Conversion Calculator: Convert Units Instantly

A recipe in cups, a speed limit in mph, a hard drive in terabytes, an oven in Fahrenheit: sooner or later a number turns up in the wrong unit. This free conversion calculator handles 72 units in 8 categories (length, weight, temperature, volume, time, speed, digital storage, and energy). Pick a category, enter a value, and it returns the converted result, a reverse check, the conversion ratio, and the formula, and it keeps a short history of your recent conversions.

How the Conversion Calculator Formula Works

The calculator measures one quantity in different units. To keep 72 units consistent, every value passes through a common base unit for its category: metres for length, kilograms for weight, litres for volume, seconds for time, kilometres per hour for speed, bytes for storage, and joules for energy. Temperature is the exception, and it goes through Celsius.

Factor is how many base units one unit equals (for example 1 mi = 1,609.344 m and 1 km = 1,000 m). Because every unit is tied to the same base, any unit in a category can be converted to any other without a table of every pair.

Step-by-step calculation walkthrough

Step 1: Identify the inputs. A runner wants to know how far a marathon is in kilometres. The commonly rounded figure is 26.2 miles, so the value is 26.2, the From unit is miles, and the To unit is kilometres.

Step 2: Apply the formula. Result = 26.2 × Factor(mi) ÷ Factor(km), with Factor(mi) = 1,609.344 m and Factor(km) = 1,000 m.

Step 3: Perform the calculation. 26.2 × 1,609.344 = 42,164.8128 m. Then 42,164.8128 ÷ 1,000 = 42.1648128 km, which the calculator displays as 42.1648 because values from 10 to 999 are shown to four decimals. Its reverse check converts 42.1648 km back to 26.2 mi.

Step 4: Interpret the result. 26.2 miles is about 42.16 km. The official marathon distance is 26 miles 385 yards, which is 42.195 km, so the rounded 26.2 figure is a few tens of metres short.

Assumptions and limitations: every factor is a fixed constant, so the calculator cannot handle quantities that depend on conditions, such as the speed of sound at a given temperature or currency rates. Temperature values are treated as readings, not differences. To convert a temperature difference, multiply by 9/5 (or 5/9) and skip the 32.

Metric vs Imperial: The Two Systems

Most measurement worldwide is built on the metric system, formally the International System of Units (SI), which scales by powers of 10. The United States mostly uses US customary units for everyday measurement, while the UK and Canada mix metric and imperial in daily life. NIST says the popular claim that only the US, Liberia, and Myanmar “don’t use” metric is an oversimplification: every country has recognized the SI, and the US signed the Treaty of the Meter in 1875. The real difference is whether metric use is required or voluntary. The practical friction is still real, from recipes to engineering drawings to medicine doses.

Temperature Conversion Formulas

Digital Storage Units Explained

Storage is the category most likely to surprise you. Here, KB, MB, GB, and TB are treated as multiples of 1,024: 1 KB = 1,024 bytes, 1 MB = 1,048,576 bytes, and 1 GB = 1,073,741,824 bytes. KiB and MiB give the same values as KB and MB. That is the traditional computing convention, but it is not what the standards say. NIST’s page on the binary prefixes defines 1 MB as 1,000,000 bytes and 1 MiB as 1,048,576 bytes, and likewise 1 GB as 10⁹ bytes and 1 GiB as 2³⁰ bytes. If you need the decimal (SI) figure, a gigabyte is 1,000 megabytes rather than 1,024, so multiply by 1,000 at each step yourself.

3 Real-Life Examples

Three everyday conversions, each in a different category. The figures are what the calculator returns for these inputs.

These conversions use the same factors as the calculator above. They are for everyday planning, not a substitute for the units in a specification.

Important Notes

• Storage uses 1,024, not 1,000. KB, MB, GB, and TB are binary multiples here, and KB equals KiB. The two conventions differ by about 2.4% per step (1,024 against 1,000), which compounds to about 10% at the terabyte level, so convert drive capacities by 1,000 yourself.
• Factors are not all equally precise. Most are exact or within 0.001%. The teaspoon and tablespoon are about 0.02% high, and the speed of light, electronvolt, light-year, and petabyte are rounded to about four digits. See the accuracy answer in the FAQ for the list.
• Some units depend on a convention. Mach uses 343 m/s (air at about 20°C), a month is the average 30.4375 days, and a year is 365.25 days. Calendar months and the speed of sound in real conditions vary.
• Calories are small calories. “Calories (cal)” is 4.184 J. Food labels use kilocalories, so choose kcal for food energy.
• US and UK units are listed separately. Pick “Gallons (US)” or “Gallons (UK/Imp.)”, and “Short tons (US)” or “Long tons (UK)”. The reverse-check line and history shorten names to the first word, so both gallons appear as “Gallons”.
• Display rounding. Results show two decimals from 1,000 up, four from 10 to 999, six from 1 to 10, and eight below 1. Very large or very small numbers appear in scientific notation, and trailing zeros are dropped.
• The history logs every recalculation. A line is added whenever the result updates, including while you type and when the window is resized, so you may see repeats.
• The Length insight repeats a common simplification. It says only the US, Myanmar, and Liberia use imperial units. As noted above, NIST calls that oversimplified.
• Privacy. Conversions and the PDF are created in your browser. Nothing you enter is sent to a server.

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Frequently Asked Questions