Race Time Predictor

This tool uses Riegel's Formula, a widely used method for predicting endurance performance across distances, published by exercise physiologist Peter Riegel in 1977. Below is exactly how the math works, a worked example you can check by hand, and where a prediction like this stops being reliable.

How the Calculation Works

This tool takes one recent finish time and projects what an equivalent effort would produce at a different distance, using Riegel's Formula:

T2 = T1 × (D2 ÷ D1)^1.06

T1 and D1 are the time and distance you enter; D2 is each target distance (5K, 10K, half marathon, marathon); and 1.06 is the fatigue exponent this calculator uses — a number greater than 1 that reflects the fact that pace naturally slows as distance increases, even for a well-trained, evenly-paced effort.

Worked Example

Enter a 25:00 5K (1,500 seconds) and the calculator runs the formula three more times, once per target distance:

• 10K: 1,500 × (10 ÷ 5)^1.06 = 1,500 × 2.085 = 3,127 seconds → 52:07
• Half Marathon: 1,500 × (21.0975 ÷ 5)^1.06 = 1,500 × 4.600 = 6,900 seconds → 1:55:00
• Marathon: 1,500 × (42.195 ÷ 5)^1.06 = 1,500 × 9.591 = 14,387 seconds → 3:59:47

Notice the ratio between distances (2×, 4.2×, 8.4×) doesn't translate into an identical multiplier on time — because of the 1.06 exponent, doubling the distance more than doubles the predicted time, which is the formula's way of building in expected fade over longer efforts. If a 25-minute 5K sounds like your current fitness, our 5K times by age page shows how that stacks up against other runners your age before you commit to a training block chasing one of these predictions.

What the Predictions Mean

A Riegel prediction is best read as "here's roughly what this same level of fitness is worth at a different distance," not as a guarantee. It assumes you'd train specifically for the target distance and execute a similarly disciplined pacing strategy — going out too fast is the single most common reason predicted times don't hold up on race day. If you want to build a pacing plan around one of these numbers rather than just eyeball it, our pace calculator converts any of the predicted finish times above into per-kilometer or per-mile splits.

Accuracy Over Distance

The 1.06 exponent was fit to model the general relationship between distance and record performance, and it holds up best when the two distances involved are relatively close together — a 5K predicting a 10K, for instance. Extrapolating a 5K time all the way out to a marathon asks the formula to model months of marathon-specific endurance, fueling, and pacing discipline using nothing but a single short race — factors Riegel's formula has no way to see. Runners preparing for that jump often build up through a structured plan first; our 10K training plan for beginners is a common starting point before attempting a half or full marathon.

Limitations

A few things this predictor cannot know from a single time and distance: your current training volume, how well-rested you were for the input race, hills or heat on the input course, and whether you have the specific endurance to hold marathon pace for hours even if your 5K speed says you could. Treat longer-distance predictions, especially the marathon figure, as a rough ceiling rather than a target time to write on your bib. Comparing your predicted times against typical results for your age group — see our average marathon time and half marathon time by age pages — can help you judge whether a prediction looks realistic before you build a training cycle around it.

Frequently Asked Questions

How accurate is a race time predictor?

Riegel's formula is a mathematical model based on how endurance performance typically scales with distance, not a guarantee for any individual race. It tends to be most accurate between distances that are close together (5K to 10K) and least accurate stretching a short race out to the marathon, where fueling and specific endurance matter far more than raw speed.

What formula does this race time predictor use?

Riegel's Formula: T2 = T1 × (D2 ÷ D1)^1.06, where T1 and D1 are your entered time and distance, D2 is the target distance, and 1.06 is the fatigue exponent, first proposed by exercise physiologist Peter Riegel in 1977.

Can I predict a marathon time from a 5K?

Yes, this calculator will generate that prediction, but treat it as an optimistic ceiling rather than an expected outcome — a 5K only tests a few minutes of speed, while a marathon demands hours of fueling, pacing, and endurance the formula can't measure from one short race.

Why does my predicted marathon time seem too fast?

Riegel's formula assumes you'd train specifically for the marathon and pace it perfectly. Runners who are fast over 5K but haven't built marathon-specific mileage typically run slower than the raw prediction, because endurance and fueling become the limiting factor long before speed does.

What is a good half marathon time based on a recent 10K?

Enter your 10K time and distance (10 km) into this calculator and it computes a half marathon prediction using the same T2 = T1 × (D2/D1)^1.06 formula — this distance pair (10K to half marathon) tends to be one of the more reliable predictions since the distances are relatively close.

Does this calculator account for hills or weather?

No — it works purely from the time and distance you enter, with no way to know the course profile or conditions of that race. A hilly or hot input race will understate your flat-course potential, and vice versa.

What is Riegel's formula exponent and why is it 1.06?

The exponent (1.06 in this calculator) models how much pace slows as distance increases for a well-executed effort. A value around 1.06 is the commonly cited figure from Peter Riegel's original research on endurance performance across many distances and sports.

How do I use my predicted time to train for a race?

Once you have a target time, converting it to a pace per kilometer or per mile makes it usable on training runs, and a structured plan helps you build toward it rather than just hoping the prediction holds on race day.