The conversion-factor method
Every unit in a given category (weight, time, length, and so on) can be expressed as some multiple of a single base unit. For weight, the base unit is commonly the kilogram: 1 g = 0.001 kg, 1 lb ≈ 0.453592 kg, and 1 oz ≈ 0.0283495 kg.
To convert a value from one unit to another, multiply it by the source unit's factor, then divide by the target unit's factor: converted value = value × (source factor ÷ target factor). This works because both factors are expressed relative to the same base unit, so converting to the base and back out again is really one multiplication.
A worked weight example
Convert 5 kilograms to pounds. The kilogram factor is 1 and the pound factor is 0.45359237, so 5 × (1 ÷ 0.45359237) ≈ 11.02 lb.
Going the other direction works the same way: to convert 10 pounds to kilograms, compute 10 × (0.45359237 ÷ 1) ≈ 4.54 kg. The source and target factors simply swap position.
Applying the same method to time
Time units convert the same way, using seconds as a common base: 1 minute = 60 seconds, 1 hour = 3,600 seconds, and 1 day = 86,400 seconds.
To convert 3.5 hours into minutes: 3.5 × (3,600 ÷ 60) = 210 minutes. To convert 210 minutes back into hours: 210 × (60 ÷ 3,600) = 3.5 hours — the same factor pair, just inverted.
Why precision and rounding matter
Because most conversion factors between systems (like pounds and kilograms) are not round numbers, converting back and forth repeatedly can introduce small rounding errors if intermediate results are rounded too early.
For the most accurate result, keep full precision through the multiplication and only round the final answer to the number of decimal places that matters for your use — for most everyday purposes, two to four decimal places is more than enough.
Frequently asked questions
Why isn't 1 kg exactly 2.2 lb?
2.2 is a rounded approximation used for quick mental estimates. The precise factor is 1 kg ≈ 2.20462262 lb, and using the rounded 2.2 figure introduces a small error that grows with larger values.
Does the conversion-factor method work for temperature too?
Not directly — temperature scales like Celsius and Fahrenheit have different zero points, not just different scales, so they need an offset formula (like F = C × 9/5 + 32) rather than a simple multiplying factor.
Why use seconds as the base unit for time instead of, say, hours?
Any consistent base works mathematically; seconds are commonly chosen because they are the smallest common unit, which avoids fractional base values when converting from very small units.
How many decimal places should I keep when converting?
It depends on the use case — for cooking or casual reference, one or two decimal places are usually enough, while scientific or financial calculations may need more precision to avoid compounding rounding errors.