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How It's Calculated

Pace is the simplest relationship in running math: pace (min/km) = time ÷ distance, and speed inverts it — speed (km/h) = distance ÷ time × 60. A 27:30 5K works out to 5:30/km, or roughly 10.9 km/h, and this tool converts freely between minutes-per-km, minutes-per-mile and km/h or mph so you never have to do the division by hand mid-run.

The more useful feature is the Riegel race-time prediction, built on a real exercise-physiology relationship rather than a rule of thumb: T₂ = T₁ × (D₂/D₁)¹·⁰⁶, where T₁ and D₁ are a known time and distance and T₂ is the predicted time for a new distance D₂. The exponent 1.06 is the key detail — if endurance performance scaled perfectly linearly with distance, the exponent would be exactly 1, and doubling the distance would exactly double the time. It does not: fatigue accumulates faster than distance does, so the extra 0.06 captures the mild, well-documented slowdown every runner experiences as races get longer. Take a runner who covers 10 km in 50 minutes: T₂ = 50 × (42.195/10)¹·⁰⁶ ≈ 245 minutes, or about 4 hours 5 minutes for the full marathon — noticeably slower than simply multiplying the 10K pace by 4.2, which is exactly the point of using the formula.

Riegel's formula is a statistical estimate fitted to large samples of real race results, not a guarantee for any individual. It assumes the runner is genuinely trained for the target distance; a fast 5K time built on speed work alone will not carry through 42 km without the underlying mileage and long-run endurance to support it. Race-day pacing follows from the same math: running the first half at or slightly slower than the predicted even pace is the reliable strategy, since "banking" time early by running faster almost always costs far more time to fatigue in the second half.

What You Should Know

• The formula works best between 5K and marathon distance — extrapolating far outside that range (ultramarathons, or very short sprints) introduces more error since fatigue and physiology behave differently.
• Training volume matters more than raw speed — the prediction assumes adequate long-run mileage for the target distance; a speed-only runner will typically finish slower than Riegel predicts for a marathon.
• Even or slightly negative splits perform closest to the prediction — starting faster than goal pace consistently produces worse finishing times than the formula suggests, because late-race fatigue compounds non-linearly.
• Course, weather and terrain shift the actual result — heat, hills and altitude all add time beyond what a flat, cool prediction accounts for.
• Speed and pace are two views of the same number — multiplying pace in min/km by distance always returns total time, so both figures are useful depending on whether you plan by clock or by distance.