A race time predictor answers one of the most common questions in running: if I can run this distance in this time, what could I run over another distance? Coaches and runners use the answer to set realistic goals, choose a target race, and pick training paces. This free tool uses Riegel’s formula, a well-known method that has predicted race times for decades. Enter one real race result, and it estimates your times for 5K, 10K, half marathon, and marathon in seconds.
Riegel’s formula works on a simple idea: as distance doubles, your sustainable pace gets a little slower, by a predictable amount. The formula takes your known time, multiplies it by the ratio of the new distance to the old distance, and raises that ratio to the power of 1.06. That exponent captures how endurance fades with distance for a typical trained runner. It is not magic, and it is not perfect, but across thousands of runners it lands close enough to plan around.
Here is a worked example. A runner with a 20-minute 5K wants a 10K prediction. Doubling the distance gives a ratio of 2, and 2 raised to 1.06 is about 2.085. Multiplying 20 minutes by 2.085 gives about 41 minutes and 42 seconds for 10K. Notice the pace per kilometre is slightly slower than the 5K pace, which matches real racing: you cannot hold 5K speed for twice the distance. The tool does this arithmetic for all four distances at once.
Predictions are most reliable when the two distances are close together. A 10K result predicts a half marathon far better than a 5K result predicts a marathon, because the marathon adds fuel, hydration, and durability demands that short races never test. If your only result is a 5K and you want a marathon prediction, treat it as an optimistic ceiling rather than a promise. A half marathon result is the best single predictor most runners have for the marathon.
Use the predictions to set training paces, not just race goals. Tempo runs, long runs, and interval sessions all work best when they are tied to a realistic current fitness level rather than a dream time. If the predictor says your half marathon pace is 5 minutes per kilometre, build your long runs and tempo sessions around that reality. Training at paces from a fantasy goal only produces exhaustion, while training at honest paces produces steady improvement.
Course and conditions change real times, and the formula cannot see them. A hilly course, strong wind, heat, or a poorly paced race all make your known result slower than your true ability, which makes every prediction slower too. Likewise, a fast downhill course flatters your result and makes predictions optimistic. For the best predictions, use a result from a flat course in good conditions where you paced evenly and finished strong.
Recheck your predictions after every race, because fitness changes through a season. A spring 10K and an autumn 10K can easily differ by minutes, and each new result refreshes all four predictions. Keep a simple log of your race results and the predictions they produced, and watch how the gap between prediction and reality shrinks as your training becomes more specific. That shrinking gap is itself a sign of improving endurance.
Do not let a prediction bully you into a race plan you are not ready for. If the tool says you could run a marathon in 3 hours 30 minutes but your longest run is 20 kilometres, the missing long runs matter more than the formula. Predictions assume you have trained for the distance, including long runs, fuelling practice, and race-day pacing. Use the number to choose a goal, then build the training that earns it.
This tool gives an educational estimate, not a guarantee and not coaching advice. Race times depend on training, health, weather, course, and race-day decisions, and no formula can capture all of that. Treat the output as a planning aid, discuss ambitious goals with a coach if you have one, and stop and seek medical advice if training or racing ever makes you feel unwell. Numbers guide you, but your body has the final say.
Try the predictor with your best recent result and see what it says about your next target. Compare its answer with how your training feels, pick the distance that excites you, and set a goal that stretches you without breaking you. A realistic target, honestly earned through training, beats an optimistic one every time. Enter your race, read your predictions, and plan your season from evidence.
Predictor de Tiempo de Carrera – Para Deportistas y Entrenadores
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