CloudCalcAppThe Formula and the Exact Factor
A pound is defined as exactly 0.45359237 kilograms, so converting pounds to kilograms is one multiplication by that number. The factor is exact, so a result goes wrong only if you shorten the factor or round too soon. The table shows what a shortened factor does to 100 lb.
The exact answer for 100 lb is 45.359237 kg, which the calculator shows as 45.3592 kg. The last column gives each shortened-factor result less 45.359237.
| Factor used | Result for 100 lb | Difference from exact |
|---|---|---|
| 0.45 | 45 kg | −0.359237 kg |
| 0.454 | 45.4 kg | +0.040763 kg |
| 0.4536 | 45.36 kg | +0.000763 kg |
| 0.45359 | 45.359 kg | −0.000237 kg |
| × 0.45359237 (exact factor) | 45.359237 kg | none (exact) |
Worked Examples
In each example the full 0.45359237 does the multiplying; only the last figure is rounded, half up, to four decimals unless stated.
A decimal value. 2.5 lb × 0.45359237 = 1.133980925 kg exactly, which the calculator shows as about 1.134 kg.
A larger value. 1000 lb × 0.45359237 = 453.59237 kg exactly. The calculator rounds that to 453.5924 kg.
Checking a result. Divide the kilograms by 0.45359237 to get back to pounds: 68.0389 kg ÷ 0.45359237 ≈ 150.0001 lb. The small gap from 150 comes from rounding 68.0389.
How Many Decimals to Keep
Four decimals is what the calculator displays, and fewer are often enough day to day. Taking 150 lb, which is exactly 68.0388555 kg, each error below is rounded minus exact.
| Decimals kept | Result | Error |
|---|---|---|
| 4 | 68.0389 kg | +0.00004 kg |
| 3 | 68.039 kg | +0.00014 kg |
| 2 | 68.04 kg | +0.00114 kg |
| 1 | 68.0 kg | −0.03886 kg |
Keep every digit while you are still calculating and shorten only the figure you report.
Common Mistakes
Dividing instead of multiplying
Dividing by 0.45359237 turns kilograms into pounds. Applied to 150 it gives about 330.69, far more than the 150 you started with. A pounds-to-kilograms answer should come out a little under half of the pounds.
Rounding before the next step
Rounding 150 lb to 68 kg and converting that back gives about 149.9143 lb, which is 0.0857 lb short of the original. Converting 68.0389 kg back gives about 150.0001 lb.
Typing a short factor
Entering 0.45 or 0.454 in place of the exact factor moves the answer before any rounding. For 100 lb the first table shows a miss of about 0.36 kg and 0.04 kg.