How to Predict Your Race Time: The Most Accurate Methods for Runners

You just ran a 5K in 22:30 and you want to know if you can break 4 hours in a marathon next spring. Or you are targeting a sub-1:45 half marathon and want to know what 10K time would tell you you are on track. Race time prediction does not give you a guarantee, but it gives you a starting point that is a lot better than guessing.

This post covers how race time prediction works, which methods are most accurate, and where the predictions fall apart.

Why race time prediction works at all

Running performance at different distances is related but not in a simple linear way. A runner who covers 5K in 20 minutes cannot just double the distance and expect a 40 minute 10K. They will run slower per kilometre because the body switches energy systems as distance increases, aerobic capacity becomes more limiting, and fatigue compounds differently over longer efforts.

Prediction formulas model exactly how much slower performance gets as distance increases. The relationship was first described scientifically by researcher Peter Riegel in 1977, and his formula has been the basis of race prediction ever since.

The Riegel formula

The most widely used race time predictor is the Riegel formula:

T2 = T1 × (D2 / D1) ^ 1.06

Where T1 is your known time, D1 is the known distance, T2 is the predicted time, and D2 is the target distance. The exponent 1.06 captures the fatigue factor — the mathematical relationship that describes how pace naturally slows as distance increases.

So if you ran a 5K in 25:00 and want to predict your 10K time:

T2 = 25 × (10 / 5) ^ 1.06 = 25 × 2.084 = 52:06

Our free race time predictor uses the Riegel formula and handles all the calculations automatically. Enter any recent race result and get predictions for 5K, 10K, half marathon, and marathon.

How accurate is the Riegel formula?

For well-trained runners comparing similar distances — 5K to 10K, or 10K to half marathon — predictions are typically accurate within 2 to 5 percent. That is close enough to set a meaningful goal and plan your race pacing strategy.

Accuracy drops in two key scenarios. First, when the distances being compared are very different. Predicting a marathon time from a 5K is less reliable than predicting it from a half marathon result, because the physiological demands diverge significantly. Second, when training has not been distance-specific. A runner with a strong 5K who has done no marathon-specific long runs will typically run slower than the formula predicts because their aerobic endurance is not developed for the effort required.

The VDOT approach

Jack Daniels developed a different approach using VO2max equivalence. The idea is that all your race times should correspond to the same underlying aerobic capacity, expressed as a VDOT value. You use a recent race result to find your VDOT, then use the VDOT table to find equivalent times at other distances and to set your training paces.

In practice, Riegel and VDOT give very similar race time predictions for most runners. Where VDOT has an advantage is in training prescription: Daniels assigns specific Easy, Marathon, Threshold, Interval, and Repetition paces based on your VDOT, which makes it more useful than Riegel for planning training rather than just predicting race times.

If you follow Jack Daniels' 2Q program, you can sync those workouts to your Garmin using VDOT-based paces.

The Tanda formula for marathon

For the marathon specifically, Giovanni Tanda developed a model that uses training data rather than a race result. His formula inputs your average weekly mileage and average training pace over the preceding four weeks to predict marathon finish time.

The Tanda approach is useful mid-training when you do not have a recent race result but want an honest assessment of whether your current fitness is on track. The limitation is that it requires accurate tracking of both distance and pace, and the model assumes consistent training rather than a peaking block.

Equivalent race times reference

These are approximate equivalent times calculated using the Riegel formula starting from common 5K benchmarks:

5K	10K	Half marathon	Marathon
18:00	37:22	1:23:17	2:53:39
20:00	41:29	1:32:23	3:11:57
22:00	45:37	1:41:45	3:31:10
25:00	51:51	1:55:27	3:59:27
28:00	58:05	2:09:05	4:27:40
30:00	1:02:13	2:18:36	4:47:32
35:00	1:12:35	2:41:44	5:35:44

Enter your exact time in our race predictor for a precise calculation.

Where predictions go wrong

Heat. Hot conditions slow marathon performance by 4 to 10 percent depending on temperature. A prediction based on a spring race will be optimistic for a summer marathon.

Hills. The Riegel formula assumes flat courses. A personal best on a hilly half marathon will not predict flat marathon performance the same way a flat half marathon will.

Distance-specific training. Marathon pace depends heavily on long run volume. A 5K specialist who has never run more than 16 miles in training will run significantly slower than their 5K time predicts. The formula assumes your training has been appropriate for the target distance.

Course profiles. Two runners with the same 5K time will not necessarily have the same marathon potential. A runner with a higher proportion of fast-twitch muscle fibre tends to be faster at shorter distances but slower at longer ones. The formula uses average relationships and cannot account for individual physiology.

Recent fitness. A race result from six months ago at peak fitness does not predict current capability. Use the most recent result from conditions closest to your target race.

Using predictions in training

The most useful application is working backwards from your goal. If you want to run sub-3:30 for the marathon, the Riegel formula suggests you need a half marathon time around 1:39 and a 10K around 46:00. If your recent 10K was 52:00, you know exactly how far you are from your goal and roughly how much fitness improvement you need.

Predictions also help set training paces. If your race predictor puts your marathon time at 3:45, you should not be doing marathon pace runs at 4:45 per kilometre just because that is what the goal pace chart says for 3:30. Work with where your fitness actually is.

Try the race time predictor with your most recent race result to get your current equivalent times and set realistic goals for your next race.