Unit Converter: Free Online Metric & Imperial

This free unit converter changes a value from one unit of measurement to another, covering length, weight, temperature, volume, area, speed, data size and time. Choose a category, type a number, pick the From and To units, and you get a quick result plus a table showing the same value in every other unit in that category.

What Is Unit Conversion?

Unit conversion is the process of expressing the same physical quantity in a different unit, such as showing 5 kilograms as pounds. The quantity does not change. Only the number and the label change.

For most units, conversion is multiplication or division by a fixed factor. One inch is exactly 25.4 millimeters, one foot is 0.3048 meters, and one mile is 1,609.344 meters. Because every unit in a category relates to the same standard, you can convert between any pair, even if you only know each unit's relationship to a common one.

Temperature is the exception. Celsius, Fahrenheit and Kelvin have different zero points, so a simple factor is not enough. The formulas are °F = °C × 9/5 + 32 and K = °C + 273.15. That is why 0 °C is 32 °F, not 0 °F.

How to Use the Unit Converter

You can finish a conversion in under ten seconds. Follow these steps in the order the fields appear on the page.

1. Open the Measurement Category menu and choose Length, Weight / Mass, Temperature, Volume, Area, Speed, Data Size or Time.
2. Type your number in the Value field. Decimals work, and the field starts with a placeholder of 1.
3. Pick the unit you are starting with in the From menu, for example Pound (lb).
4. Pick the unit you want in the To menu, for example Kilogram (kg).
5. Click Convert. The Quick Result line shows your answer, and the All Conversions table lists the same value in every other unit of that category.
6. Click Swap to reverse the From and To units when you need the opposite direction, such as going from kilograms back to pounds.

Worked Example: Four Common Conversions

Here is what the converter returns for four everyday inputs, and how you can check each answer by hand.

Take the first row. You choose Weight / Mass, enter 5, set From to Kilogram (kg) and To to Pound (lb), then click Convert. The Quick Result shows roughly 11.02 lb. The All Conversions table also lists 5,000 g, 5,000,000 mg, about 176 oz and about 0.79 stone, so you may find the unit you actually needed without a second conversion.

How Do You Read and Act on the Results?

The Quick Result answers the exact From and To pair you chose, while All Conversions shows every other unit in the same category for the same starting value. Use the first for a single answer and the second when you are not sure which unit you need.

Run a quick sanity check

Before you copy a number, ask whether it points the right way. Converting to a smaller unit should give a bigger number: 1 meter is 100 centimeters. Converting to a larger unit should give a smaller number: 1 meter is 0.001 kilometers. If the direction is wrong, you probably swapped From and To, and the Swap button fixes that in one click.

Memorize a few anchor values

These widely used approximations help you catch input mistakes:

• 1 kg is about 2.2 lb, and 1 lb is about 454 g.
• 1 inch is 2.54 cm, and 1 mile is about 1.61 km.
• 1 liter is about 4.2 US cups, and 1 US gallon is about 3.785 liters.
• Room temperature of 20 to 22 °C is about 68 to 72 °F, and body temperature of about 37 °C is about 98.6 °F.

Decide how many decimals to keep

The converter can show long decimal strings. Round to match your task. Recipes rarely need more than one decimal place. A shipping weight is usually rounded to two. Machining or lab work may need the full figure, so keep it and round only at the final step.

Watch the ton and gallon variants

Several unit names mean different amounts depending on the country. A metric ton is 1,000 kg, a US ton (short ton) is about 907 kg, and a UK ton (long ton) is about 1,016 kg. A US gallon is about 3.785 liters, while a UK gallon is about 4.546 liters. Pick the exact label in the menu, not just the word "ton" or "gallon".

Who Uses a Unit Converter, and When?

A unit converter helps anyone who works with numbers from two measurement systems at once. These are the situations where it saves the most time.

• Cooking and baking: turn milliliters into cups, or tablespoons into teaspoons, when a recipe comes from another country. The Volume category has teaspoon, tablespoon, fluid ounce, cup, pint, quart and gallon options.
• Travel: convert miles to kilometers, kilometers per hour to the speed you are used to, and Fahrenheit to Celsius for a weather forecast.
• Fitness and health: move between kilograms, pounds and stone when you track weight or read a gym chart.
• Home and property: compare square feet to square meters, or acres to hectares, when you read a listing or a land survey.
• Science and homework: convert nanometers, micrometers and millimeters, or Celsius to Kelvin, in physics and chemistry problems.
• Tech and storage: switch between data size units when you compare file sizes or storage plans. If you work with raw binary values, the binary calculator is a useful companion.

Best Practices and Common Mistakes

Most wrong answers come from picking the wrong unit, not from the math. These habits prevent the common errors.

1. Check the category first. If you want ounces of weight, choose Weight / Mass. The Volume category also has fluid ounces, and they are not the same thing.
2. Read the unit labels in full. Cup (US) and Gallon (UK) are labeled for a reason. Using the wrong regional version changes the answer by several percent or more.
3. Convert once, from the original number. Converting a rounded result a second time stacks rounding errors. Start from your original value and use the All Conversions table to find other units.
4. Use the Value field for negatives where they make sense. A temperature of -10 °C converts to 14 °F. Negative lengths or weights do not describe real measurements, so double-check those inputs.
5. Compare small differences carefully. A tiny difference in the last decimal places between two calculators usually comes from rounded conversion constants, not an error in your input.

If you convert a list of measurements and then need a typical value, paste the converted numbers into the average calculator. If you convert image or print dimensions between inches and centimeters, the aspect ratio calculator helps you keep the proportions intact when you resize.

What Can This Converter Not Do?

The converter works within one category at a time, so it cannot turn a length into a weight or a volume into a mass. That limit is intentional, because those conversions need extra information.

• Milligrams to milliliters: this depends on the density of the substance. Water is about 1 mg per 0.001 ml, but oil, syrup and medicine are not. Check the product label or a density table, and ask a pharmacist for medication doses.
• Currency: exchange rates change constantly, and this page converts physical units only. The same goes for airline miles or loyalty points, which have no fixed unit value.
• Spreadsheet formulas: the converter gives you the number to type, but it does not write formulas for you. Use the factors shown in the worked example to build your own.
• Professional-grade rounding: for engineering, medical or legal work, confirm the result against the standard your field requires.

For time values, the converter turns units such as hours and days into one another. If you need an exact age or the gap between two calendar dates, the age calculator handles months and leap years better than a flat conversion.

Convert Your First Value Now

You now know how the factors work, which results to double-check and where the tool stops. Pick a category above, enter your number, choose the From and To units, and click Convert to get your answer in seconds. For more everyday math helpers, browse the online calculators and utilities collection.