Race Time Predictor

By RECATOOLS Editorial Updated 11 Jun 2026

About this tool

Riegel formula across distances

Predict race times across distances using the Riegel formula. Enter your known race, get predictions for 1 mile, 5K, 10K, half, marathon, 50K, 100K. Used by Strava + Garmin.

RT-HLT-012 · Health & Fitness

Known race distance (km)

Known race time (h : m : s)

: :

Fatigue exponent

Distance	Predicted Time	Pace
Enter your known race time + distance above

Enter a known race to predict times at other distances

How to use the Race Time Predictor

Enter a recent race result

Use your best race time from the last 6 months. Ideal source distances: 5K, 10K, or half marathon — these are the most accurate baselines. Time-trial efforts on a track work too, but should be true-max effort with reasonable pacing.

Read the predicted times across distances

The table shows predicted times for 1 mile, 5K, 10K, 15K, half marathon, marathon, 50K, 100K. Your input row is highlighted. Use predictions for race-day pacing strategy + goal-setting. Caveat: predictions assume similar training base + race-day effort. A 5K-trained runner won\'t hit Riegel-predicted marathon time without endurance training.

Adjust the fatigue exponent if needed

Default 1.06 is the original Riegel value, accurate for trained runners. Try: 1.07-1.08 for less-trained runners or first-time marathoners (more slowdown at distance); 1.05 for elite-trained runners (less slowdown); 1.10 for tropical heat conditions or first marathon ever.

Use predictions for pacing + planning

The pace column tells you per-km pace for each distance. Use this to: (1) plan race-day pacing strategy; (2) calibrate training pace targets (Daniels-style training paces tied to race performance); (3) set realistic goal times for upcoming races. Predicted time + 5-10% buffer is a sensible first-attempt goal for any new distance.

Riegel formula — how Strava + Garmin predict your race times

Pete Riegel\'s 1981 paper "Athletic Records and Human Endurance" introduced the formula T₂ = T₁ × (D₂ / D₁)^1.06 for predicting race times across distances. The exponent 1.06 represents the "fatigue cost" of running further — a 10K takes ~2.07× your 5K time (not 2×) because endurance fatigue accumulates. Riegel validated the formula empirically against thousands of race results from amateur to elite athletes. Forty-plus years later, the formula remains the most widely-used race-prediction tool: Strava, Garmin Connect, COROS, Polar, TrainingPeaks, and most coaching software embed Riegel or a variant. This calculator implements the classic version with an adjustable exponent for different runner types + conditions.

Why the exponent 1.06 works

If running were purely linear (no fatigue), the exponent would be 1.0 — a 10K would take exactly 2× your 5K time. Reality: longer races involve more aerobic vs anaerobic energy, more accumulated fatigue, more pacing challenges, more mental load. The 1.06 exponent is the empirical best-fit across data: 5K → 10K ratio is 2^1.06 ≈ 2.085 (instead of 2.0); 10K → marathon ratio is 4.2^1.06 ≈ 4.54 (instead of 4.2); half → marathon ratio is 2^1.06 ≈ 2.085 (instead of 2.0). For most trained amateur runners, these ratios match real-race performance within 3-5%. Elite athletes often run sub-1.06 (smaller slowdown) because they\'ve trained the endurance ceiling higher; first-time marathoners often run super-1.06 (bigger slowdown) due to wall + pacing inexperience.

Riegel's formula predicts within 3-5% for trained runners. It's used inside Strava, Garmin Connect, COROS, Polar, TrainingPeaks — basically every endurance platform.

Where Riegel breaks down

The formula has known limitations. (1) Distance ratio beyond 3×: predicting marathon time from a 5K is less accurate because the energy systems differ too much (5K is heavily aerobic-glycolytic; marathon is primarily aerobic-oxidative). (2) Untrained athletes: someone who can do a 22-min 5K but has never run more than 10K will likely run substantially slower than Riegel predicts for marathon — they lack endurance base. (3) Heat + humidity: tropical conditions penalise longer races more than shorter (heat-stress accumulates over time). ASEAN-marathon Riegel predictions should bump the exponent to 1.07-1.08. (4) Trail + hills: Riegel assumes flat road; trail races + significant elevation gain require different prediction models. (5) Ultra distances: 50K+ ultras involve nutrition, sleep, terrain — Riegel underestimates how slow these can get. (6) Elites and recreational runners differ: elites typically run sub-1.06; recreational runners often super-1.06. Adjust the exponent if you know which camp you\'re in.

The ASEAN race-time reality

Tropical race conditions in ASEAN markets significantly shift race-time predictions. Singapore Marathon (December, ~25-28°C, 80%+ humidity), KL Marathon (September, 28-32°C), Bangkok Marathon (November, 26-30°C), Jakarta Marathon (October, 28-32°C) all run measurably slower than equivalent-effort cool-condition races. Heat penalty estimates: ~3-5% time loss at 25°C vs 15°C ideal; 7-12% at 30°C; 12-20%+ at 32-35°C. For Riegel predictions: use exponent 1.07-1.08 for ASEAN long-distance races; goal times should bake in the heat penalty. Early-morning starts (3-5am) help significantly — Singapore Marathon and most regional events use early starts for this reason. Trail + altitude: ASEAN trail races (Cameron Highlands, Da Lat, Bali) at altitude or with elevation gain require additional adjustments — Riegel doesn\'t capture climb effects. For long ASEAN ultras, factor in nutrition + heat strategy heavily.

10 Things to Know About Race Prediction

Riegel formula (1981): T₂ = T₁ × (D₂/D₁)^1.06. The most-used race prediction formula in endurance sports.

Used inside Strava, Garmin, COROS, Polar, TrainingPeaks. Standard tool across the endurance platform ecosystem.

Accuracy: ±3-5% for trained runners within 3× distance ratio (5K→10K, half→marathon).

5K → 10K = ~2.07× (not 2× — endurance fatigue accumulates).

Half → Marathon = ~2.10×. Most amateurs add 5-15% buffer due to "the wall" at mile 18-22.

Exponent 1.05 for elites (less slowdown); 1.08-1.10 for first-time marathoners (more slowdown).

Heat + humidity add ~3-5%/5°C above 20°C. ASEAN marathon predictions need exponent 1.07-1.08.

Predicting marathon from a 5K is less accurate than predicting from half marathon — energy systems differ.

Formula doesn\'t capture elevation, trail, ultra-specific challenges. Use only for road races.

Other prediction models exist (Cameron, Vickers + Vertosick) but Riegel remains the simplest + most widely-validated.

Frequently Asked Questions

Within 3-5% for trained runners predicting across distances within a 3× ratio (e.g., 5K predicting 10K, half predicting marathon). Less accurate at the extremes: predicting marathon from 5K can be off by 10-15% if you don\'t have endurance base; predicting 5K from marathon often underestimates short-distance speed by 5-10%. The formula assumes consistent training across distances — gaps in your training will show up as bigger prediction errors.
Common causes: (1) The wall: first-time + under-prepared marathoners hit glycogen depletion around mile 18-22 and slow dramatically. Riegel doesn\'t model the wall. (2) Pacing errors: starting too fast based on training paces, blowing up later. (3) Heat: 25°C+ races cost 3-10% vs cool conditions. (4) Insufficient long runs: marathon prediction assumes 16-20mi long runs in training; if your longest run was 14mi, expect to run slower than Riegel predicts. (5) Fueling: under-fueled marathons hit the wall earlier. Re-run with exponent 1.08 for realistic first-marathon expectations.
For predictions, 10K is usually best — it\'s long enough to reflect endurance + short enough to be a true effort. 5K predictions for marathon often overestimate (you\'re using a heavily anaerobic baseline to predict an aerobic event). Half-marathon time is the gold standard for marathon prediction — almost the same energy system, just twice the distance. Use 5K only when you don\'t have a recent 10K or half-marathon result.
Riegel doesn\'t handle them well. Trail races involve technical terrain, elevation gain, and pacing strategies that differ from road. Ultras (50K+) involve nutrition, sleep, fatigue management — all variables Riegel ignores. For ultras, use ultra-specific calculators or estimate as: flat-road Riegel prediction × 1.15-1.30 depending on elevation gain. For 100K + 100mi: prediction is highly individual — depends on nutrition + pacing + experience. Use only as a rough planning baseline.
Yes for half marathons + marathons. Tropical heat (25-32°C) penalises long races more than short. Suggested exponents: cool (under 18°C) — use 1.05-1.06; temperate (18-24°C) — use 1.06; warm (24-28°C) — use 1.06-1.07; hot (28°C+) — use 1.07-1.08. Singapore Marathon, KL Marathon, Bangkok Marathon, Jakarta Marathon all benefit from heat-adjusted predictions. The short-distance races (5K, 10K) are less affected — exponent stays close to 1.06 even in heat.
Riegel assumes your training base supports the predicted distance. A fast 5K runner without long-run training won\'t hit predicted marathon time. Calibration check: do your weekly long runs match what marathon training requires (16-20mi peak)? Does your weekly mileage match the goal distance (40-60mi for sub-3:30 marathon)? If training doesn\'t match the distance, increase the exponent to 1.07-1.08 for realistic predictions.
Several exist with marginal accuracy gains. Cameron formula: uses different exponent depending on race time; slightly better for sub-elite athletes. Vickers + Vertosick (Slate Runners): more sophisticated regression model published 2016; slightly better accuracy across distance range. Jack Daniels VDOT: alternative system mapping race times to a "VDOT" number, then to predicted paces across distances. All are within ±1-2% of Riegel for most amateur runners; Riegel\'s simplicity + 40-year validation keep it dominant.
Not directly — cycling has different fatigue dynamics (more aerodynamic drag, less impact load, drafting effects in groups). For cycling, use power-based prediction: FTP (Functional Threshold Power) and intensity-duration curves. Try our Cycling FTP Calculator (RT-HLT-015) for cycling-specific metrics.
No. All calculations run in your browser via JavaScript. Open DevTools → Network and confirm zero outbound requests. Race time + distance stay on your device. Safe for personal training logs + race planning.
Pair with: VO2 Max Calculator (RT-HLT-011) for fitness-level baseline; Running Pace Calculator (RT-HLT-013) for goal-pace splits; Sweat Rate Calculator (RT-HLT-014) for race-day hydration; Heart Rate Calculator (RT-HLT-009) for training zones. External: Daniels Running Formula book; McMillan Running Calculator (paid version with more features); Jack Daniels VDOT tables for training-pace prescription.

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Race Time Predictor