An American client sends you a file specifying dimensions in inches and pounds. You work in centimeters and kilograms. A recipe calls for 2 cups of flour and 350°F — but your measuring cups are metric and your oven shows Celsius. A fitness app tracks your running distance in miles, but your local race is marked in kilometers.
Unit conversion is a daily need for anyone working, traveling, cooking, studying, or communicating across the metric-imperial divide — which is essentially anyone interacting with both American and international standards.
The Unit Converter on AllTools handles length, weight, temperature, volume, area, speed, pressure, energy, data storage, and more. Type a value, pick your units, get the result instantly. No formula memorization, no account, no upload. Everything runs in your browser.
The Metric vs Imperial Problem
Only three countries officially use the imperial system: the United States, Myanmar, and Liberia. The rest of the world uses metric. But because the US is a global economic powerhouse, imperial units appear everywhere — in product specifications, scientific publications (sometimes), engineering documents, cooking recipes, and everyday conversation.
This creates constant friction:
- International business — Product specs, shipping dimensions, and weight limits mix metric and imperial depending on the market
- Science and engineering — While science standardized on metric/SI, many engineering fields (especially in the US) still use imperial, leading to unit confusion that has caused real-world failures (the Mars Climate Orbiter was lost due to a metric-imperial conversion error)
- Cooking — American recipes use cups, tablespoons, and Fahrenheit; international recipes use grams, milliliters, and Celsius
- Travel — Speed limits, distances, and temperatures vary by country
- Fitness — Running apps, gym equipment, and health data mix miles/kilometers and pounds/kilograms depending on your device settings
A reliable unit converter eliminates the mental math and the risk of conversion errors.
Unit Categories Covered
The AllTools Unit Converter supports conversions across all major unit categories:
Length and distance
Millimeters, centimeters, meters, kilometers, inches, feet, yards, miles, nautical miles, and more. Whether you’re converting building dimensions (feet to meters) or travel distances (miles to kilometers), every common length unit is covered.
Weight and mass
Milligrams, grams, kilograms, metric tons, ounces, pounds, stones, and US tons. Essential for cooking (grams to ounces), shipping (kilograms to pounds), and health (stones to kilograms for BMI calculations).
Temperature
Celsius, Fahrenheit, and Kelvin. Temperature conversion is the one that trips people up most because it’s not a simple multiplication — the scales have different zero points. The converter handles the offset correctly every time.
Volume
Milliliters, liters, cubic meters, teaspoons, tablespoons, fluid ounces, cups, pints, quarts, gallons (US and Imperial). Critical for cooking, chemistry, and fuel calculations. Note: US gallons and Imperial gallons are different sizes (3.785L vs 4.546L) — the converter distinguishes between them.
Area
Square millimeters, square centimeters, square meters, hectares, square kilometers, square inches, square feet, square yards, acres, square miles. Used in real estate, agriculture, landscaping, and construction.
Speed
Meters per second, kilometers per hour, miles per hour, knots, and feet per second. Convert between the speed units used in different countries, transportation modes, and weather reporting.
Pressure
Pascals, kilopascals, bar, atmospheres, PSI, mmHg, and torr. Used in tire pressure, weather reporting, medical devices, and engineering.
Energy
Joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU. Relevant for nutrition (calories), electricity (kWh), and HVAC (BTU).
Data storage
Bits, bytes, kilobytes, megabytes, gigabytes, terabytes, petabytes. Convert between data units — useful when comparing storage capacities, download speeds (bits) vs file sizes (bytes), and cloud storage plans.
Quick Reference Conversion Table
These are the most commonly needed conversions. Bookmark this table or memorize the approximate multipliers:
| From | To | Multiply by | Example |
|---|---|---|---|
| Kilometers | Miles | 0.6214 | 10 km = 6.21 miles |
| Miles | Kilometers | 1.6093 | 10 miles = 16.09 km |
| Kilograms | Pounds | 2.2046 | 70 kg = 154.32 lbs |
| Pounds | Kilograms | 0.4536 | 150 lbs = 68.04 kg |
| Celsius | Fahrenheit | ×1.8 + 32 | 25°C = 77°F |
| Fahrenheit | Celsius | (F-32) ÷ 1.8 | 77°F = 25°C |
| Liters | US Gallons | 0.2642 | 10 L = 2.64 gal |
| US Gallons | Liters | 3.7854 | 5 gal = 18.93 L |
| Centimeters | Inches | 0.3937 | 30 cm = 11.81 in |
| Inches | Centimeters | 2.54 | 12 in = 30.48 cm |
| Meters | Feet | 3.2808 | 2 m = 6.56 ft |
| Feet | Meters | 0.3048 | 6 ft = 1.83 m |
| Hectares | Acres | 2.4711 | 1 ha = 2.47 acres |
| Kilograms | Stones | 0.1575 | 80 kg = 12.6 stone |
Quick mental approximations
For when you need a rough answer without a calculator:
- km to miles: multiply by 0.6 (close enough for directions)
- kg to lbs: double it, then add 10% (70 kg → 140 + 14 = 154 lbs — actual: 154.3)
- cm to inches: divide by 2.5 (close to 2.54)
- liters to gallons: divide by 4 (close to 3.785)
- Celsius to Fahrenheit: double it, add 30 (25°C → 50 + 30 = 80°F — actual: 77°F, close enough)
These approximations work for everyday use. For precise work, use the Unit Converter.
Common Mistakes in Unit Conversion
Confusing US and Imperial gallons
A US gallon is 3.785 liters. An Imperial (UK) gallon is 4.546 liters — about 20% larger. If a British recipe calls for “a gallon of water,” that’s more than an American gallon. Similarly, fuel economy figures differ: “30 miles per gallon” means something different depending on which gallon you’re using.
Temperature offset errors
Temperature conversion isn’t a simple multiplier because Celsius and Fahrenheit have different zero points. You can’t just multiply Celsius by some number to get Fahrenheit. The correct formula is: °F = (°C × 1.8) + 32. A common error is forgetting the +32 offset, which gives dramatically wrong results at typical temperatures.
Mixing up mass and weight
Technically, kilograms measure mass and pounds measure force (weight). On Earth, 1 kg of mass weighs about 2.205 pounds. In everyday use, this distinction doesn’t matter. But in engineering and physics contexts, confusing mass units (kg) with force units (Newtons, pounds-force) can cause serious calculation errors.
Forgetting squared and cubed conversions
1 meter = 3.281 feet, but 1 square meter ≠ 3.281 square feet. It’s 10.764 square feet (3.281²). Similarly, 1 cubic meter = 35.315 cubic feet (3.281³). Area and volume conversions require squaring or cubing the linear conversion factor.
Using outdated conversion factors
Some conversion factors have been refined over time. Using rounded or outdated values accumulates error in precise calculations. The Unit Converter uses the most current standard conversion factors.
Use Cases by Audience
Travelers
- Convert miles to kilometers for road signs and maps
- Convert Fahrenheit to Celsius for weather reports
- Convert pounds to kilograms for luggage weight limits (typically 23 kg / 50 lbs per checked bag)
- Convert gallons to liters at gas stations
- Convert feet/inches to meters for height descriptions
For currency conversion while traveling, use the Currency Converter alongside the unit converter.
Students
- Physics problems requiring SI unit conversions
- Chemistry calculations with moles, grams, and liters
- Engineering homework mixing metric and imperial specifications
- Math exercises involving unit analysis and dimensional analysis
- Nutrition science courses converting calories to joules
The Scientific Calculator pairs well with the unit converter for complex calculations.
Engineers and architects
- Converting architectural plans between metric and imperial
- Material specifications in different unit systems
- Pressure and temperature conversions for HVAC and piping
- Electrical energy conversions (kWh, BTU, joules)
- Structural load calculations crossing unit systems
Cooks and bakers
- Cups to milliliters for liquid ingredients
- Ounces to grams for dry ingredients
- Fahrenheit to Celsius for oven temperatures
- Tablespoons to milliliters for small quantities
- US cups to metric cups (US cup = 236.6ml; metric cup = 250ml)
Fitness enthusiasts
- Miles to kilometers for running and cycling distances
- Pounds to kilograms for weight tracking and gym equipment
- Stones to kilograms (common in the UK)
- Calories to kilojoules for nutrition labels in different countries
The BMI Calculator uses these conversions internally if you input weight in pounds and height in feet/inches.
Related Math Tools
- Currency Converter — Convert between world currencies with current exchange rates
- Scientific Calculator — Advanced calculations with trigonometry, logarithms, and constants
- Percentage Calculator — Calculate percentages, percentage change, and percentage difference
- BMI Calculator — Body Mass Index with metric and imperial input support
- Fuel Calculator — Calculate fuel consumption, cost per trip, and efficiency
- Speed Calculator — Calculate speed, distance, or time given the other two values
FAQ
What are the most common unit conversions?
Based on search volume and usage data, the most frequently needed conversions are: Celsius to Fahrenheit (and reverse), kilograms to pounds, kilometers to miles, centimeters to inches, and liters to gallons. These cover the majority of everyday conversion needs for people working across metric and imperial systems.
Does the converter support scientific notation?
Yes. For very large or very small values, the converter displays results in scientific notation when appropriate. This is useful for data storage conversions (bytes to petabytes) and scientific units where values span many orders of magnitude.
How accurate are the conversions?
The converter uses standard conversion factors defined by the International Bureau of Weights and Measures (BIPM) and NIST. Conversions are calculated with full floating-point precision. For everyday use, the accuracy far exceeds what you need. For laboratory or engineering precision requirements, verify against your field’s specific standards.
Can I convert between non-standard units?
The converter covers all common units within each category. Highly specialized units (light-years, astronomical units, barleycorns) may not be available. If you need a conversion that isn’t listed, you can chain conversions — convert to a common intermediate unit and then to your target unit.
Is there a mobile app?
No separate app needed. The Unit Converter works in any mobile browser. Bookmark it for quick access. Since it runs entirely in the browser, it works offline after the initial page load — useful when traveling without reliable internet.
What’s the difference between US and Imperial units?
Most length and weight units are the same between US and Imperial systems (inches, feet, yards, miles, ounces, pounds). The main differences are in volume: a US gallon is 3.785 liters while an Imperial gallon is 4.546 liters. This affects all derived volume units (pints, quarts, cups). The converter distinguishes between US and Imperial volume units to avoid this confusion.
Convert Now
Open the Unit Converter and start converting. Type a value, pick your units, get instant results. No account, no upload, works offline.
For related calculations: convert currencies, use the scientific calculator, calculate percentages, or check your BMI. Explore the full Math tools category.
