How does the Riegel formula work?

The Riegel formula predicts race times using the equation T2 = T1 x (D2/D1)^1.06, where T1 is your known race time, D1 is the known distance, and D2 is the target distance. The 1.06 exponent accounts for the fact that pace slows as distance increases.

How accurate is the marathon time prediction?

The Riegel formula is most accurate when predicting from similar distances. A half marathon result gives a better marathon prediction than a 5K result. Most runners find the prediction accurate within 5-10 minutes, though training, nutrition, and race-day conditions all affect actual performance.

What is a good marathon time for a beginner?

For a first-time marathon runner in the UK, finishing in under 5 hours is a solid achievement. The average London Marathon finish time is around 4 hours 30 minutes. parkrun 5K results are a great starting point for predicting your marathon potential.

What are easy and tempo training paces?

Easy pace is your marathon pace plus about 60 seconds per km, used for recovery and long runs. Tempo pace is your marathon pace minus about 15 seconds per km, used for threshold training. These paces help structure your training plan for marathon preparation.

Should I use my parkrun time to predict my marathon?

A parkrun 5K time can give a rough marathon estimate, but predictions from longer races (10K or half marathon) tend to be more accurate. If your only recent race is a parkrun, use it as a starting point but expect the actual marathon to be slightly slower than predicted.

Marathon Time Calculator

Predict your marathon finish time from a known race time using the Riegel formula

Published by UltraCalculatorsLast reviewed: 29 Mar 2026v1.0Sources:Runner's World UK parkrun UK NHS Couch to 5KEditorial PolicyEstimates only. Always consult a qualified professional.Report an issue

This calculator provides estimates only. Consult your GP or a healthcare professional for personalised advice.

This marathon time calculator predicts your marathon finish time from a known race result using the Riegel formula, the most widely used endurance performance prediction model. Whether you have a recent parkrun 5K, a 10K time, or a half marathon result, this calculator estimates what you could achieve over the full 42.195 km marathon distance. The Riegel formula was developed by researcher Peter Riegel and published in 1977. It accounts for the fact that pace naturally slows as race distance increases, using an exponential factor of 1.06. The formula has been validated across decades of race data and remains the standard prediction tool used by coaches and runners worldwide. Marathon running has grown enormously in the UK, with events such as the London Marathon, Edinburgh Marathon, and Manchester Marathon attracting tens of thousands of runners each year. parkrun, with over 700 locations across the UK, provides an accessible entry point for runners looking to benchmark their fitness. The NHS Couch to 5K programme has helped millions take their first steps toward running. This calculator also provides training paces to help you prepare for your target marathon time. The easy pace is suitable for recovery runs and long training runs, while the tempo pace is designed for threshold sessions that build endurance and speed.

How to Use This Calculator

To predict your marathon time: 1. Select the distance of a recent race you have completed. Choose from 5K, 10K, Half Marathon, or enter a custom distance in kilometres. 2. Enter your finishing time for that race. Use the hours, minutes, and seconds fields. For example, a 10K time of 50 minutes would be 0 hours, 50 minutes, 0 seconds. 3. View your predicted marathon time. The calculator will show your estimated finish time in H:MM:SS format, along with predicted times for all standard race distances. 4. Check your marathon pace. The pace per kilometre and pace per mile are provided for race-day planning. Compare this with your current training paces to assess readiness. 5. Use the training paces. The easy pace (marathon pace plus 60 seconds per km) is for recovery and long runs. The tempo pace (marathon pace minus 15 seconds per km) is for threshold training sessions. These paces form the foundation of most marathon training plans.

How It Works

The Riegel formula predicts race performance using the equation: T2 = T1 x (D2 / D1) ^ 1.06 Where: T1 = your known race time (in seconds) D1 = the known race distance (in km) D2 = the target race distance (in km) T2 = the predicted time for the target distance The exponent 1.06 reflects the physiological reality that runners slow down as distance increases. This is due to factors including glycogen depletion, muscle fatigue, and the shift from aerobic to anaerobic energy systems at higher intensities. For example, a 10K time of 50:00 (3,000 seconds) predicts a marathon time of: 3000 x (42.195 / 10) ^ 1.06 = approximately 13,801 seconds = 3:50:01. Marathon pace is calculated by dividing the predicted marathon time by 42.195 km. Mile pace is converted using the factor 1.60934 km per mile. Training paces are derived from marathon pace: Easy pace = marathon pace + 60 seconds/km (for aerobic base and recovery) Tempo pace = marathon pace - 15 seconds/km (for lactate threshold development) Predictions from longer races (half marathon) tend to be more accurate than those from shorter races (5K), because the physiological demands are more similar to the marathon.

Worked Examples

Example 1: 10K in 50:00

Inputs: Race distance: 10K. Time: 0 hours, 50 minutes, 0 seconds.

T1 = 3,000 seconds. D1 = 10 km. D2 = 42.195 km.
T2 = 3000 x (42.195 / 10) ^ 1.06.
T2 = 3000 x 4.2195 ^ 1.06 = 3000 x 4.6003 = 13,801 seconds.
Marathon time: 13,801 seconds = 3:50:01.
Marathon pace: 13,801 / 42.195 = 327 seconds/km = 5:27/km.
Easy pace: 5:27 + 1:00 = 6:27/km.
Tempo pace: 5:27 - 0:15 = 5:12/km.

Result: Predicted marathon time: 3:50:01. Marathon pace: 5:27/km (8:47/mile).

Example 2: Half marathon in 1:45:00

Inputs: Race distance: Half Marathon (21.0975 km). Time: 1 hour, 45 minutes, 0 seconds.

T1 = 6,300 seconds. D1 = 21.0975 km. D2 = 42.195 km.
T2 = 6300 x (42.195 / 21.0975) ^ 1.06.
T2 = 6300 x 2.0 ^ 1.06 = 6300 x 2.0848 = 13,135 seconds.
Marathon time: 13,135 seconds = 3:38:55.

Result: Predicted marathon time: 3:38:55. A half marathon result gives a more reliable marathon prediction than a 5K time.

Example 3: parkrun 5K in 25:00

Inputs: Race distance: 5K. Time: 0 hours, 25 minutes, 0 seconds.

T1 = 1,500 seconds. D1 = 5 km. D2 = 42.195 km.
T2 = 1500 x (42.195 / 5) ^ 1.06.
T2 = 1500 x 8.439 ^ 1.06 = 1500 x 9.586 = 14,387 seconds.
Marathon time: 14,387 seconds = 3:59:47.

Result: Predicted marathon time: 3:59:47. A sub-25 parkrun suggests a sub-4-hour marathon is achievable with proper training.

The Riegel formula assumes adequate training for the target distance. A fast 5K time does not guarantee a fast marathon without the necessary long-run base and endurance training. Most marathon training plans span 12-20 weeks and include progressively longer runs, with a peak long run of 32-35 km (20-22 miles). Factors that affect marathon performance beyond fitness include race-day temperature, course elevation, nutrition strategy, and pacing discipline. Hot weather can add 10-20 minutes to marathon times. Taking on fluids and energy gels at regular intervals is essential for maintaining performance beyond the 30 km mark. In the UK, the London Marathon ballot opens each October, with the race held in April. Other popular UK marathons include Edinburgh (May), Manchester (April), Brighton (April), and Loch Ness (October). Many runners use the Manchester or Brighton marathon as preparation for London. parkrun provides a free, timed 5K every Saturday morning at locations across the UK. It is an excellent way to track fitness improvements and generate a benchmark time for marathon prediction. Many runners progress from parkrun to 10K events, then to half marathons, and eventually to the full marathon distance.