Marathon Time Predictor — estimate your 26.2 from a recent race

The marathon time predictor estimates your likely finish time for a full 26.2-mile marathon based on a recent 5K, 10K, or half-marathon result, using a widely cited endurance formula rather than a rough guess. Enter the distance you raced and your finish time, and the calculator projects your marathon time along with predicted times for the other common distances, plus the pace per mile your marathon prediction implies.

This tool from Arb Digital is built for runners who have raced a shorter distance recently and want a realistic, evidence-based target for marathon day — not a fantasy number, and not a guess pulled from a pace chart that ignores how your particular fitness translates across distances.

What This Marathon Time Predictor Does

You give it one known data point — a race distance and the time you actually ran it in — and it projects what that same underlying fitness would produce over other distances, including the full marathon. Behind the scenes it applies Riegel's formula, a well-established relationship between race distance and finish time that has been used by coaches and exercise scientists for decades to estimate performance at untested distances from a known result.

The output includes your predicted marathon time as the headline number, plus predicted 5K, 10K, and half-marathon times derived from the same fitness baseline, along with the exact pace per mile your marathon prediction requires — useful for planning training paces well before race day.

How to Use It

1. Select your recent race distance. Choose 5K, 10K, or half marathon — whichever you have raced most recently at genuine effort.
2. Enter your finish time. Use hours, minutes, and seconds for the time you actually ran, not a goal or estimate.
3. Predict My Marathon Time. The calculator instantly shows your projected marathon finish time, predictions for the other distances, and the marathon pace per mile implied by that time.

For the most reliable prediction, use a time from an all-out race effort rather than a comfortable training run — the formula assumes the input reflects close to your current maximum sustainable effort for that distance.

The Formula — How Riegel's Formula Works

Riegel's formula, developed by researcher Peter Riegel and published in a 1977 paper on endurance performance, states that T2 = T1 × (D2 ÷ D1)^1.06, where T1 is your known time at distance D1, and T2 is the predicted time at a new distance D2. The exponent, 1.06, captures a simple physiological reality: as race distance increases, your average pace naturally slows, and that slowdown follows a predictable curve rather than a straight line. A small exponent above 1.0 means the time penalty for going longer compounds gradually rather than linearly — which matches how endurance performance actually degrades with distance in trained athletes.

This calculator applies the same formula in both directions from whichever distance you enter, projecting times for 5K, 10K, half marathon, and full marathon all from one baseline result. The formula has held up well in sports science literature and is referenced across coaching resources and endurance research, including discussion in publications like Runner's World, precisely because it is simple, requires only one input race, and produces predictions that track closely with real-world marathon outcomes for runners who have trained appropriately for the distance.

Why the Prediction Assumes You've Done the Training

Riegel's formula is remarkably accurate for trained runners predicting from a shorter distance to a longer one they have specifically prepared for — but it has one critical assumption baked in: that you have actually built the endurance base the longer distance demands. The formula extrapolates your current pace-versus-distance curve; it does not know whether you have run a single long run over 15 miles, whether your weekly mileage supports 26.2, or whether you have practiced fueling during a multi-hour effort.

A runner who posts an outstanding 5K time on strong natural speed but has never run more than 8 miles at once will very likely miss this calculator's marathon prediction — often badly, and often specifically during the last 10K of the race, where glycogen depletion and inadequate long-run preparation cause a sharp slowdown that no formula can predict from a 5K result alone. The shorter your input race relative to the marathon, the more the prediction depends on you having done real marathon-specific training between now and race day — long runs, back-to-back training days, and practiced fueling — rather than just raw speed.

For this reason, a half-marathon input generally produces a more reliable marathon prediction than a 5K input, since the half marathon already tests some of the endurance qualities the marathon demands, while a fast 5K time reflects speed and VO2 max more than the fat-burning, glycogen-management, and muscular endurance that carry a runner through the final miles of 26.2.

How Training Between Now and Race Day Changes the Number

This prediction is a snapshot of your current fitness, not a fixed destiny. If your race day is still months away and you follow a structured marathon training block — building weekly mileage, running a progression of long runs up to 18–20 miles, and adding marathon-pace segments — your actual fitness on race day will likely be better than what today's shorter race implies, and your real result could beat this prediction. Conversely, if you stop training or reduce mileage significantly between now and the marathon, expect to finish slower than this number suggests, since the prediction assumes the endurance-specific work gets done.

Re-run this calculator with a fresh race result every few weeks during a training cycle — as your 10K or half-marathon time improves, your marathon prediction updates with it, giving you a moving target that reflects your current fitness rather than a number frozen at the start of training.

Using the Predicted Pace in Training

Once you have a marathon time prediction, the pace-per-mile figure this calculator provides becomes your most practical training tool. Rehearse that exact pace during marathon-pace segments of your long runs so it feels automatic rather than foreign on race day, and use your predicted half-marathon and 10K times as intermediate checkpoints to gauge whether your training is on track. If a tune-up race midway through training beats this calculator's prediction for that distance, your marathon prediction — and your goal pace — should be recalculated upward with the better number.

Many coaches build an entire training cycle around this kind of checkpoint racing: an early-cycle 10K or half marathon establishes a baseline prediction, a mid-cycle tune-up race confirms whether fitness is trending in the right direction, and the final few weeks are dedicated to tapering rather than chasing new data. Treat each recalculated prediction as a data point in a trend, not a single verdict — a runner whose predicted marathon time improves by even a few minutes across two or three checkpoint races over a training block is a runner whose fitness is genuinely moving in the right direction, regardless of what any single number says in isolation.

How Course and Weather Affect the Real Result

Riegel's formula predicts what your fitness can produce under reasonably neutral conditions — it does not know whether your marathon has 1,000 feet of net elevation gain, whether the forecast calls for 80-degree heat and high humidity, or whether the course is notoriously crowded at the start and costs you several minutes weaving through slower runners in the opening mile. Hilly courses, hot weather, and high humidity can each add meaningful minutes to a marathon finish time regardless of how accurate the underlying fitness prediction is, so treat this calculator's output as your fitness ceiling under fair conditions rather than a guaranteed clock time on any specific course.

Common Mistakes to Avoid

• Using a training-run time instead of a race time. Riegel's formula assumes close-to-maximal effort; a comfortable jog will produce an inflated, unrealistic prediction.
• Predicting a marathon from a 5K with no endurance base. The shorter the input race, the more the prediction depends on you having built long-run mileage before race day.
• Treating the prediction as guaranteed. It's a fitness-based estimate, not a promise — weather, course terrain, pacing strategy, and fueling all affect the real result.
• Never updating the prediction. Re-check it with a fresh race time periodically through training so your goal pace reflects your current fitness.
• Ignoring the implied pace. The marathon time is only useful if you actually train at the pace it implies — check the pace-per-mile figure and rehearse it in long runs.

Related Free Tools From Arb Digital

Turn your predicted pace into finish-time and split planning with the Running Pace Calculator, or switch between min/mile, min/km, mph, and km/h with the Pace Converter. Training on a treadmill? Use the Treadmill Pace Calculator to match your treadmill speed to an outdoor-equivalent pace, and check your energy needs with the Running Calorie Calculator. See every calculator we've built in the free online tools hub.

Frequently Asked Questions

This tool provides general estimates for educational purposes only and is not medical advice. Consult a doctor before starting a new training program.