Race Time Predictor
All you need is one recent, genuinely hard effort.
Riegel and Cameron — we show both formulas at once. Pick the one that fits you and set your goal.
Predicted-time comparison Both formulas at once
| Distance | Riegel | Cameron | Average | Pace |
|---|---|---|---|---|
| 1500m 1.5 km | — | — | — | — |
| 3km 3 km | — | — | — | — |
| 5K 5 km | — | — | — | — |
| 10K 10 km | — | — | — | — |
| 15K 15 km | — | — | — | — |
| Half 21.1 km | — | — | — | — |
| 30K 30 km | — | — | — | — |
| Full 42.2 km | — | — | — | — |
The two formulas — which is more accurate?
Both answer "how does time change when you run a different distance at the same effort?" — but they take different approaches.
Riegel formula (1981)
T₂ = T₁ × (D₂ / D₁)^1.06- The most widely used formula. Simple and fast.
- Weakness: short distance (5K) → full-marathon predictions are a bit optimistic. Assuming a constant 1.06 exponent across all distances is unrealistic.
Cameron formula (1998)
- Applies a different pace-decay rate depending on distance
T₂ = T₁ / D₁ × D₂ × ((a − b × log₁₀(D₁)) / (a − b × log₁₀(D₂)))
where a = 13.49681, b = -0.048865 × ((D₁ × D₂)^-0.5 + 0.39477)
(D is in miles)- Strength: long-distance (full-marathon) predictions are more conservative and realistic than Riegel's
- Weakness: the formula is complex, and for short distances it barely differs from Riegel
Which one?
- 5K → 10K, 10K → half: both are similar, use Riegel
- 5K → full marathon: Cameron or the average of the two is recommended
- Real-world caveat: whichever formula you use, the actual full marathon takes 5–10% longer than predicted is common wisdom — because of "the wall."
Why are longer distances harder to predict?
- After 15 km, glycogen depletion; after 25 km, muscle damage; after 30 km, "the wall"
- These are not part of a pace-decay curve but separate variables — nutrition, pacing, and course conditions
- That is why whether you have done 30 km+ LSD training is decisive
Sources: Riegel P. (1981) Athletic Records and Human Endurance · Cameron D. (1998) · Costill DL. Inside Running: Basics of Sports Physiology
Predicted pace = your yardstick for finding running partners
Aiming for a 4-hour full? There is a group with the same goal. Chasing a 25-minute 5K? There is a pace group for that. You get there faster together than alone. Find your pace group in a crew.
