Calculator
Weight loss and running pace calculator
Enter your body weight and how much you would lose, and we show how much faster your pace would be. Add a recent time to see your new race time too.
Your details
Optional, only for the health check.
5% of your body weight
Your pace would be about 3% faster (theoretical maximum: 5%)
At your pace (6:00/km) that is 5:49/km, 11 s faster per kilometre.
Distance
Your current time for this distance (hh:mm:ss)
Estimate
Expected new time
58:12
now: 1:00:00
You would be faster by
1:48
Pace
6:00 → 5:49 /km
Theoretical maximum
57:00 (−3:00)
Upper limit: what you would get if your fitness stayed exactly the same and only your weight went down.
About 27 s per kilo on this distance (2.7 s/km).
How much does it bring?
| Weight lost | New time | Faster by | Faster by |
|---|---|---|---|
| −1 kg | 59:33 | 0:27 |
|
| −2 kg | 59:06 | 0:54 |
|
| −3 kg | 58:39 | 1:21 |
|
| −5 kg | 57:45 | 2:15 |
|
| −8 kg | 56:24 | 3:36 |
|
| −10 kg | 55:30 | 4:30 |
|
The estimate only holds for losing excess fat. If you lose muscle too, or you are already lean, losing weight will not make you faster and can make you slower.
How we calculate
When you run, you carry your own body weight with every step. If you lose fat while keeping your muscle and endurance, you move less mass with the same amount of oxygen, so you can run faster.
In theory, 1% less body weight would mean about 1% more speed. In the experiments the gain was smaller: a 1% change in weight changed times by about 0.5–0.8%. These experiments used weighted vests or a harness that slightly lifted the runners, not actual weight loss, so the calculator stays cautious at 0.6% and shows the theoretical upper limit next to it.
Example: you weigh 80 kg and run 10 km in 1:00:00. If you lose 4 kg (5% of your body weight), the expected new time is 58:12, which is 1:48 faster. The theoretical maximum is 57:00.
The estimate assumes a flat course and roughly unchanged fitness. On hilly terrain body weight matters more, so the effect can be larger there.
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Frequently asked questions
Will I really get faster if I lose weight?
If you have excess weight and you lose fat, yes. Your time improves roughly in proportion to the weight you lose, only less than you might expect. If you lose muscle or lose weight too fast, the benefit can disappear.
What is 1 kg worth?
It depends on your weight and your time. For an 80 kg runner who runs 10 km in 60 minutes, losing 1 kg means about 25 to 30 seconds. The calculator works it out for your own numbers.
How much weight is it worth losing?
Down to the healthy weight range. Below it performance drops, the risk of injury and stress fractures rises, and your hormones can be disrupted (low energy availability, RED-S). Runners should lose weight slowly: at most 0.5–1% of body weight per week, so the muscle stays.
Why are there two values?
Because the exact effect differs from runner to runner. The theoretical upper limit assumes your fitness stays exactly the same and only your weight goes down. The measured value comes from running experiments with weighted vests and harnesses and is more cautious, so we show it as the main result.
The result is an informative estimate, not medical or nutritional advice. Before losing a larger amount of weight, ask a doctor or sports dietitian.
Sources: the weighted-vest running experiments of Cureton et al. (1978, Medicine and Science in Sports) and Cureton & Sparling (1980, Medicine and Science in Sports and Exercise); a 3 km harness body-weight-reduction experiment with 11 club-level runners; Teunissen, Grabowski and Kram (2007, Journal of Experimental Biology); Jack Daniels’ VO2max scaling (Daniels’ Running Formula).