How to Convert Units Online

Metric to imperial, Celsius to Fahrenheit, gigabytes to megabytes: unit conversions are one of those things you need to do often enough that memorizing every ratio is impractical, but not often enough that you remember them all. A browser-based converter handles the math instantly across dozens of units and runs entirely on your device.

Supported unit categories

How to convert units

1. Select the category: choose length, weight, temperature, or another unit type.
2. Pick your units: select the source unit and the target unit from the dropdowns.
3. Enter a value: type a number in either field. The conversion is bidirectional and updates in real time.

A brief history of measurement standardization

Humans have measured things for at least 5,000 years, but the units were a mess until very recently. Ancient Egypt used the cubit (the length of the pharaoh's forearm). Medieval England used the foot (length of a king's foot) and the inch (three barleycorns end-to-end). Different villages, different trades, different countries all had their own units.

The metric system was created during the French Revolution (1795) as a rational, decimal alternative. The meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris. The kilogram was the mass of a liter of water. Both definitions have since been refined: the meter is now defined by the speed of light (the distance light travels in 1/299,792,458 of a second), and the kilogram by Planck's constant (since 2019, replacing the platinum-iridium reference cylinder in Sèvres, France).

Most of the world adopted metric in the 19th and 20th centuries. Notable holdouts: the United States (still uses imperial for most consumer-facing measurements), Liberia, and Myanmar (officially metric but with mixed daily use). The UK is a hybrid: metric for science and most commerce, imperial for road distances (miles), beer (pints), and human weight (stones).

Even within metric there are subtleties: the SI (International System of Units) standardizes seven base units (meter, kilogram, second, ampere, kelvin, mole, candela), but informal units like the liter, hectare, and tonne are widely used alongside.

Common conversions to know

Some conversions come up so often they are worth having a rough sense of:

• 1 mile ≈ 1.6 km: multiply km by 0.62 to get miles
• 1 kg ≈ 2.2 pounds: multiply kg by 2.2 to estimate pounds
• 1 inch = 2.54 cm: this one is exact
• 1 foot = 30.48 cm: also exact
• 0°C = 32°F: water freezes at 32°F
• 100°C = 212°F: water boils at 212°F (at sea level)
• 1 liter ≈ 0.26 US gallons: roughly 4 liters per gallon
• 1 US gallon = 3.785 liters: exact
• 1 acre ≈ 0.4 hectares: 1 hectare ≈ 2.5 acres
• 1 GB = 1024 MB: in computing (1000 MB in the SI definition)
• 1 mile = 5,280 feet: the obscure-looking imperial constant
• 1 nautical mile ≈ 1.852 km: used in aviation and shipping
• 1 light-year ≈ 9.46 trillion km: distance light travels in a year

Where unit conversion matters

• Cooking recipes: American recipes use cups, ounces, Fahrenheit; European use grams, ml, Celsius. Converting reliably matters for baking where ratios are precise.
• Travel: speed limits, fuel prices, distances, temperatures all switch units at borders.
• Engineering and construction: international projects mix metric and imperial drawings constantly. NASA famously lost the $327 million Mars Climate Orbiter in 1999 because one team used pounds-seconds and another used Newton-seconds.
• Health and fitness: body weight in kg vs pounds, height in cm vs inches, running distances in km vs miles.
• Trade and shipping: ocean cargo measured in TEU (twenty-foot equivalent unit) regardless of country.
• Science and research: SI units universally in publication, but raw data may come from imperial-instrumented suppliers.
• Computing: file sizes in GB vs GiB, network speeds in Mbps (megabits) vs MB/s (megabytes; 8x larger).

SI vs binary prefixes (the GB vs GiB problem)

The most confusing unit area is digital storage:

• SI (decimal) prefixes: kilo = 1000, mega = 1,000,000, giga = 1,000,000,000
• Binary prefixes: kibi = 1024, mebi = 1,048,576, gibi = 1,073,741,824

Hard drive manufacturers use SI (1 TB = 10^12 bytes), so a "1 TB" drive shows as ~931 GiB in your OS (which uses binary). The IEC standard names these binary units KiB, MiB, GiB, TiB explicitly, but most people still say "GB" for both.

For RAM and operating systems, "GB" almost always means GiB. For storage and network speeds, "GB" usually means SI gigabytes. Always check context.

Conversion formulas worth memorizing

For when you do not have a converter handy:

• Celsius to Fahrenheit: F = C × 9/5 + 32. Quick shortcut: double the Celsius, subtract 10%, add 32. (20°C → 40 - 4 + 32 = 68°F. Actual: 68°F.)
• Fahrenheit to Celsius: C = (F - 32) × 5/9. Shortcut: subtract 30, divide by 2. (75°F → 45 / 2 = ~22°C. Actual: 23.9°C.)
• km to miles: multiply by 0.6 (rough) or 0.621 (precise).
• miles to km: multiply by 1.6.
• kg to pounds: multiply by 2.2.
• pounds to kg: divide by 2.2, or multiply by 0.45.
• liters to US gallons: divide by 3.8.
• inches to cm: multiply by 2.5.

Common pitfalls

• Imperial vs US customary: they look similar but differ. 1 US gallon = 3.785 L; 1 UK (imperial) gallon = 4.546 L. 20% difference. Always check which.
• Fluid ounce vs ounce (weight): 1 fl oz is volume (~30 mL); 1 oz is mass (~28 g). Different units, same name.
• Long ton vs short ton vs metric tonne: short ton (US) = 2000 lb, long ton (UK) = 2240 lb, metric tonne = 1000 kg ≈ 2204.6 lb. Three different "tons."
• Calorie vs kilocalorie: food "calories" on labels are actually kilocalories (kcal). 1 food Calorie = 1000 chemistry calories.
• Horsepower variants: mechanical horsepower (745.7 W), metric horsepower (735.5 W), electric horsepower (746 W), boiler horsepower (9809.5 W). All called "hp."
• Knots vs km/h: 1 knot = 1.852 km/h. Used in shipping, aviation, weather forecasts in some regions.
• Pascal vs psi vs bar vs atmosphere: 1 atm = 101,325 Pa = 14.696 psi = 1.01325 bar. Easy to confuse "1 bar" with "1 atm" (close but not equal).

Tips

• Type in either field: the converter works both ways. If you know the target value and want to find the source, just type in the other box.
• Temperature is not a ratio: unlike length or weight, temperature conversions use an offset (adding 32 for Fahrenheit). You cannot simply multiply. Always use the formula or a converter.
• GB vs GiB: in computing, 1 GB can mean 1000 MB (SI) or 1024 MB (binary/GiB). Know which definition your context uses.
• Works on any device: the converter runs in your browser on desktop, tablet, or phone.
• Round appropriately for context: a recipe needs 2-3 significant figures; an engineering tolerance might need 5-6. Do not assume precision beyond what your source measurement provides.
• Double-check international shipping: package dimensions in inches vs cm, weight in lb vs kg, can all matter for tariffs and shipping costs.

Privacy

The unit converter runs entirely in your browser. The numbers you enter, your category choices, and any custom inputs all stay on your device. Unit conversion is rarely sensitive on its own, but the values you convert can reveal context (engineering specifications, recipe quantities, financial figures with units, health measurements). A browser-only converter has zero exposure to any of that, and works offline once the page is loaded.