9 stones is 57.15263862 kg. The exact arithmetic is 9 × 6.35029318 = 57.15263862 kilograms, using the defined factor rather than a rounded one, so this matches what a calibrated instrument would give you.
At 57.1526 kg, BMI works out to 22.3 at 1.60 m, 19.8 at 1.70 m, 17.6 at 1.80 m and 15.8 at 1.90 m. The healthy band tops out at 25, so the same weight can sit in three different categories depending on height — which is the whole limitation of the index.
The result sits 2.153 kg past 55 kg, so if you need a round figure to quote, 55 kg is the honest nearest one. Plenty of people do this in their head: multiply by 6.4 instead of using the exact factor 6.35029318. At 9 that shortcut gives 57.6 kilograms, off by 0.4474 kilograms or about 0.78%. Fine for a rough estimate, not fine for anything you are cutting to size or dosing.
The formula
9 st- The quantity being converted
6.35029318- Exact kilograms in one stone
57.15263862 kg- The converted result
How it works, step by step
- Take the stone value, 9.
- Apply the defined factor: one stone is 6.35029318 kilograms, so multiply by that.
- The product is 57.15263862 kilograms.
- Going the other way, divide rather than multiply — 57.15263862 kg ÷ 6.35029318 brings you back to 9 st.
Worked examples
Working it through
Because this is a straight proportion, 9 st × 6.35029318 = 57.15263862 kg — no offset, no second term, nothing else to account for.
Proving it both ways
57.15263862 kg ÷ 6.35029318 = 9 st. Recovering the original number is the fastest way to confirm the factor was not applied upside down.
How to read your score
Frequently asked questions
What is 9 stones in kilograms?
It comes to 57.15263862 kg. The unrounded figure is 57.15263862 kilograms, if you need to carry it forward.
How do you convert stones to kilograms?
Multiply by 6.35029318. So 9 × 6.35029318 = 57.15263862 kilograms. The factor comes from a definition rather than a measurement, so it never changes.
What is 57.15263862 kilograms back in stones?
9 stones. Dividing by 6.35029318 undoes the conversion exactly.
What does 9 st actually look like?
At 57.1526 kg, BMI works out to 22.3 at 1.60 m, 19.8 at 1.70 m, 17.6 at 1.80 m and 15.8 at 1.90 m. The healthy band tops out at 25, so the same weight can sit in three different categories depending on height — which is the whole limitation of the index. The reference table shows the values either side of it for comparison.
What is a stone?
Fourteen pounds — still the customary way to state body weight in the UK and Ireland.
What is a kilogram?
The SI base unit of mass, redefined in 2019 in terms of the Planck constant.
9 st in kilograms — nearby values
| Stones | Kilograms | Unrounded |
|---|---|---|
| 3 st | 19.05088 | 19.05087954 |
| 4 st | 25.401173 | 25.40117272 |
| 5 st | 31.751466 | 31.7514659 |
| 6 st | 38.101759 | 38.10175908 |
| 7 st | 44.452052 | 44.45205226 |
| 8 st | 50.802345 | 50.80234544 |
| 9 st | 57.152639 | 57.15263862 |
| 10 st | 63.502932 | 63.5029318 |
| 11 st | 69.853225 | 69.85322498 |
| 12 st | 76.203518 | 76.20351816 |
| 13 st | 82.553811 | 82.55381134 |
| 14 st | 88.904105 | 88.90410452 |
| 15 st | 95.254398 | 95.2543977 |
Exact factor: 1 stone = 6.35029318 kilograms. The row for 9 st is the value this page is about.