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Race Time Predictor

Predicts finish times for 5 km, 10 km, half marathon and marathon from one known race result using Riegel's endurance formula T2 = T1 × (D2/D1)^1.06, with the corresponding paces.

When to use

You have a recent race time (or time trial) and want equivalent-effort predictions for other standard distances, e.g. to set a marathon goal from a half-marathon result.

Do not use when: You only need the pace or constant-pace splits of a run (use running-pace), the known effort was a trail or very hilly race, or it was shorter than about 1.5 km or longer than a marathon.

Formula

T2 = T1 × (D2 / D1)^exponent, exponent = 1.06 (Riegel 1981); pace = T2 / D2

Riegel's power law was fitted to world-record performances from about 3.5 minutes to 4 hours and assumes comparable training for the target distance. Predictions from short races to the marathon are usually optimistic for non-elite runners.

Inputs

Parameter	Type	Required	Description
`distance`	number	yes	Distance of the race you already ran, in distance_unit. Range: > 0, ≤ 500
`distance_unit`	enum: km \| mi	default km	Unit of the distance input.
`time`	string	yes	Finish time of the known race as h:mm:ss, mm:ss or minutes.
`exponent`	number	default 1.06	Riegel's exponent; 1.06 fits elite and well-trained runners, 1.07–1.10 is often more realistic for recreational marathoners. Range: ≥ 1, ≤ 1.3

Outputs

Output	Type	Unit	Description
`predicted_5k`	string	h:mm:ss	Predicted 5 km time.
`predicted_10k`	string	h:mm:ss	Predicted 10 km time.
`predicted_half_marathon`	string	h:mm:ss	Predicted time for 21.0975 km.
`predicted_marathon`	string	h:mm:ss	Predicted time for 42.195 km.
`predictions`	list		Per race: distance_km, time (h:mm:ss), pace_min_per_km and pace_min_per_mile.

Example

10 km in 50:00: {"distance":10,"distance_unit":"km","time":"50:00"} → {"predicted_5k":"0:23:59","predicted_10k":"0:50:00","predicted_half_marathon":"1:50:19","predicted_marathon":"3:50:01"}

Half marathon in 1:45:00: {"distance":21.0975,"distance_unit":"km","time":"1:45:00"} → {"predicted_5k":"0:22:50","predicted_10k":"0:47:35","predicted_marathon":"3:38:55"}

GET https://tttkmbb.com/api/v1/calculate/race-time-predictor?distance=10&distance_unit=km&time=50%3A00

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/race-time-predictor (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/race-time-predictor · Markdown: https://tttkmbb.com/fitness/race-time-predictor.md · JSON definition: https://tttkmbb.com/fitness/race-time-predictor.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="race-time-predictor"
OpenAPI operationId: predict_race_time
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

FAQ

Why 1.06?

Riegel found that record times scale with distance to the power 1.06 for running (the 'fatigue factor'): doubling the distance takes about 2.085 times as long, i.e. pace slows by roughly 4 % per doubling.

How accurate is the prediction?

Typically within a few percent between neighbouring distances (10 km ↔ half marathon). Marathon predictions from a 5 km are often 5–10 % too fast unless mileage is high; raising the exponent to 1.07–1.10 compensates.

Related calculators

Running Pace Calculator — Constant-pace splits and speed for a given distance and time.
VO2 Max Calculator — Estimate aerobic capacity from a 12-minute run.