What this tool does
Conversions that mix the two systems are common in recipes, product specifications, and travel, and most online converters round too early or round to the wrong number of significant figures. This one converts from the base unit directly, so the result stays accurate even when the input is a long decimal.
How it works
Each unit is defined as a fixed multiple of a base unit: metres for length, kilograms for mass, and so on. Converting between two units means dividing by the first unit's factor and multiplying by the second. Doing it in one step avoids the rounding error that appears when a value passes through an intermediate unit.
Temperature is the exception that needs a formula rather than a multiplier, because the scales do not share a zero. Celsius to Fahrenheit is value times nine-fifths plus thirty-two, and Fahrenheit to Celsius reverses that. Freezing is 0°C and 32°F, not 0°F.
Precision is reported to a sensible number of significant figures rather than a fixed number of decimals, because a value like 3.4 metres does not carry twelve meaningful digits. Pick the precision that matches the accuracy of your input.
A common source of wrong answers is treating a conversion as a single multiplication. Going from pounds to kilograms is not the same factor as kilograms to pounds, because one of them is a division, and the two differ by more than rounding: 1 kilogram is 2.20462 pounds, so 100 pounds is 45.3592 kilograms, not 220.462. The safe habit is to convert to the base unit first, then out again, which is what this tool does and what keeps a chained conversion such as feet to centimetres to inches exact.
Worked example
Converting 180 centimetres to feet and inches.
- 180 cm / 2.54 = 70.8661 inches
- 70.8661 / 12 = 5 whole feet
- Remainder = 70.8661 - 60 = 10.8661 inches
180 cm is approximately 5 ft 10.87 in, or 5.91 ft as a decimal.
Accuracy and limitations
- Conversions are for everyday and technical reference. For regulated, legal, medical, or trade measurement, use an official conversion standard for your jurisdiction.
Frequently asked questions
- How many centimetres are in an inch?
- Exactly 2.54. This has been the international definition since 1959, and it is why some common conversions produce repeating decimals, such as 1 inch being 2.54 cm and 1 foot being 30.48 cm exactly.
- Why is the temperature conversion not a simple ratio?
- Because the Celsius and Fahrenheit scales have different zero points. Celsius zero is the freezing point of water, Fahrenheit zero is roughly minus 17.8°C, so the scales are offset as well as scaled. Any conversion that treats it as a pure ratio is wrong.
- How precise are these results?
- The base factors are exact by international agreement, so precision is limited only by your input. The result is rounded to a fixed number of significant figures, which is more meaningful than a fixed number of decimal places.
- How many feet are in a metre?
- One metre is 3.28084 feet, and one foot is 0.3048 metres exactly. A useful rule of thumb is that 5 feet 9 inches is about 1.75 metres and 6 feet is about 1.83 metres, which is close enough for a quick estimate but not for a legal or medical measurement.
- Why does converting 100 pounds to kilograms not match 100 kilograms to pounds?
- Because the relationship is not symmetric. One kilogram is 2.20462 pounds, so the conversion in one direction is a multiplication and in the other a division. 100 pounds is 45.36 kilograms, and 100 kilograms is 220.46 pounds, and neither is the inverse of the other.