Race Time Predictor
Predict your 5K, 10K, half-marathon and marathon finish times from one recent race result using Peter Riegel's published endurance formula.
Formula: Riegel formula: predicted time = recent time × (target distance ÷ recent distance)^1.06
Worked examples
- Recent 5K in 25:00 → predict 10K, half and full marathon
A common starting point — using a recent 5K result to set realistic goal paces for longer races.
- Recent 10K in 48:00 → predict shorter and longer distances
The Riegel formula also works backwards, predicting a faster 5K time from a 10K result.
- Recent half marathon in 1:45:00 → predict marathon time
Half-to-full marathon prediction is one of the most common uses of the Riegel formula among marathon-block training plans.
What this calculator estimates
Given one recent race result, this tool projects what you could run for the four standard race distances — 5K, 10K, half marathon and marathon — using a formula that has been a staple of running coaching for nearly five decades. It's a fast way to set a realistic goal pace for an upcoming race based on fitness you've already demonstrated, rather than guessing.
The Riegel formula
Peter Riegel, an engineer and long-time runner, published his endurance-performance formula in 1977 after analyzing how race times scale with distance across a wide range of running (and other endurance) performances: predicted time = recent time × (target distance ÷ recent distance)^1.06. The exponent, 1.06, captures the fact that pace naturally slows as distance increases — running twice as far takes more than twice as long, because endurance, fueling and pacing all become harder to sustain over longer efforts. It's a public-domain formula, not tied to any brand or proprietary dataset, which is why it appears across nearly every running calculator and coaching app.
Why it works best over similar distances
The formula assumes your aerobic fitness scales predictably with distance, which holds up well when you're predicting a nearby distance — a 10K result predicting a half marathon, for example — but gets less reliable across bigger jumps. Predicting a marathon from a 5K result assumes your endurance base (long runs, fueling, mental pacing) scales the same way your raw speed does, which isn't guaranteed; a fast 5K runner without marathon-specific training will often run slower than the formula suggests, while a well-trained marathoner may run faster than a bare 5K time would predict.
How to use the prediction
Treat the predicted times as a goal-setting starting point, not a guarantee. If your recent result was in a similar training block to your target race, and both are efforts you've trained specifically for, the prediction tends to land close. If conditions differ significantly — you ran your recent race on a hot day, or you're targeting a race with much more elevation gain — adjust your goal pace accordingly rather than trusting the number literally. Many coaches use Riegel predictions as one data point among several (recent workout paces, heart rate response, perceived effort) rather than the sole basis for a race plan.
Limits
The formula doesn't know anything about your specific training history, injury status, race-day weather, course profile or fueling strategy — all of which meaningfully affect real race outcomes. It also tends to be less accurate for very new runners, whose fitness is changing quickly, and for ultra-distance events far beyond the marathon, where different physiological limits (glycogen depletion, joint fatigue) start to dominate. This tool gives a general training estimate, not medical or coaching advice — build your race plan with input from your own training data and, if you have a health condition, a qualified professional.
Frequently asked questions
- What formula does this use?
- Peter Riegel's 1977 endurance-performance formula: predicted time = recent time × (target distance ÷ recent distance)^1.06. It's a public-domain formula widely used across running coaching and training-plan software, derived from analyzing how race performance scales with distance.
- How accurate is the prediction?
- It's most accurate when your recent race and target race are reasonably close in distance and you're similarly trained for both — predicting a marathon from a 5K assumes your endurance base scales in the way the formula expects, which isn't guaranteed. Treat predictions from very different distances as a rough guide, not a guarantee.
- Can I predict a 5K time from a marathon result?
- Yes, the formula works in both directions — enter your marathon distance and time as the 'recent' result and the table still shows all four standard distances, including 5K, calculated the same way.
- Why might my actual race time differ from the prediction?
- The formula doesn't account for race-day conditions (heat, wind, course terrain), pacing strategy, taper quality, or endurance-specific training gaps (e.g. a strong 5K runner who hasn't built marathon-distance long runs). Use it to set a realistic goal range, not an exact target.
These calculators give general estimates from published formulas. They are not medical advice and do not account for your individual health; talk to a qualified professional before changing your diet or training.