Race Time Predictor

About the Race Time Predictor

You have just run a 10K in 50 minutes — so what could you run for a half marathon, or a marathon? Race time prediction helps runners set realistic goals, choose a starting pace, and decide whether a target such as a sub-4-hour marathon is within reach. The most widely used method is Riegel's formula, published by the American engineer Peter Riegel in 1977, which describes how much runners slow down as races get longer.

This race time predictor takes one recent race — 1 mile, 5K, 10K, 15K, half marathon, marathon or a custom distance — and predicts your time for a target distance, with the pace you would need. It also lists predictions for all the standard distances, and lets you adjust the fatigue exponent for your level of endurance training.

How to Use the Race Time Predictor

Choose the distance of your recent race, or Other and enter it in kilometres.

Enter your time as h:mm:ss or m:ss.

Choose the distance to predict.

Optionally, change the fatigue exponent from the standard 1.06.

Riegel's Formula

  T₂ = T₁ × (D₂ ÷ D₁)^1.06

  T₁, D₁ = time and distance of the race you ran
  T₂, D₂ = predicted time and target distance

If the exponent were exactly 1, doubling the distance would double the time — the same pace for any distance. The exponent of 1.06 adds the slowing that comes with longer races: doubling the distance multiplies the time by 2^1.06 ≈ 2.085.

Step-by-Step Example

A 10K in 50:00, predicting the marathon.

  Distance ratio:   42.195 ÷ 10 = 4.2195
  Raised to 1.06:   4.2195^1.06 = 4.6003
  Predicted time:   50:00 × 4.6003 = 230.0 min = 3:50:01
  Pace:             5:27 per km, 8:46 per mile

The same 10K predicts about 1:50:19 for a half marathon and 23:59 for a 5K.

A 5K in 25:00, predicting the 10K: 25:00 × 2^1.06 ≈ 52:07.

How Accurate Is It?

Riegel's formula works best between distances that are not too different, for runners who are equally well trained for both. Predictions from 5K or 10K up to the half marathon are usually good. Marathon predictions from short races are often optimistic, because the marathon depends heavily on endurance and fuelling: many runners finish 5 to 10 percent slower than predicted unless they have done plenty of long runs. The best guide to a marathon is a recent half marathon.

The Fatigue Exponent

Riegel's 1.06 was derived largely from well-trained athletes. Later studies found that recreational runners often fit a slightly higher exponent, around 1.07 to 1.10, because they slow more over long distances. Raising the exponent gives more cautious predictions for longer races. With 1.08, the 50-minute 10K runner's marathon prediction becomes about 3:56:44 instead of 3:50:01. Use the standard value for a first estimate, then adjust it to match your own results across several distances.

Other Prediction Methods

Riegel's formula is the simplest and most widely used, but not the only one. Coach Jack Daniels' VDOT tables estimate an aerobic fitness score from a race and read off equivalent performances at other distances; the Cameron formula uses a different curve; and some online tools blend several models or use data from thousands of runners. For most distances the methods agree within a minute or two, and all share the same limitation: they predict what your fitness could produce, not what your training for a particular distance will allow. Treat any prediction as a guide, not a promise, and update it as new race results come in.

Setting a Race Pace

Once you have a prediction, the predicted pace is a sensible target. For a marathon, many coaches advise starting a few seconds per kilometre slower than target pace for the first few kilometres, then settling in, and aiming for an even or negative split — running the second half as fast as or faster than the first. Going out too fast is the most common way to miss a marathon goal.

Improving Your Prediction

Predictions improve as you do. Short, fast training — intervals and tempo runs — lifts your 5K and 10K times, which raises every prediction. But to actually run the longer times predicted, you need the endurance to match: regular long runs, gradual increases in weekly distance, and practice with race-day fuelling. Comparing your actual results with predictions shows whether your speed or your endurance needs more work.

Understanding Your Result

The headline gives the predicted time for the target distance.

The pace line gives the pace per km and per mile for that prediction.

The all distances line lists predictions for the other standard distances.

The based on line restates the race used and the exponent.

The worth knowing line reminds you what the prediction assumes.

When Should You Use This Calculator?

Use it to set a realistic goal for your next race.

Use it to choose a target pace for a half marathon or marathon.

Use it to compare performances across different distances.

Use it to see how much improvement a new goal requires.

Common Mistakes

Predicting from a training run. Use a race or all-out time trial.

Trusting a marathon prediction from a 5K. Endurance matters more for long races.

Ignoring course and conditions. Hills, heat and wind slow you down.

Starting at the predicted pace without the training. Build endurance first.

Using an old result. Predictions are only as current as the race they come from; use one from the last few months.