How to Predict Your Race Time

To predict your race time, take a recent race result and scale it to your target distance with the Riegel formula: T2 = T1 x (D2/D1)^1.06, where T1 is your known time over distance D1, D2 is the distance you want to predict, and 1.06 is a fatigue exponent that captures how runners slow as races get longer. Enter a recent result into our race time predictor to predict your times and you'll get finish estimates across the standard distances; this guide explains where those numbers come from, how far to trust them, and how to turn a prediction into a race-day plan. It's general information for healthy runners, not medical advice.

The Riegel formula, in plain terms

In a 1981 American Scientist paper, engineer Peter Riegel showed that across running, swimming and walking, time and distance follow a consistent power law. Double the distance and you don't simply double the time — you take a little more. His equation captures that:

• T2 = T1 x (D2/D1)^1.06
• T1 = your known finish time (from a recent, honest race effort)
• D1 = the distance of that race
• D2 = the distance you want to predict
• T2 = the predicted finish time

Because the two distances appear as a ratio, the units cancel — you can use kilometres or miles as long as both distances use the same unit. Only the times need to be in consistent units, and seconds are easiest.

What the 1.06 fatigue exponent means

If running were perfectly linear, the exponent would be 1.00: twice the distance, twice the time, same pace throughout. Endurance doesn't work that way. The exponent of 1.06 says time grows slightly faster than distance, so your pace per kilometre (or per mile) gets a little slower as the race lengthens. Doubling the distance multiplies your time by 2^1.06, which is about 2.085 rather than 2.0 — roughly a 4% pace penalty each time you double up. That small, compounding surcharge is Riegel's shorthand for everything that makes longer races harder: glycogen depletion, rising core temperature, accumulating muscle fatigue and the mental cost of holding pace. The exponent doesn't explain why you slow; it's an empirical average of how much runners slow across a wide span of distances.

How reliable is a prediction?

The single most important rule: predictions are most reliable when your input distance and target distance are close, and least reliable across big jumps. Predicting a 10K from a 5K, or a half marathon from a 10K, is usually accurate to within a percent or two for a trained runner, because the physiology and pacing are similar. Predicting a marathon from a 5K is a leap across a very different event — one that lasts hours rather than minutes and is limited by fuel and endurance far more than by raw speed — so the error grows. Use a recent result over the closest distance you can when you want the tightest estimate.

The marathon caveat: 1.06 runs a touch fast

Riegel derived 1.06 largely from elite and world-record performances by athletes superbly prepared for every distance. For the average recreational runner, the classic exponent tends to predict a marathon that is slightly too fast — it assumes you'll hold form over 42.195 km (26.22 miles) about as well as a well-drilled competitive athlete does. In practice most amateurs fade more than that. A common, practical fix is to nudge the exponent up to 1.07 or 1.08 for marathon predictions, which lengthens the estimate and better matches how age-group runners actually finish. Well-trained runners with high mileage may sit closer to 1.06 or even below; newer runners often fit 1.08. The exponent is a dial you can tune to your own history — if past races show you fading, dial it up.

It assumes you're actually trained for the distance

Every race predictor shares one big assumption: that you are equally well prepared for the target distance as for the one you ran. The formula reads your current fitness from a 5K and projects it, but it cannot know whether you've done the long runs a marathon demands. A fast 5K only becomes a matching marathon time if you've built the endurance base to support it. If your longest run in training is 10 km (6.21 miles), a Riegel marathon prediction measures your speed, not a realistic finish time — treat it as a ceiling you'd need months of specific training to approach.

Pacing and fuelling decide the marathon's second half

Even a perfectly trained runner can miss a predicted marathon by going out too hard. The race is effectively decided in its second half. Start faster than goal pace and you'll burn through glycogen early, hit the wall, and finish far slower than any formula suggested. Two habits protect a prediction: even or slightly negative pacing (running the back half no slower than the front), and fuelling — taking in roughly 30-60 grams of carbohydrate per hour and drinking to thirst on efforts longer than about 90 minutes. No exponent can rescue a race you've paced or fuelled badly, so treat the prediction as the reward for executing well, not a guarantee.

The standard distances

Race predictions are usually shown across the classic road-racing ladder. Knowing the exact distances helps you read any calculator's output:

• 5K — 5 km (3.11 miles)
• 10K — 10 km (6.21 miles)
• 10 mile — 16.09 km (10 miles)
• Half marathon — 21.10 km (13.11 miles)
• Marathon — 42.195 km (26.22 miles)

A worked example: predicting from a 25:00 5K

Suppose your recent 5K is 25:00 (a 5:00 per km / 8:03 per mile pace). Plugging D1 = 5 km and T1 = 1,500 seconds into the Riegel formula gives the predictions below. The table also shows the same target distances at 1.07 and 1.08, so you can see how much the marathon estimate stretches when you tune the exponent for a recreational fade.

Predicted finish times from a 25:00 5K using the Riegel formula T2 = T1 x (D2/D1)^1.06, with 1.07 and 1.08 shown for comparison. Notice how close the 10K and half-marathon columns stay, while the marathon spreads by more than ten minutes — that spread is exactly the uncertainty a recreational runner should respect. Times rounded to the nearest second.

Read the table the right way and it tells a clear story. The 10K prediction (about 52:07) is trustworthy because it's a small jump from your 5K. The marathon prediction of 3:59:47 at the classic exponent is the optimistic, well-trained-athlete number; if you're a typical recreational runner, the 4:04-4:10 range from the 1.07-1.08 columns is a more honest planning target — and only if you've done the marathon-specific training to back it up.

Turning a prediction into a race plan

A predicted time is only useful once it becomes a pace you can hold. Take your target finish, divide by the distance, and you have your goal pace per kilometre and per mile — the number you'll actually run to. Our pace calculator does that split for you and can work backwards from a goal time to the pace you need, or forwards from a training pace to a finish time. From the 25:00-5K example, a 1:55:00 half marathon works out to about 5:27 per km (8:46 per mile), a pace you can rehearse in training long before race day.

Effort is the other half of pacing. Prediction formulas assume you're running at an appropriate, sustainable intensity for the distance — hard for a 5K, comfortably controlled for a marathon. Checking your heart rate against your training zones is a good sanity test that you're not banking time you'll pay back later; our target heart rate calculator shows the ranges that correspond to easy, threshold and hard efforts so your predicted pace and your actual effort line up.

Getting the most accurate prediction

• Use a recent race — fitness drifts, so a result from the last few weeks predicts far better than one from last year.
• Use a genuine effort — a tempo run or a race you jogged will lowball your fitness; the formula needs a real, near-maximal performance for that distance.
• Stay close — predict up or down one or two rungs of the distance ladder rather than across the whole thing.
• Tune the exponent — if your own past races show you fading over longer distances, use 1.07-1.08 for the marathon instead of 1.06.
• Match your training to the target — a prediction is a statement of potential, unlocked only by distance-specific preparation, smart pacing and fuelling.

Used this way, race-time prediction sets realistic goals, shapes your training paces, and gives you a number to chase. Just remember it's an estimate built on averages, not a promise. This article is general educational information, not medical advice; if you're new to running, returning from injury, or managing a health condition, it's worth a word with a doctor before targeting a hard race effort.

Key sources: Riegel P., "Athletic Records and Human Endurance," American Scientist 69(3):285-290 (1981), the origin of the 1.06 endurance exponent; World Health Organization Guidelines on Physical Activity and Sedentary Behaviour (2020); U.S. CDC Physical Activity Basics for Adults; NHS Physical Activity Guidelines for Adults; American College of Sports Medicine (ACSM) preparticipation health screening recommendations; International Society of Sports Nutrition (ISSN) position stand on nutrient timing and endurance carbohydrate intake; and American Heart Association (AHA) recommendations for physical activity in adults. See the full list below.