How to Predict Your Marathon Time From a Shorter Race

Most runners have a recent race result sitting in their watch history — a 10K from a club series, a parkrun 5K, a half from a spring event. That single time is enough to estimate a marathon finish with surprising accuracy, provided you know what the math can and cannot see. This guide walks through the formula behind those predictions, shows a worked example, and explains why the marathon is the one distance where the number on the screen needs the most caution.

The Idea: Fatigue Scales With Distance

Run twice as far and you slow down — but not by a fixed percentage. Pace decays as distance grows, and that decay follows a remarkably stable curve across the population of trained runners. Pete Riegel, an engineer and marathoner, captured it in 1981 with a single power law that has been the backbone of race prediction ever since.

The formula is:

T2 = T1 × (D2 / D1)^1.06

Where T1 is your known time over distance D1, D2 is the target distance, and 1.06 is the fatigue exponent. If pace held perfectly across distances, the exponent would be 1.0. The extra 0.06 is the price you pay for endurance: the longer you go, the more each kilometre costs.

The exponent is the whole story. A value of 1.06 describes an average trained runner. A beginner with a thin aerobic base behaves more like 1.08 or even 1.10 — they fade harder over distance, so a prediction built on 1.06 will flatter them. A seasoned high-mileage runner can sit closer to 1.05. This is why a prediction is only ever as good as the assumption that your training matches the curve.

A Worked Example: From a 50-Minute 10K to a Marathon

Let's run the numbers on a clean case. Say you ran a 10K in exactly 50 minutes and want to know what that implies for a marathon.

First, convert to consistent units. T1 = 50 minutes, D1 = 10 km, D2 = 42.195 km. Plug them in:

T2 = 50 × (42.195 / 10)^1.06
T2 = 50 × (4.2195)^1.06
T2 = 50 × 4.527
T2 ≈ 226.3 minutes

That works out to about 3:46:20 for the full marathon. So a 50-minute 10K runner, on paper, is a sub-3:50 marathoner.

Notice what that prediction quietly assumes: that you can hold a 5:21/km pace for four times the distance you actually raced. If your longest run this month was 12 km, the formula has no way of knowing that — it sees only the 10K. The number is honest about the speed your engine produced on that day. It is silent about whether your fuel tank is big enough to last 42 kilometres. The marathon time predictor runs Riegel alongside the Cameron and Jack Daniels VDOT models and blends them, which softens the optimism of any single formula, but no calculator can see your weekly mileage from a finish time alone.

Why the Marathon Is the Hardest Race to Predict

Predicting a 10K from a 5K is easy. Predicting a half from a 10K is reliable. The marathon breaks the pattern, and there are two physiological reasons.

The first is the wall. Your body stores roughly 90 to 120 minutes of glycogen at race pace. Most runners finishing under three hours skate close to that ceiling; everyone slower than about 3:30 will run out before the line unless they fuel during the race. When glycogen runs low, pace collapses non-linearly — not a gentle fade but a cliff, often around the 30 to 35 km mark. Riegel's smooth power curve has no term for a cliff. It assumes you decay gracefully the whole way.

The second is fueling and hydration, which simply do not exist as variables in any shorter race. A 10K is over before your stomach matters. A marathon turns gel timing, sodium, and carbohydrate intake into performance limiters. Two runners with identical 10K times and identical training can finish a marathon fifteen minutes apart purely on how well they fueled. The formula cannot reach those variables, which is exactly why the prediction band for a marathon should be read as ±5% rather than the ±2% you'd trust for a half.

This is also why a longer input race always beats a shorter one. A half marathon prediction for a full is a tighter prior than a 10K prediction, because the half already contains some of the endurance signal — the glycogen draw, the mental grind — that the marathon will demand.

From Prediction to Pacing

A predicted time is only useful if it turns into a pacing plan you can execute on race morning. The single most common way runners blow up is going out faster than their fitness, riding the early adrenaline, then paying for it in the final ten kilometres.

Take our worked example. A 3:46 marathon is an average pace of about 5:21/km, or roughly 8:36 per mile. The disciplined plan is to run the first half a touch slower than target — say 5:25/km — and let the back half come to you. Negative splitting a marathon (running the second half faster than the first) is how almost every personal best is actually set. To convert your predicted finish into exact per-kilometre and per-mile splits, drop the time into a dedicated running pace calculator and print the band you intend to hold.

When I tested this on my own training block, I had a 10K that predicted a 3:42 marathon. I ignored it on race day and went out at 3:35 pace because I felt good through 15 km. By 32 km the wall arrived exactly on schedule, and I limped in at 3:51 — nine minutes slower than the prediction I had thrown away. The formula was right; my pacing discipline was not. That race taught me to treat the predicted time as a ceiling for the first half, never a target for the first kilometre.

Reading the Prediction Honestly

Use the number as a planning anchor, not a promise. Three habits keep it useful:

• Check your mileage against the goal. If the prediction says sub-4:00 but you're running 30 km a week, the speed is there and the endurance is not. Build the volume before you commit to the time.
• Weight your longest race most. If you have both a 10K and a half on record, trust the half-derived prediction. It carries more of the endurance information the marathon will test.
• Subtract for inexperience. First-time marathoners should mentally add five to ten minutes to any formula output to account for fueling mistakes and pacing nerves that veterans have already worked out.

A recent race time is the best single predictor of marathon fitness we have, and Riegel's formula remains the cleanest way to extract it. Just remember the formula measures the engine, not the fuel tank — and the marathon is the one race where the tank decides the finish.

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Made by Toolora · Updated 2026-06-13