Unit Converter

You have a recipe in cups but a kitchen scale in grams, a height in feet but a form that wants centimeters, or a speed limit in km/h while your dashboard reads mph. A unit converter turns one measurement into another instantly, without the mental math or the chance of slipping a decimal point. Enter 98.6 °F and it returns 37 °C; enter 12 inches and it returns 30.48 cm; enter 150 pounds and it returns 68.04 kg. This page covers length, weight, temperature, volume, area, and speed - the six categories people convert most.

How unit conversion works

Almost every conversion is just a multiplication. Each category defines one base unit, and every other unit is stored as how many base units it equals. Length uses the meter, weight uses the gram, volume uses the milliliter. To convert, the tool turns your value into the base unit, then divides by the target unit's factor:

For example, to convert 5 feet to inches: 1 ft = 0.3048 m and 1 in = 0.0254 m, so the factor is 0.3048 ÷ 0.0254 = 12, and 5 × 12 = 60 inches. The only category that breaks this rule is temperature, because Celsius, Fahrenheit, and Kelvin have different zero points (offsets), so they need formulas instead of a single factor.

The temperature formulas

Temperature is special. There is no single number you can multiply by, because the scales do not share a zero. Use these exact formulas:

So body temperature 98.6 °F = (98.6 − 32) × 5/9 = 37 °C, and water boils at 100 °C = 100 × 9/5 + 32 = 212 °F. Kelvin simply shifts Celsius by 273.15, so 0 °C = 273.15 K and absolute zero (0 K) = −273.15 °C.

How to use this unit converter

Getting an answer takes three steps:

1. Pick a category - tap Length, Weight, Temperature, Volume, Area, or Speed. Each one loads a useful default pair so you can see how it works immediately.
2. Choose your units - set the "From" unit (what you have) and the "To" unit (what you want). The swap button flips them in one click.
3. Type the value - enter the number to convert. The result, the conversion factor, and the value of a single unit all update instantly.

There is no "calculate" button to press and nothing to install - everything runs in your browser as you type.

Who this converter is for

• Students doing physics, chemistry, or math homework where the answer must be in SI (metric) units.
• Cooks and bakers switching between cups, tablespoons, milliliters, and grams across US and metric recipes.
• Travelers reading km/h speed limits, Celsius forecasts, or kilogram luggage limits abroad.
• DIY and trades reconciling a tape measure in feet and inches with materials sold in metric.
• Health and fitness users converting pounds to kilograms or feet/inches to centimeters before running the figures through a BMI Calculator or a dosing formula.
• Anyone who just needs a fast, reliable answer without trusting their head-math at the checkout, gym, or stove.

Key terms explained

• Base unit: the reference each category converts through (meter for length, gram for weight, milliliter for volume).
• Conversion factor: the single number you multiply by to go from one unit to another, e.g. 2.54 for inches to centimeters.
• SI / metric: the International System of Units (meters, kilograms, liters, Celsius) used by most of the world and all of science.
• US customary: the everyday US system (inches, pounds, gallons, Fahrenheit). Similar-sounding to imperial but not identical.
• Significant figures: the digits in a measurement that carry real precision - your answer should not be more precise than your input.
• Offset: the shift in a scale's zero point (why temperature needs a formula, not just a factor).

Worked example 1: inches to centimeters

You measure a shelf at 30 inches and need centimeters for a metric bracket. Multiply by the exact factor 2.54: 30 × 2.54 = 76.2 cm. If you only need a round number, 76 cm is fine - but never report 76.20000 cm, because your tape measure was not that precise.

Worked example 2: gallons to liters

A US gas tank holds 15 gallons and you want liters. One US gallon = 3.785411784 liters, so 15 × 3.785 = 56.78 liters. Watch the unit system here: an imperial (UK) gallon is about 4.546 liters, so the same "15 gallons" abroad would be roughly 68 liters. This converter uses US gallons.

Worked example 3: km/h to mph for a road trip

A sign abroad reads 120 km/h. Divide by 1.60934 (or multiply by 0.621371): 120 × 0.621371 = 74.6 mph. Going the other way, a US 65 mph limit is 65 × 1.60934 = 104.6 km/h. The "per hour" cancels on both sides, so only the distance unit changes.

Length and distance conversions

Length is the category people convert most, usually because a form, a tape measure, or a foreign road sign uses a different system than they do. The base unit is the meter, and the everyday US-to-metric links are worth knowing: 1 inch = 2.54 cm (exact), 1 foot = 30.48 cm, 1 yard = 0.9144 m, and 1 mile = 1.609344 km (exact). Going the other way, a meter is about 3.28 feet (just over a yard) and a kilometer is about 0.621 miles. Heights are a frequent case - 5 ft 10 in is 70 inches, which is 70 × 2.54 = 177.8 cm. Because these factors are exact by definition, length conversions never introduce rounding error of their own; any imprecision comes from how you round the final display. If you are combining a converted length into a wider sum or proportion, the Ratio Calculator handles the scaling once everything is in one unit.

Weight and mass conversions

Strictly speaking, pounds and kilograms measure different things - a pound is a unit of weight (force) and a kilogram is a unit of mass - but in everyday Earth-surface use the conversion is treated as fixed: 1 pound = 0.45359237 kg (exact), and 1 kg = 2.20462 lb. The mental shortcut "divide pounds by 2.2" gets you within about 1%, which is fine for luggage limits or a rough body-weight figure but not for a recipe or a medication dose. Smaller units chain off the same base: 1 ounce = 28.3495 g, and there are 16 ounces in a pound. Cooks often need ounces-to-grams for dry ingredients, while travelers need the kilogram airline allowance - a 50 lb checked-bag limit is 22.68 kg. If you are converting a body weight to plug into a health tool, do the unit step here first, then carry the metric figure into a BMI or calorie calculation.

Volume and cooking conversions

Volume is where unit confusion causes the most real damage, because the US kitchen system, the US "fluid" system, and the UK imperial system all reuse the same words. In US customary units, 1 cup = 236.588 ml, 1 tablespoon = 14.787 ml, 1 teaspoon = 4.929 ml, 1 fluid ounce = 29.574 ml, 1 pint = 473.176 ml, 1 quart = 946.353 ml, and 1 gallon = 3.785411784 liters (exact). A British recipe that calls for a "pint" means 568 ml, not the US 473 ml, so the same word is about 20% larger across the Atlantic. This converter uses US customary volumes throughout. One extra wrinkle for bakers: cups measure volume, while a kitchen scale measures weight, and the two only line up if you know the ingredient's density - a cup of flour and a cup of honey weigh very different amounts, so a pure volume converter cannot turn cups into grams without that density.

Area and volume: square and cube the factor

Area and three-dimensional volume follow the same base-unit logic as length, but with one trap: the conversion factor is squared for area and cubed for volume. Because 1 foot = 0.3048 m, one square foot is 0.3048² = 0.09290304 m², and one cubic foot is 0.3048³ = 0.0283168 m³. Common area links: 1 acre = 4,046.86 m² = 0.404686 hectares, and 1 square mile = 2.59 km². The most common error is reusing the linear factor (multiplying floor area by 0.3048 instead of 0.0929), which understates the result by more than three times. The Scientific Calculator is handy when you need to square or cube a factor by hand to check the tool.

Speed conversions

Speed is a length divided by a time, so converting it only changes the distance part - the "per hour" or "per second" stays the same on both sides of the equation. The everyday cases are mph ↔ km/h for driving (1 mph = 1.60934 km/h; 60 mph ≈ 96.6 km/h) and meters-per-second for physics homework (1 m/s = 3.6 km/h = 2.237 mph). A useful sanity check: km/h numbers are always larger than the same speed in mph, because a kilometer is shorter than a mile, so it takes more of them to cover the same ground per hour. Knots, used in aviation and at sea, are 1.852 km/h each.

Metric prefixes: kilo, centi, milli

Most metric conversions inside a single category (km to m, cm to mm, kg to g, ml to L) are just powers of ten, which is the whole point of the metric system. The prefix tells you the multiplier: kilo = 1,000, hecto = 100, deca = 10, the base unit = 1, deci = 0.1, centi = 0.01, and milli = 0.001. So 2.5 km = 2,500 m, 750 ml = 0.75 L, and 1,500 g = 1.5 kg - you are only shifting the decimal point. This is why metric-to-metric conversions almost never need a calculator, while US customary units (12 inches to a foot, 3 feet to a yard, 16 ounces to a pound) require remembering an irregular factor at each step.

Why the US and the metric world use different units

Nearly every country has standardized on the metric (SI) system, but the United States still runs daily life on US customary units - a descendant of British imperial measures that predates the metric reforms. The two systems were defined against each other decades ago, which is why so many conversion factors are exact rather than approximate: the inch was officially fixed at exactly 2.54 cm in 1959, and the pound at exactly 0.45359237 kg. In practice you only need a converter at the boundary between the systems - reading a metric spec, following a foreign recipe, or reporting homework in SI. Within either system, the conversions are either trivial powers of ten (metric) or a handful of memorized factors (customary).

Common conversion factors

Keep these handy reference factors in mind. Multiply by the factor to go in the listed direction; divide to reverse it.

Quick reference: temperature points

Some everyday temperatures are worth memorizing in both scales:

Significant figures: how precise should your answer be?

A converter can show eight decimals, but that does not mean your answer should. The rule: round your result to match the precision of your measurement. If you weighed something as 5 lb (one significant figure), report about 2 kg, not 2.26796 kg. Here is a quick guide:

Tips for accurate conversions

• Convert once, not twice. Going inches → feet → meters introduces extra rounding; convert straight to the unit you want.
• Mind the system. US and imperial gallons, pints, and tons differ - confirm which one a recipe or spec means.
• Square and cube carefully. Area factors are the linear factor squared, and volume factors are cubed (1 ft = 0.3048 m, but 1 cu ft = 0.3048³ m³).
• Keep full precision until the end. Round only the final answer, not intermediate steps, to avoid compounding errors.
• Sanity-check the direction. A meter is longer than a yard, so meters → yards should give a slightly bigger number; if it shrinks, you divided the wrong way.

Limitations and what this tool does not do

A unit converter is precise within its scope but deliberately narrow. Keep these boundaries in mind:

• It uses US customary units for weight and volume. Imperial (UK) gallons, pints, and tons are a different size and are not offered here.
• It cannot convert volume to weight (cups to grams) on its own, because that depends on the ingredient's density - a cup of flour and a cup of water weigh different amounts.
• It does not handle currency, fuel economy (mpg ↔ L/100km), data storage, time, or angles; those are out of scope for this tool.
• Displayed results are rounded for readability. Carry full precision in any multi-step calculation and round only at the end.
• It is a measurement tool, not engineering certification - for safety-critical work, verify against the controlling standard.

How it compares to related calculators

Conversion is a building block, not a destination - it is usually the first step before another calculation. If your real question is different, a sister tool fits better:

• For a proportion or "what percent of" question rather than a unit change, use the Percentage Calculator.
• To scale a recipe, a map, or a model where two quantities stay in proportion, use the Ratio Calculator - conversion's close cousin.
• After converting several measurements into the same unit so they can be combined, use the Average Calculator.
• For the squares, cubes, and roots behind area and volume factors, use the Scientific Calculator.
• To add or simplify the fractional measurements common in US cooking and carpentry, use the Fraction Calculator.

Whatever you are measuring, the goal is the same: express the number in the unit that the task, the form, or the rest of the math actually needs.

Sources

• National Institute of Standards and Technology (NIST) - Unit Conversion and the International System of Units (SI).
• NIST Special Publication 811 - Guide for the Use of the International System of Units (exact factors for inch, pound, mile).
• Bureau International des Poids et Mesures (BIPM) - The International System of Units (SI Brochure).