A 45 lb plate weighs 20.4117 kg and a 20 kg plate weighs 44.0925 lb, so no loading that is round in pounds is round in kilograms, and none that is round in kilograms is round in pounds. Converting a lift between gyms means converting the total on the bar and rebuilding it from whatever is on the rack — not matching plate for plate.
This catches people out because both systems look tidy from the inside. A pound gym counts in 45s and 25s; a kilo gym counts in 20s and 15s. Neither set reproduces the other, and the gap is large enough to move a personal best.
What does each plate weigh in the other unit?
Every common plate converts to an untidy figure. The pound is defined as exactly 0.45359237 kilograms, fixed by international agreement in 1959, so these are not estimates — they are the whole answer to as many decimals as anyone needs.
| Pound plate | In kilograms | Kilo plate | In pounds |
|---|---|---|---|
| 45 lb | 20.4117 kg | 25 kg | 55.1156 lb |
| 35 lb | 15.8757 kg | 20 kg | 44.0925 lb |
| 25 lb | 11.3398 kg | 15 kg | 33.0693 lb |
| 10 lb | 4.5359 kg | 10 kg | 22.0462 lb |
| 5 lb | 2.2680 kg | 5 kg | 11.0231 lb |
| 2.5 lb | 1.1340 kg | 2.5 kg | 5.5116 lb |
The near-miss between the 45 lb plate and the 20 kg plate causes most of the confusion. They differ by 0.41 kg — under a kilogram per plate, and several kilograms across a loaded bar.
Why is a 225 lb bench not a 100 kg bench?
Because 225 lb is 102.0583 kg, a little over two kilograms heavier than the round figure people quote. The number 225 exists for a mechanical reason rather than a mathematical one: it is a 45 lb bar with two 45 lb plates a side, the first loading a pound gym can build entirely out of its largest plate.
Going the other way, 100 kg is 220.462 lb, which a pound gym cannot make at all. The closest it builds is 220 lb — 99.79 kg — from a 45 lb bar plus 45, 35, 5 and 2.5 on each side.
Does the bar weigh what the label says?
A standard Olympic bar is either 20 kg or 45 lb, and those are different bars. Twenty kilograms is 44.0925 lb, so a bar sold as 45 lb is genuinely heavier than the 20 kg bar used elsewhere, by about a pound.
Most kilo-market bars marked 45 lb are 20 kg bars with a rounded label. If your gym is metric and the bar is stamped in pounds, weigh it or assume 20 kg — assuming 45 lb adds a pound of imaginary weight to every set you log.
How do you hit a kilogram target in a pound gym?
Convert the target to pounds, subtract the bar, halve what is left, and build that from the plates on the rack. Round once, at the end, and write down the true total rather than the target you aimed at.
For a 100 kg squat: 100 kg is 220.462 lb. Take off a 45 lb bar and 175.462 lb remains, or 87.73 lb a side. The rack can make 87.5 — a 45, a 35, a 5 and a 2.5 — which puts the bar at 220 lb, or 99.79 kg. That is 0.21 kg light, and worth recording as 220 lb rather than as 100 kg.
The reverse trip is easier, because metric increments are finer at the bottom. A 315 lb deadlift is 142.8816 kg, and a kilo gym building 142.5 kg lands within 0.4 kg.
Which total should you log?
Log the one you actually lifted, in the unit the plates were marked in, and convert only when you need to compare. A log that stores 220 lb keeps its meaning forever; one that stores "100 kg, roughly" has already lost the 0.21 kg and loses more each time it is copied.
Competition removes the choice. The international federations for weightlifting and powerlifting both run in kilograms, so a meet total is a kilogram figure and any pound number attached to it is a translation for the audience.
What about dumbbells and machines?
Dumbbells are worse, because the increments differ as well as the units. A pound rack steps 5, 10, 15, 20 and upward in fives, which is 2.27, 4.54, 6.80 and 9.07 kg — so a metric lifter moving from 10 kg to 12.5 kg has no equivalent jump available, and the pound rack forces a 2.27 kg step where the programme wanted 2.5.
Selectorised machines are the least transparent of all. The stack is marked in the local unit with no stamp on the individual plates, and the pin position tells you which plate rather than what it weighs. Plate-loaded machines add a further complication: the load at the handle is not the load on the pins, because the lever geometry changes it, so the number on the plates is an input rather than a resistance.
The workable habit is to treat each machine as its own scale. Record the pin number or the plates loaded, note which machine it was, and stop trying to translate between rooms — the translation is not defined even when both rooms use the same unit.
Questions people ask
Is a 45 lb plate ever exactly 20 kg? No. It is 20.4117 kg by definition. Some manufacturers sell a 20 kg plate labelled 44 lb, which is the same plate honestly described.
How much error does plate matching add? Loading 20 kg plates where the programme called for 45 lb plates loses 0.41 kg per plate. Across three plates a side that is 2.47 kg, or about 5.4 lb — enough to turn a hard triple into an easy one.
What tolerance do plates have? Calibrated competition plates are held to tight limits. Commercial gym cast iron is not, and can sit several per cent from its stamp. If the arithmetic matters, the plates need weighing first.
Do the collars count? Competition collars weigh 2.5 kg for the pair and are included in the total. Training spring clips weigh almost nothing and are ignored.
Convert the total, not the plates, and the two systems stop fighting. Pounds to kilograms and kilograms to pounds both run live as you type, and if you are converting a body weight rather than a bar, the BMI calculator takes either unit directly.