What formula does the predictor use?

It uses the Riegel formula, a power-law model that scales race time by distance and a fatigue exponent.

Can a 5K predict a marathon?

It can give a rough mathematical estimate, but it is often optimistic unless your marathon-specific endurance is strong.

What is a good Riegel exponent?

1.06 is the classic default. For marathon planning, 1.07 or 1.08 can be a more conservative scenario.

Race Time Predictor Calculator

Predict equivalent race times for 1K, 5K, 10K, half marathon and marathon from a recent result using the Riegel formula and a fatigue exponent.

Known race distance

Race time (h / min / sec)

Riegel exponent: 1.06

1.06 is the classic default; 1.07-1.08 is more conservative for marathon projections. Shorter distances often use 1.04-1.05.

Predicted race times

Distance	Time	Pace
1K	3:55	3:55/km
5K	21:35	4:19/km
10K (input)	45:00	4:30/km
Half marathon	1:39:17	4:42/km
Marathon	3:27:01	4:54/km

Long-distance extrapolation

A marathon projection from a 5K or 10K can be optimistic. A half marathon is usually a stronger input, and without long runs, race fueling practice and late-race experience, choose a more conservative target.

Time vs distance curve (log-log)

1 km51021.142.2 km

The green line is the Riegel projection. The red point is your input result. The farther the target is from the point, the larger the uncertainty.

Training pace estimates

Zone	Use	Pace /km
Easy (E)	Recovery and base mileage	5:34 – 5:49
Marathon (M)	Marathon-pace work	4:49 – 4:59
Threshold (T)	Tempo threshold work	4:31 – 4:37
Interval (I)	VO2 max intervals, 3-5 min reps	4:14 – 4:24
Repetition (R)	Speed and economy, 200-400 m	3:54 – 4:01

The Riegel model with exponent 1.06 is a mathematical estimate, not a guarantee. It does not model hills, weather, surface, fueling, fatigue or distance-specific preparation.

Predict another race distance

The English SERP is broader than a literal marathon forecast. Users usually enter a recent 5K, 10K, half marathon or marathon time and expect equivalent predictions for other common race distances.

T_{2} = T_{1} (\frac{D _{2}}{D _{1}})^{k}

T1 - known race time, T2 - predicted time, D1 - known distance, D2 - target distance, k - fatigue exponent.

How to read the projection

A recent all-out race on a measured course is the best input.
A half marathon usually predicts a marathon better than a 5K does.
Long-distance predictions can be optimistic without long runs, fueling practice and distance-specific preparation.
Use the exponent slider to compare classic and conservative scenarios.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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