Enter a recent race result and get predicted finish times for the 5K, 10K, half marathon, and marathon — with the pace you'd need to hit each one. Based on the Riegel formula used by coaches for decades.
Recent race
Distance
Predicted time
Pace /mi
Pace /km
5Kyour result
25:00
8:03
5:00
10K
52:07
8:23
5:13
Half Marathon
1:55:00
8:46
5:27
Marathon
3:59:47
9:09
5:41
How race time prediction works
The predictor uses Pete Riegel's formula, published in 1977 and still the standard for race equivalency: T₂ = T₁ × (D₂ ÷ D₁)^1.06. In plain English: your pace slows by a predictable amount (about 6% per doubling of distance) as races get longer. A runner who holds 8:00 per mile for a 5K won't hold it for a marathon — but the formula tells you what they can hold.
Getting the most reliable prediction
Use a recent, all-out effort. A tempo run or training PR underestimates your fitness; a race from last year may overestimate it.
Predict adjacent distances. 5K → 10K and 10K → half marathon are very reliable. 5K → marathon stretches the formula the furthest.
Respect the training caveat.The prediction assumes you've trained appropriately for the target distance — for the marathon that means consistent 16–20 mile long runs and practiced fueling.
Adjust for conditions. Heat, hills, and wind all add time. On a hot day, add 1–3% to the prediction.
From prediction to race plan
Once you have a realistic goal time, turn it into an actionable plan: use the running pace calculator to get your goal pace and even splits, and keep your easy days genuinely easy with the heart rate zone calculator. The biggest predictor mistake isn't the math — it's racing the first half faster than the fitness the formula measured.
Frequently asked questions
How accurate is the Riegel race predictor?
For distances up to the half marathon, Riegel predictions are typically within 2–3% for trained runners. Marathon predictions assume you've done the long-run volume — without marathon-specific training, expect your real time to be slower than predicted, sometimes by 10 minutes or more.
What 5K time predicts a 4 hour marathon?
Roughly a 24:30–25:00 5K predicts a 4-hour marathon using the Riegel formula, assuming proper marathon training. Enter your exact 5K time above to see your personal prediction.
Why is my marathon slower than the prediction?
The formula assumes equivalent training for both distances. Marathons punish missing endurance more than short races: insufficient long runs, poor fueling, or hot weather can add 10–30 minutes. Treat the prediction as a best-case target.
What result should I use as the input?
Use your most recent all-out race, ideally within the last 6–8 weeks and closest in distance to your goal race. A fresh 10K predicts a half marathon better than a year-old 5K does.