Metric conversion chart

A chart is best for quick checks and common numbers. If you are copying a measurement into a shipping label, a recipe, or a product listing, use a calculator and match the precision of the original measurement.

When a definition is exact (like inches and centimeters), the chart can be exact too. When a factor creates a long decimal (like kilometers to miles), rounding is normal.

Scale a chart value before rounding

Start with the factor shown in the table, multiply by the value you have, and round once at the end. A 55 cm carry-on is 55 × 0.393701 = 21.653555 inches, which is 21.65 inches to two decimal places. A 10 km run is 10 × 0.621371 = 6.21371 miles, or 6.21 miles. For 500 mL, divide by 236.588 mL per US cup to get 2.11338 cups, then report 2.11 cups if hundredths are useful.

Do not repeatedly round an intermediate number. Using 0.39 instead of 0.393701 for every centimeter makes the 55 cm result 21.45 inches, about two tenths of an inch low. That may not matter for a rough estimate, but it can matter for luggage limits, product dimensions, or material cuts.

Temperature needs an offset

Length, mass, volume, area, and speed can be scaled with multiplication or division. Temperature is different because Fahrenheit and Celsius have different zero points. Convert Celsius to Fahrenheit with °F = (°C × 9/5) + 32. Convert Fahrenheit to Celsius with °C = (°F − 32) × 5/9. For example, 20°C becomes 68°F; multiplying by 1.8 without adding 32 would give the wrong result.

Exact factors and quick estimates serve different jobs

Some US customary units are defined from metric units. One inch is exactly 2.54 centimeters, so one foot is exactly 0.3048 meters and one mile is exactly 1,609.344 meters. The decimal shown in the reverse direction may still repeat, which is why one kilometer is displayed as 0.621371 miles rather than an endless string of digits.

For mental math, fewer digits are often enough. The NIST approximate conversion card rounds one pound to about 0.45 kilograms and one US gallon to about 3.79 liters. Use those values to check an answer quickly. Use the fuller factors in the calculators when the converted number will be copied into a recipe, shipment, specification, or quote.

US customary and imperial volumes are not interchangeable

The volume table on this page uses US customary gallons and cups. A label that says only “gallon” or “cup” can be ambiguous because UK imperial measures and metric cooking cups use different amounts. Check the country or standard named by the recipe, container, or document before converting. This site does not currently calculate UK imperial gallons, so do not use the US gallon result as a substitute.

Choose the number of decimal places from the original value

A converted result should not imply more certainty than the starting measurement. If a room is measured as 4.2 meters, reporting 13.7795276 feet looks exact but is not more informative than about 13.8 feet. Keep extra digits during the calculation, then round the final result to a level that matches the source. The dedicated converter pages show both the factor and the calculation so the rounding can be checked.

Sources for the factors

The chart uses conversion factors published in the NIST Guide to the SI conversion-factor appendix. Definitions and notation for the International System of Units come from the BIPM SI Brochure. These primary references are preferable when a conversion must be documented for technical work.

Metric conversion chart FAQ

Is this metric conversion chart exact? Some relationships are exact by definition, including 1 inch = 2.54 cm. Other directions produce long decimals, so the displayed chart value is rounded to a practical number of digits.

What if the number I need is not in the chart? Use the metric conversion calculator. Pick the measurement type and units, enter the number, and keep the formula with the result if you need to verify it later.

Which gallons and cups does the chart use? The table uses US customary gallons and cups. It does not convert UK imperial gallons, which are a different size.