How do you convert kg to lbs?

The factor is a definition, not a measurement. The same 1959 international yard and pound agreement that fixed the yard at 0.9144 m fixed the pound at exactly 0.45359237 kg, so:

1 pound     =  0.45359237 kg          exactly
1 kilogram  =  1 ÷ 0.45359237 lb  =  2.2046226218487758… lb

Eight decimal places one way, no end of them coming back. As a fraction a kilogram is 100000000/45359237 pounds, and 45359237 is 7 × 11 × 97 × 6073, none of which a decimal system can divide out. The 7 is not an accident of the digits, it is useful: a pound is 7000 grains, so the grain comes out at exactly 0.45359237 ÷ 7000 kg, which is 64.79891 mg, with nothing left over.

The two directions

Kilograms to pounds: divide by 0.45359237. A 70 kg person:

70 ÷ 0.45359237    =  154.323583529414…   →  154.32 lb

Pounds to kilograms: multiply by 0.45359237. A 150 lb person:

150 × 0.45359237   =  68.0388555 kg       exact, no rounding needed

Multiplying by 2.20462 instead of dividing by 0.45359237 is easier to type and lands in the same place to two decimals: 70 × 2.20462 is 154.3234. Just remember that 2.20462 is itself rounded, so it is a shortcut and not an equal sign. What that costs is worked out further down. It is not much.

Pounds and ounces, and the 7.7 trap

A pound is 16 ounces, not 10. So the decimal part of a pounds figure has to be multiplied by 16 before it can be read as ounces. Take a 3.5 kg newborn:

3.5 ÷ 0.45359237      =  7.716179176… lb
Whole pounds          =  7
Decimal part × 16     =  0.716179176… × 16  =  11.4588… oz

3.5 kg                =  7 lb 11.5 oz

The trap is reading 7.7 lb as "seven pounds seven". It is not. Seven tenths of a pound is 11.2 ounces, so 7.7 lb is 7 lb 11.2 oz, and the misreading takes 4.2 ounces off the baby. The same mistake turns 70 kg into 154 lb 3 oz when the right answer is 154 lb 5.2 oz, because 0.3236 × 16 is 5.18. It is the twelve inches problem from meters to feet, with sixteen instead of twelve.

For the reverse, put everything in ounces first. An ounce is exactly 0.45359237 ÷ 16 kg, which is 0.028349523125 kg, or 28.349523125 g:

8 lb 6 oz   =  8 × 16 + 6  =  134 oz   →  134 × 28.349523125  =  3798.83609875 g

Doing it in your head: double it, then add ten percent

Two and a bit pounds to the kilogram. The bit is very nearly a tenth of the two, which gives a two step estimate:

 70 kg   →    70 × 2  =  140   →  140 + 14   =  154 lb
 Exact        70 ÷ 0.45359237               =  154.3236…

100 kg   →   100 × 2  =  200   →  200 + 20   =  220 lb
 Exact       100 ÷ 0.45359237               =  220.4623…

That is multiplying by 2.2 when the true factor is 2.20462, so the estimate always runs low, by the same proportion every time: 2.2 × 0.45359237 = 0.997903214, which is 0.21% under. On 100 kg that is 0.46 lb, about 7.4 ounces. For comparison, the five eighths trick for kilometers to miles is off by 0.58%, nearly three times as much.

Going the other way, halve the pounds and then subtract a tenth of the half. That is multiplying by 0.45, which runs 0.79% low: 150 lb gives 75 − 7.5 = 67.5 kg against a true 68.04 kg, and 200 lb gives 100 − 10 = 90 kg against 90.72 kg.

Stones, for the British bathroom scale

A stone is 14 pounds, which makes it exactly 14 × 0.45359237 = 6.35029318 kg. Convert to pounds, then divide by 14 and keep the remainder as pounds:

70 kg   =  154.3236 lb   =  11 × 14 + 0.3236   →  11 st 0.3 lb
90 kg   =  198.4160 lb   =  14 × 14 + 2.4160   →  14 st 2.4 lb

The remainder is in pounds, not tenths of a stone, so 11.02 stone and 11 st 0.3 lb are the same weight written two ways, and 11.5 stone is 11 st 7 lb.

Conversion table: kg to lbs

Conversion table: lbs to kg

Every row here is exact. Multiplying a whole number of pounds by 0.45359237 never needs more than eight decimal places, so there is nothing to round:

The 45 and 20 rows are the gym pair. A 45 lb plate is 20.41 kg and a 20 kg plate is 44.09 lb, so they are close enough to share a bar and different enough, 0.91 lb apiece, that a bar loaded with four of one is 3.63 lb off a bar loaded with four of the other.

Where the rounded factor costs you

On 100 kg, each popular factor against the exact one:

Exact      100 ÷ 0.45359237   =  220.4622621… lb
2.20462    100 × 2.20462      =  220.462 lb      short by  0.0003 lb
2.2        100 × 2.2          =  220 lb          short by  0.46 lb
2          100 × 2            =  200 lb          short by  20.46 lb

So 2.20462 is safe for anything that is not a pharmacy, 2.2 is fine for a suitcase, and 2 is for people who would rather not know.

The binary arithmetic underneath has its own opinion. Ten pounds is 10 × 0.45359237, which is 4.5359237 kg and nothing else. A machine working in binary floating point answers 4.535923700000001, because 0.45359237 has no exact binary form. That stray 1 is the same fault behind 0.1 + 0.2 = 0.30000000000000004, and it is why a chain of conversions drifts. Prism holds 0.45359237 as the exact number it is, so 10 lb converts to 4.5359237 kg, 3 lb to 1.36077711 kg, and 0.1 + 0.2 is 0.3.

One pedantic footnote, since someone will raise it: kilograms measure mass, not weight, and so does this pound, the avoirdupois pound. The conversion is the same on the Moon. Only the bathroom scale would disagree.

Or let the unit converter hold the factor

Prism converts kilograms, pounds, ounces, stones, grams, milligrams and tonnes in the weight category, which is free, along with length and temperature. No ads, no subscription. See the app.