Race time predictor
From a single race time, estimate your times on every distance from 5 km to 100 km, pace included. No exponent formula.
Updated 16 September 2026
One recent race time says a lot about the others — provided the conversion still holds beyond the marathon, where most tools simply stop.
These estimates assume training suited to the target distance. A predictor tells you what your engine allows, not what your legs will absorb: nobody runs their predicted marathon without having done the long runs.
Free, no sign-up. Everything is computed in your browser: nothing you type here is sent anywhere.
How it is computed
Your time first gives a VMA by inverting the sustainable-%VMA table. That same table is then applied to the target distance: 95 % at 5 km, 90 % at 10 km, 85 % at half marathon, 80 % at marathon, 75 % at 50 km, 65 % at 100 km, interpolated in between. Because it works both ways, predicting your reference distance returns exactly the time you entered.
Why not Riegel’s law, the 1.06 exponent found everywhere? We measured it against our table: the two stay within 1.2 % of each other on the half and diverge by 3.2 % on the marathon, Riegel being the more optimistic. It is calibrated on trained runners and knows nothing of the marathon wall or of walking in an ultra.
Past 50 km, treat the result as an order of magnitude. Aid stations, night, heat and elevation then matter more than physiology — for a trail, use the effort-kilometre calculator instead.
The plans built from this number
Now the plan that goes with it
These numbers are exactly the ones Endu uses to build a full preparation: phases, volume, detailed sessions, exported to your watch. 14-day trial, cancel any time.
Create my planFrequently asked questions
- Can I target my predicted marathon time?
- Only if your training supports it. The prediction assumes volume and long runs consistent with the distance. On a first marathon, allow 5 to 10 minutes over the predicted time: that is the price of inexperience with pacing and fuelling.
- Why are my predicted short-distance times pessimistic?
- If your reference time is long (half or full marathon), converting down to 5 km assumes raw speed you may not train. A very enduring but slow runner will be under-estimated at 5 km — and the converse holds too.
- Does the predictor account for age or weight?
- No, deliberately: those factors are already baked into your race time. Age-grading tables exist to compare runners with each other, not to predict your own times from your own performance.
- Which reference distance predicts best?
- The one closest to your target, and the most recent. To prepare a marathon, an all-out half marathon within the previous six weeks is the best possible reference; a 5 km from a year ago is the worst.