KSI: natty or not?
Likely natty YouTuber, rapper, and cruiserweight boxer
Short answer. Verdict: likely natty. At the peak physique on this page, KSI is 6'0" (183 cm) and 198 lbs (90 kg) at roughly 12% body fat. That works out to an FFMI of 23.5, against the ~25 mark cited as the natural ceiling.
No official record settles this. What follows is opinion and analysis based on public numbers — how we judge.
The numbers (estimated)
| Peak physique | Cruiserweight fighting condition |
|---|---|
| Height | 6'0" (183 cm) |
| Weight | 198 lbs (90 kg) |
| Body fat | ~12% |
| FFMI | 23.5 — ~25 is the cited natural ceiling |
Era by era (estimated)
| Era | Year | Age | Weight | Evidence | Body fat | FFMI |
|---|---|---|---|---|---|---|
| Cruiserweight fighting condition | 2023 | 30 | 198 lbs (90 kg) | reported | ~12% | 23.5 |
Note. Weight and body fat are estimates from footage, not measurements. The verdict is opinion. See the methodology page for how we judge.
KSI got in shape in public, over several years of boxing preparation: professionally since November 2019, twice against Logan Paul as an amateur and then a pro, with a 2023 loss to Tommy Fury. He trained for those fights under professional coaches including Viddal Riley and Jeff Mayweather.
The numbers
At 6’0” and fighting at cruiserweight, the FFMI on this page is unremarkable — below the drug-free ceiling our methodology page cites, and below plenty of people who have never been accused of anything.
He got visibly fitter, not visibly bigger. That distinction is the whole of this page: losing fat and adding conditioning transforms how someone looks without adding the muscle mass that would show up in the arithmetic.
Verdict
Likely-natty — a normal amount of muscle on a normal frame, with no record or allegation of any kind. The change people remember is a fat-loss change.
Common questions
Is KSI natty or on steroids?
This site rates KSI as likely natty. That is an opinion drawn from public numbers, not a statement of fact: the peak physique on this page works out to an FFMI of 23.5, against the ~25 mark cited as the natural ceiling.
Has KSI ever failed a drug test or admitted to using PEDs?
Nothing of the sort turned up in the sources on this page. Had it, the verdict would read confirmed instead of likely natty.
What is KSI's FFMI?
About 23.5. That comes from 6'0" (183 cm) and 198 lbs (90 kg) at roughly 12% body fat, normalized to 1.8 m. Drug-free lifters in the Kouri 1995 study topped out near 25.