar: 100g ÷ 200g/cup (granulated sugar density) = 1/2 cup. Or by weight: 100g ÷ 28.35 = 3.5 oz.

80g butter: 80g ÷ 113g per stick (US butter stick = 4 oz = 113g) = 0.7 sticks, or about 5.5 tablespoons.

200°C oven: 200 × 9/5 + 32 = 392°F. Round to 390°F (most US ovens have 25°F increments).

25cm round pan: 25 × 0.394 = 9.85 inches. Use a standard 10-inch round cake pan.

The recipe converts cleanly into US units. She bakes the tarte and it works perfectly. The minor approximations (390 vs 392°F, 10 inch vs 9.85) don't affect the recipe.

Variation: she's traveling to Vienna and reads a fuel economy listing for the Volkswagen Polo at 4.7 L/100km. To compare to a US Toyota Corolla rated 32 MPG: 4.7 L/100km equivalent = 235.215 ÷ 4.7 = 50 MPG equivalent. The European Polo gets about 50 MPG; the US Corolla gets 32 MPG. Significant difference — the Polo is roughly 56% more fuel efficient. But the Polo isn't sold in the US, and the test cycles differ slightly between EPA and European NEDC/WLTP. Useful for context, not directly comparable.

Related resources

For cooking-specific unit conversions, see Cooking Converter. For temperature in cooking specifically, the Cooking Temperature Converter. For currency conversions, the Currency Converter. For clothing and shoe sizes, the Clothing Size Converter and Shoe Size Converter. The NIST Office of Weights and Measures is the US federal authority on measurement standards; NIST Reference on Constants, Units, and Uncertainty publishes exact defined values for all SI units.

Related calculators

percentage calculatortip calculator

Frequently asked questions

Why is the unit field free-text instead of a dropdown?

A free-text field lets you type unit abbreviations directly (ft, kg, mph) and supports the full set across all categories with one keyboard. The help text under each field lists the valid units for the selected category.

Which gallon — US or Imperial?

This calculator uses US gallons (3.78541 liters). The Imperial gallon (used in the UK) is 4.54609 liters, about 20% larger. If you need Imperial conversion, multiply the US result by 0.832674.

What is the difference between weight and mass?

Mass is the amount of matter (kilograms); weight is the force gravity exerts on that mass (newtons). On Earth at sea level the two are commonly conflated, and this tool follows the everyday convention.

Why does the US use different units from most of the world?

Historical inertia plus high conversion costs. The US adopted English customary measurements as a British colony and never formally converted. The metric system was officially endorsed in the Metric Conversion Act of 1975 but never made mandatory; voluntary conversion stalled when manufacturers and consumers resisted relabeling and re-equipping. Today, the US is one of only three countries (with Liberia and Myanmar) that hasn't officially adopted metric for everyday use. US science, medicine, and military use metric internally; consumer products show both. The estimated cost to fully convert US to metric would be $500 billion to $1 trillion — politically and practically not feasible at this point.

Is a US gallon the same as a UK gallon?

No. US gallon = 3.785 liters (128 US fl oz). UK Imperial gallon = 4.546 liters (160 UK fl oz). UK gallon is 20% larger than US gallon. UK fl oz is also slightly different from US fl oz (28.41 ml vs 29.57 ml — UK is smaller). This causes recipe and measurement errors when crossing systems. A British recipe calling for 'a pint of milk' (568 ml) doesn't match a US pint (473 ml). Modern recipes typically list metric volumes (ml, L) to avoid the confusion.

How precise are unit conversions?

Most common conversions are defined exactly. 1 inch = 2.54 cm exactly (defined since 1959 international agreement). 1 pound = 453.59237 g exactly. 1 US fl oz = 29.5735 ml exactly (with longer decimal expansion). Temperature conversions use exact mathematical relationships. The calculator typically shows 2-6 decimal places, sufficient for any practical use. For scientific work requiring more precision: NIST publishes the exact defined values.

What about converting old or obscure units?

Many old units are still used in specific contexts. Furlongs (220 yards, used in horse racing). Hands (4 inches, used to measure horse height). Stones (14 pounds, used in UK body weight). Carats (200 mg, used for gemstones and gold purity). Fathoms (6 feet, used in maritime). Most calculators don't include these niche units but most can be looked up on Wikipedia or NIST handbooks. The standard converters focus on the 80% of daily-use cases (length, weight, volume, temperature, common derivatives).

How do I convert between metric prefixes?

Each prefix changes by powers of 10. Common SI prefixes: nano (10⁻⁹), micro (10⁻⁶), milli (10⁻³), centi (10⁻²), deci (10⁻¹), [base unit], deca (10¹), hecto (10²), kilo (10³), mega (10⁶), giga (10⁹). So 1 km = 1,000 m = 100,000 cm = 1,000,000 mm. Or 1 mg = 0.001 g = 0.000001 kg. The factor of 1,000 between commonly-used prefixes (kg/g/mg, km/m/mm) makes most conversions just decimal point shifts. For less-common prefixes (deca, hecto, deci), be careful; people rarely use those in everyday units except in measurements like deciliters (used in European cooking).