Free Race Time Predictor
Predict your finish time for any race distance using the Riegel formula.
What is a race time predictor?
A race time predictor is a free online tool that helps you predict your finish time for any race distance based on a recent training result using Riegel's formula. It runs entirely in your browser — no signup, no app, and no data is ever sent to a server.
A race time predictor estimates your finishing time for any running distance based on a recent performance at a different distance. It uses the Riegel formula, which is the most widely used and validated prediction method for running performances from 1 mile to marathon. Rather than guessing what you can run for a half-marathon based on your 5K time, the predictor gives a data-backed target based on established endurance science and thousands of race results.
How it works
The Riegel formula: T₂ = T₁ × (D₂/D₁)^1.06. The exponent 1.06 means running twice the distance takes about 2.08× the time — the extra 0.06 accounts for fatigue. The formula converts your reference time to seconds, applies the factor, and converts back. Pace per mile or km is also calculated from the predicted time.
When to use a race time predictor
Setting goal times for upcoming races. Planning pacing strategy for a longer event. Gauging fitness improvement by comparing equivalent performances over time. Choosing which distance to train for based on where you're strongest. Estimating marathon potential from a recent half-marathon or 10K result. The predictor is a staple tool for every serious runner during race season planning.
Common mistakes with race time prediction
The biggest mistake runners make is treating the predicted time as a guarantee rather than a target range. The Riegel formula is accurate to about ±5% for well-trained runners, but that 5% on a 3:30 marathon is over 10 minutes. I tell runners to use the prediction as a pacing guide, not a finish line promise. Weather, elevation gain, and race-day conditions can easily add 5-10% to your time. Aim for the predicted pace and adjust in the first few miles based on how you feel.
Another common error is using a PR from years ago as the reference time. Fitness changes significantly over 6-12 months. A 5K time from two seasons ago does not reflect your current marathon potential. The predictor should use a race result from the last 6-8 weeks for the most accurate projection. If you don't have a recent race, use a recent time trial or a hard training run of at least 5K.
I also see runners predicting from a distance that's too different from the target. Riegel is most accurate within about 3:1 distance ratio — predicting a half-marathon from a 5K is reasonable, but predicting a marathon from a mile introduces more error because the mile relies more on anaerobic capacity. For the best accuracy, use a reference race that's within 2-3 times your target distance or use a half-marathon time for marathon prediction.
How to use this calculator
- Enter your reference time — Input a recent race time for any distance.
- Enter the distance — Select or enter the distance of your reference race.
- Select your target distance — Choose the distance you want to predict a time for.
Frequently asked questions
How does the Riegel race time prediction formula work?
Riegel's formula: T₂ = T₁ × (D₂/D₁)^1.06. If you run a 5K in 22:00, predicted marathon time = 22:00 × (42.195/5)^1.06 = 3:24:46. The exponent 1.06 accounts for fatigue over longer distances — doubling the distance takes more than double the time. The formula is most accurate for distances from 1 mile to marathon.
How accurate is race time prediction?
Riegel's formula is about ±5% for well-trained runners. It's most accurate when the prediction distance isn't too far from the input distance (predicting marathon from a half-marathon is more accurate than from a mile). It tends to over-predict for beginners and under-predict for elite athletes. Weather, elevation, and course difficulty can add ±10%.
What is a good equivalent performance?
Equivalent performances are times across different distances that require the same fitness level. For a 40-minute 10K: 5K = 19:14, half-marathon = 1:28:24, marathon = 3:04:06. The calculator shows equivalent times for all standard race distances so you can set realistic goals across distances.
How do I predict my 5K time from a 10K?
A 10K in 50:00 using Riegel: T₂ = 50:00 × (5/10)^1.06 = 50:00 × 0.482 = 24:06. Running a shorter distance from a longer one gives the reciprocal of the Riegel factor. The predictor handles both directions — longer from shorter and shorter from longer.
What factors affect race time prediction accuracy?
Training specificity (a miler's 5K prediction is less accurate than a marathoner's). Aerobic vs. anaerobic fitness balance. Weather conditions: heat (+5-10% slower), altitude (+3-5% per 1000m). Course profile: hilly courses (+5-15% slower). Pacing strategy: even splits are optimal. Surface: trail vs. road vs. track.
Should I use chip time or gun time for the reference?
Always use chip time (net time) for both the reference and predicted race. Gun time includes the time spent crossing the starting line, which can add 30-90 seconds in large races. Using gun time inflates your reference and produces a slower predicted time. The predictor assumes net time for the most accurate result.