The marathon is the race most runners dream about, and the question almost every marathon hopeful asks is simple: what time could I actually run? This free predictor estimates your marathon finish time from a recent half marathon or 10K result, using the same Riegel formula coaches have used for decades. Enter your best recent race, and the tool gives your predicted marathon time plus your average pace per kilometre and per mile, so you can judge the goal honestly.
The method is straightforward. Your known time is scaled up to marathon distance by the ratio of the distances, raised to the power of 1.06, which accounts for the pace slowing that happens as races get longer. A half marathon result predicts the marathon much better than a 10K result, because the half already tests endurance, fuelling, and pacing over a long effort. If you only have a 10K result, treat the prediction as a best-case figure and expect reality to be slower until you build long-run endurance.
Here is a worked example. A runner with a 1 hour 30 minute half marathon wants a marathon prediction. The distance ratio is 2, and 2 raised to 1.06 is about 2.085, so the predicted marathon time is roughly 3 hours 7 minutes and 40 seconds. That is an average pace of about 4 minutes 26 seconds per kilometre, or about 7 minutes 8 seconds per mile. Notice how the pace is slower than the half marathon pace of 4 minutes 16 seconds per kilometre, which is exactly what longer racing demands.
Your longest training run matters more than any formula. Predictions assume you have trained for the marathon distance, with regular long runs, practice eating and drinking on the move, and at least one run beyond 30 kilometres. If your longest run is only 20 kilometres, no prediction will save the last 12 kilometres of the race. Build the long runs first, then let the predictor refine your goal, not replace your preparation.
Pacing is where marathon dreams usually fail, and the predicted pace is your best defence. Start at the predicted pace, not faster, even when the early miles feel easy and the crowd carries you along. Runners who start 10 seconds per kilometre too fast often pay it back three times over in the final 10 kilometres. Write your target pace on your hand or your watch, check it every few kilometres, and treat even pacing as a skill you practise in training.
Course profile and weather shift real marathon times by minutes, and the formula cannot know about them. A hilly course costs time, heat costs more, and a strong headwind on an exposed course costs the most. Adjust your goal for the course you actually entered: add time for hills and heat, and keep the prediction as your flat-course, good-weather figure. Many runners set two goals, one for perfect conditions and one for the day they actually get.
Use the prediction to plan your training paces across the whole build-up. Long runs should be slower than marathon pace, tempo runs around half marathon to marathon pace, and easy runs much slower than both. If your predicted marathon pace is 5 minutes 30 seconds per kilometre, easy runs near 6 minutes 30 seconds are not lazy, they are correct. Training too fast is the most common self-inflicted injury risk in marathon preparation.
Recheck the prediction after each tune-up race, because fitness changes over a marathon build. A half marathon eight weeks out gives a useful early estimate, and another one four weeks out sharpens it. If the two predictions differ, trust the later one and adjust your goal rather than clinging to the first number. Honest goals, updated with fresh evidence, lead to better races than old goals defended out of pride.
This predictor gives an educational estimate only, not a promise and not medical advice. Marathon training is demanding, and you should build up gradually, rest when injured, and seek medical advice before starting if you have any health concerns. If you feel chest pain, dizziness, or severe illness during training or racing, stop and get help immediately. A finish time is never worth a health risk.
Enter your best recent half marathon or 10K, read your predicted marathon time, and decide whether the goal excites you and your training supports it. If it does, build your plan around that pace and enjoy the journey to the start line. If it does not, pick a nearer goal, train well, and let the marathon wait for the season when you are ready. The marathon rewards patience more than ambition.
Predictor de Tiempo de Maratón – Solo Estimación Educativa