kg, lb or stone? Track in the units you think in

One stone is 14 pounds, which is about 6.35 kilograms, and one pound is exactly 0.45359237 kilograms. Use whichever unit you naturally think in for training and for body weight, because the maths is identical as long as you stay consistent. The trouble starts when numbers are converted over and over, or when units get mixed in a single log.

This guide gives you a conversion cheat sheet, explains why gym units matter more than you would think, and shows how to avoid the small errors that make progress look like a plateau.

How many kg in a stone, and a pound?

The pound is defined as exactly 0.45359237 kilograms, a value fixed by the International Yard and Pound Agreement that took effect on 1 July 1959 (Pound (mass), Wikipedia (opens in a new tab)). A stone is 14 pounds, so one stone comes to 6.35029318 kilograms.

To convert stone and pounds to kilograms, change everything to pounds first, then multiply by 0.4536. For 12 stone 5 pounds, that is (12 x 14) + 5 = 173 lb, and 173 x 0.4536 is about 78.5 kg.

Body weight in stone

Barbell weights

Why the unit matters in the gym

The unit is not just a label. It decides what you can load. Gyms in most countries stock kilogram plates, and many in the United States stock pound plates, so the smallest jump you can make differs. A common smallest kilogram pair adds 2.5 kg in total, which is about 5.5 lb. A common smallest pound pair adds 5 lb, which is about 2.3 kg.

That is why a programme written in kilograms can be awkward to follow with pound plates. A 20 kg bar is 44.1 lb, but a standard American barbell is 45 lb, or 20.4 kg. The difference looks tiny until you are loading 100 kg and trying to hit an exact number.

Small jumps matter for progressive overload. The method works best when you can add a small increment every few weeks. Use the units your gym's plates are marked in, so each step is something you can physically load.

Body weight and training weight can use different units

There is no rule that says you must pick one unit for everything. Plenty of people in the UK and Ireland think about body weight in stone and pounds but load barbells in kilograms, and many travellers think in pounds at home and kilograms abroad.

What matters is that each log uses one unit consistently. Your lifts should all be in the unit of the plates on the bar. Your body weight should all be in the unit you weigh yourself in. If you want to see the trend, tracking progress without the scale covers using a weekly average rather than daily readings.

Four ways unit mix-ups create false plateaus

1. Mixed units in one log. Logging 100 for one session in kilograms and 100 for the next in pounds shows a drop of more than half. Keep one unit per exercise.
2. Rounding on every conversion. Convert 100 lb to 45 kg and back, and you land on 99.2 lb. Do that a few times and your numbers drift. Convert once, from the original value.
3. Treating a pound plate as a kilogram plate. The difference between 2.5 kg and 2.5 lb is more than a kilogram of load.
4. Ignoring the bar. A 20 kg bar and a 45 lb bar are close but not equal. Know which one you are using.

The fix for all four is the same: choose your units once, set them in your tracking app, and let it do any conversion rather than doing it yourself in your head.

A simple way to choose

If you are not sure which units to use, decide in this order:

1. Lifts: match the plates and the bar in your gym. If the plates say kg, log kg.
2. Body weight: use the unit your bathroom scale shows, because you will read it without thinking.
3. Programmes and recipes from elsewhere: convert them once, at the start, to your own units, and round to something you can actually load or measure. A 135 lb target becomes 60 kg on a kilogram bar, not 61.2 kg.
4. Changing your mind later: pick a quiet week to switch, note the date, and avoid comparing numbers across the switch by eye. A week of overlap is enough to see that nothing has really changed.

Other units worth knowing

Fitness tracking is full of paired units. These are the ones that tend to cause confusion.

• Height: centimetres, or feet and inches.
• Distance: kilometres, or miles. One mile is about 1.609 km.
• Liquid: millilitres, or fluid ounces. Note that UK and US fluid ounces differ slightly.
• Energy: kilocalories (kcal), or kilojoules (kJ). In the UK, food labels must include the amount of energy in both kilojoules and kilocalories (NHS, how to read food labels (opens in a new tab)). One kcal is 4.184 kJ.

If you have ever compared a recipe from a UK website with one from an American one, you have already met the problem. A good tracker lets you choose the units you want for each kind of measurement and leaves the arithmetic to the app.

How VivFit handles units

VivFit is built for every country. You can choose Metric, US or UK, or mix your own: kilograms, pounds or stone and pounds, centimetres or feet and inches, kilocalories or kilojoules, and so on. The point is that the app shows numbers the way you think about them, so you are not converting in your head halfway through a set. You can see it listed in the FAQ on the home page. VivFit is coming soon to iPhone and Android, and you can join the waitlist for launch news.

Frequently asked questions

How many kilograms are in a stone?

One stone is 6.35 kilograms, to two decimal places, because a stone is 14 pounds and a pound is 0.45359237 kilograms. As a shortcut, multiply the stone figure by 6.35. For example, 11 stone is 69.9 kg.

How do I convert stone and pounds to kilograms?

Convert the whole weight to pounds by multiplying the stone figure by 14 and adding the pounds, then multiply by 0.4536. For 12 stone 5 pounds, that is 173 pounds, or about 78.5 kilograms. Round once at the end, not at each step.

Should I use kg or lb for lifting?

Use whichever matches the plates in your gym and the way you think. Kilograms are used in most countries and in most programmes written outside the US, while pounds are common in American gyms. The two systems work equally well for progressive overload as long as you stay consistent and can load a small increment.

Is a 45 lb barbell the same as a 20 kg barbell?

Almost, but not exactly. A 45 lb bar weighs about 20.4 kg, so it is roughly 0.4 kg heavier than a 20 kg bar. That is small, but it adds up when you compare numbers across programmes or gyms, so check which bar you are using before you log a lift.

Can I use different units for body weight and lifting?

Yes. Many people log body weight in stone and pounds and lifts in kilograms. Keep each measurement in one unit, and avoid converting between them in your log, so your trends stay clean.

Sources

• Wikipedia. Pound (mass) (opens in a new tab): definition of the international avoirdupois pound as exactly 0.45359237 kg (International Yard and Pound Agreement, 1959).
• NHS. How to read food labels (opens in a new tab).