Cardio
Race Time Predictor
Race Time predictor: predict finish times across 5K, 10K, half marathon, and marathon from any known race result using Riegel's formula.
Result
5:27 /km average pace
4:48 /km
5:00 /km
5:14 /km
Predicted finish times
Riegel: T2 = T1 × (D2/D1)^1.06 — longer races run a little slower per kilometre.
What does the Race Time Predictor calculate, and which formula does it use?
The Race Time Predictor converts one known race result into predicted finish times and paces for 1K, 1 mile, 5K, 10K, half marathon, and marathon using Riegel's endurance formula, T2 = T1 × (D2/D1)^1.06. The 1.06 exponent models how pace slows with distance, and each prediction reports how much per-kilometer pace slows versus your baseline race.
Last updated .
How to use it
- Select the race you ran (or Other for any distance) and type your finish time as m:ss or h:mm:ss — even 30 seconds of error shifts all predictions. Use an official chip time, not a watch start/stop.
- Choose the races to predict. The predictor uses Riegel's formula (T2 = T1 × (D2/D1)^1.06), which assumes you are as well trained for the target race as for the one you ran, so it is most dependable between nearby distances.
- Read each prediction with its average pace, per km or per mile with Units set to US. Riegel makes marathon pace about 14% slower per km than 5K pace; if your real marathons run slower than that, the gap is endurance training, not the formula.
- Use predicted paces as references for race-specific sessions: marathon pace for marathon-pace work, half marathon pace for tempo-style runs. Easy long runs usually go slower than marathon pace.
- Rerun after each race — your fitness evolves and a fresh data point is always more accurate than extrapolating from a 6-month-old result.
Questions people usually ask
Can this predict my half marathon time?
Yes. Half marathon (21.0975 km) is one of the four target distances the predictor outputs alongside 5K, 10K, and the marathon. Enter any one recent race result and it projects your half marathon finish using Riegel's T2 = T1 × (D2/D1)^1.06. Predictions are most dependable between nearby distances, so a 10K result gives a more reliable half marathon estimate than a 5K.
What formula is used?
Riegel's endurance model (1977): T2 = T1 × (D2/D1)^1.06. The 1.06 exponent captures the non-linear fatigue increase with longer distances — you slow down proportionally more as distance increases.
How accurate are these predictions?
Closest for well-trained runners predicting between nearby distances. Accuracy drops for first-time marathoners extrapolating from a 5K (recreational runners often run the marathon slower than Riegel predicts), and for athletes who are much stronger at one distance than another.
Why does my predicted marathon time seem too slow?
Riegel's formula assumes consistent aerobic fitness across all distances. If you are undertrained for the marathon (short long runs, not enough weekly volume), your actual time will be slower than predicted. The formula models potential, not current race readiness.
Can I use a training run instead of a race?
A hard, evenly paced time trial works. An ordinary training run does not: it was not a maximal effort, so every prediction comes out too slow. Race results give the most accurate predictions because they represent your best effort over the distance.
Is the tool free and private?
Yes. All calculations are client-side. No data leaves your browser.
The formula, worked through
- Formula Riegel Race Time Formula
Riegel race time prediction: T2 = T1 × (D2/D1)^1.06. Race-time prediction across distances. Valid 5K–marathon, breaks down beyond 30K.
- Formula Running Pace Formula: Riegel & VDOT Race Pace Conversion
Running pace formula: pace = time / distance, plus Riegel T2 = T1 x (D2/D1)^1.06 for race-equivalent paces. Worked 10K-to-half-marathon example.
- Formula Marathon Pace Elevation Formula
Marathon elevation pace correction: about 1.5 sec of finish time per metre climbed, 0.75 sec per metre descended (2:1 asymmetric). Daniels rule of thumb, Minetti 2002 consistent.
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Background on the Race Time Predictor
- Guide How to Train for a 5K
Train for a 5K with this expert guide. Learn to build endurance, boost speed, and prevent injuries for your best race day performance.
- Checklist Marathon Training Checklist: 16–20 Weeks to Race Day
A marathon training checklist from base to race day: a 16 to 20 week plan, long runs in race kit, fuel every 30 to 45 minutes, and a 2 to 3 week taper.
- Formula Race-Week Taper Formula + Carb Loading (Burke)
Race-week taper math: cut training volume 41-60% over 8-14 days while holding intensity, then carb-load 7-12 g/kg (Burke 2011) for a ~3% faster race.
- Formula Run Training Paces from a Race Time (Daniels VDOT)
Run training paces from a recent race time: get your easy, marathon, threshold, interval, and repetition paces using Daniels' VDOT model.
- Formula Treadmill to Outdoor Pace Conversion Formula
Treadmill grade adjustment: 0% treadmill = outdoor pace + 12 sec/km (no wind resistance). +1% grade adds ~3.3% energy cost (Jones & Doust 1996). 1-2% grade simulates outdoor effort.
- Formula VO₂ Max Formula (Cooper Test)
Cooper 12-min run: VO₂max = (distance_m − 504.9) / 44.73. Other estimates: Rockport walk, 1.5-mile run, Storer cycle ergometer.
Articles
- 11 min read Endurance Training: The Evidence-Based Levers
Endurance training, ranked by evidence: intensity distribution, easy-day pace, VO2 max, and the taper, each linked to its deep dive on the science.
- 12 min read VO2 Max and Longevity: The 122,000-Patient Mortality Study
VO2 max sits at the center of the 2026 longevity debate: what the 122,007-patient Mandsager 2018 mortality study found, and why fitness has no ceiling.
- 7 min read Strava + Runna Bundle vs TrainingPeaks (2026)
Strava + Runna bundle ($149.99/yr) vs TrainingPeaks Premium ($134.99/yr) in 2026: adaptive run coaching plus Claude AI versus structured-plan load tracking.
By goal
Use it from code
The same calculation as a JavaScript module you can import. It runs where you import it, with no request, key or rate limit.
import { compute } from "https://aifithub.io/engines/race-time-predictor.js";Input and output contract, TypeScript types and the guide for agents.