Convert between units of length, weight, temperature, volume, speed, and 200+ more categories. Free, instant, and accurate online unit converters.
Whether you are a student calculating physics homework, an engineer designing components for an international client, or a home cook trying to follow a recipe from another country, unit conversion is one of those tasks that never goes away. And getting it wrong can range from mildly inconvenient to genuinely dangerous.
The world runs on multiple measurement systems. The United States uses customary units. Most of the rest of the world uses metric. The UK uses an eclectic mix of both. Scientific fields use SI units exclusively, while industries like aviation, maritime, and medicine each have their own preferred conventions. If you work across any of these boundaries, you need reliable conversion tools.
That is why we built over 206 free converter tools that run directly in your browser — no downloads, no sign-ups, no data sent to servers. This article covers the most commonly needed conversions, provides quick reference formulas, and points you to the right tool for every job.
Length conversion is probably the most frequent conversion people perform. The metric system uses meters as its base unit, while the US customary and imperial systems use feet, inches, yards, and miles.
Travelers constantly convert between miles and kilometers. If your car speedometer reads in mph and the road signs are in km/h, you need this conversion immediately. Architects and engineers working on international projects convert between feet and meters dozens of times per day. Even online shopping requires length conversion when buying furniture or clothing from overseas retailers.
Our Unit Converter handles all standard length units instantly, including less common ones like nautical miles, light-years, and astronomical units.
Weight and mass conversion creates real confusion because the terms are used interchangeably in everyday language, even though they are technically different in physics. For practical purposes — grocery shopping, shipping, fitness tracking — the conversions below are what most people need.
International shipping requires weight in kilograms. Most gym equipment outside North America is labeled in kilograms. Medical dosages are calculated in milligrams and micrograms, where precision is critical. Jewelers work in carats and troy ounces. The Unit Converter supports all of these, from micrograms to metric tons.
Temperature conversion is uniquely tricky because the scales do not share a common zero point. You cannot just multiply by a single factor — you need both multiplication and addition.
| Fahrenheit | Celsius | Context |
|---|---|---|
| 32°F | 0°C | Water freezes |
| 72°F | 22.2°C | Room temperature |
| 98.6°F | 37°C | Body temperature |
| 212°F | 100°C | Water boils |
| 350°F | 176.7°C | Common baking temp |
| 425°F | 218.3°C | High-heat roasting |
The Temperature Converter handles Fahrenheit, Celsius, Kelvin, and Rankine with precision to multiple decimal places. It is particularly useful for cooking — when a recipe says "preheat to 180°C" and your oven dial is in Fahrenheit, you need 356°F, not a rough guess.
Volume conversion is where the US customary system and the imperial system diverge in confusing ways. A US gallon is not the same as an imperial gallon. A US fluid ounce differs from an imperial fluid ounce. Our tools handle both systems correctly.
Cooking across measurement systems is the most common use case. A British recipe calling for 200ml of milk does not map cleanly to US cups. The Cooking Converter is purpose-built for kitchen conversions, handling cups, tablespoons, teaspoons, milliliters, and fluid ounces with cooking-specific precision.
For scientific and industrial applications, the Unit Converter handles liters, cubic meters, cubic feet, barrels, and other volume units used in chemistry, petroleum, and manufacturing.
Speed conversion matters for travel, athletics, weather, and engineering.
Runners and cyclists tracking performance across international events need pace and speed conversions. Pilots work in knots while ground operations use km/h or mph. Weather reports in different countries use different speed units for wind. The Speed Converter handles all common velocity units including knots, Mach number, and the speed of light for physics calculations.
In the digital age, data size conversion has become essential. The confusion between binary and decimal prefixes (kibibytes vs. kilobytes) catches even experienced engineers off guard.
Hard drive manufacturers use decimal. Operating systems historically use binary. This is why a "500 GB" hard drive shows as roughly 465 GB in your file manager.
The Data Size Converter correctly distinguishes between binary and decimal prefixes, which most casual online converters get wrong.
Energy conversion spans cooking, fitness, physics, and engineering. Calories, joules, watt-hours, BTUs, and electronvolts are all units of energy used in different contexts.
Nutritionists convert between calories and kilojoules daily (most of Europe and Australia label food in kJ). HVAC engineers work in BTUs. Physicists use joules and electronvolts. Electricity bills use kilowatt-hours. The Energy Converter supports all of these conversions with scientific precision.
Time zone conversion is not a straightforward unit conversion — it involves geopolitical boundaries, daylight saving time rules that change periodically, and half-hour and quarter-hour offsets in some regions.
The Time Zone Converter and Timezone Converter account for all of these complexities, automatically applying the correct DST rules for any given date.
Currency conversion is technically a unit conversion, but with a critical difference: exchange rates change continuously. A currency converter needs real-time or near-real-time rate data to be useful.
International travelers, online shoppers buying from foreign stores, freelancers billing international clients, investors tracking foreign equities, and businesses processing cross-border payments all need currency conversion regularly. The Currency Converter provides up-to-date exchange rates for major and minor world currencies.
Understanding why we need all these conversions requires a brief look at the world's measurement systems.
Used by virtually every country for scientific purposes and by most countries for everyday use. Based on powers of ten, making conversions within the system trivial (1 km = 1,000 m = 100,000 cm). The seven SI base units are: meter, kilogram, second, ampere, kelvin, mole, and candela.
Used primarily in the United States for everyday measurements. Derived from English units but diverged from the British imperial system after American independence. Uses inches, feet, yards, miles, ounces, pounds, and tons with non-decimal relationships.
Used in the UK for certain purposes (road distances in miles, beer in pints). Largely replaced by metric for trade and science, but persists in everyday life. Imperial and US customary units share many names but sometimes differ in actual size (notably, the pint and gallon).
Traditional measurement systems persist in specific contexts worldwide. The Japanese tsubo is still used in real estate. Indian cooking uses the "cup" but it is 150ml, not 236ml. Chinese traditional units (jin for weight, li for distance) coexist with metric. Being aware of these local variations prevents costly errors.
Beyond the core conversion categories, several specialized tools solve specific professional needs:
Know your precision requirements. Cooking tolerates rounding. Pharmaceutical dosing does not. Engineering has specified tolerances. Use the right number of decimal places for your context.
Watch for system ambiguity. When someone says "ounce," do they mean fluid ounces (volume) or avoirdupois ounces (weight)? When they say "ton," is it a short ton (2,000 lb), a long ton (2,240 lb), or a metric ton (1,000 kg)?
Double-check critical conversions. The Mars Climate Orbiter was lost because one team used metric units and another used US customary. That mistake cost $327.6 million. For anything consequential, verify.
Bookmark your frequently used tools. If you convert between the same units regularly, keep the relevant converter one click away. All our tools can be added to your favorites for quick access.
Use the all-in-one option for exploration. The Unit Converter handles dozens of unit categories in a single interface. When you are not sure which specific converter you need, start there.
Every conversion on our platform happens locally in your browser. No data is sent to any server. This means:
With over 206 converter tools covering everything from basic length and weight to specialized formats like color models, CSS units, and data encodings, you should never need to hunt across multiple websites for a conversion tool again. Every tool is free, works on any device, and delivers results with the precision your work demands.
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