Running Pace Calculator — 5K, 10K, Marathon Times

The physiology is straightforward once stated. Short races run largely on anaerobic capacity and stored glycogen at high intensity. As distance grows, the sustainable intensity drops toward the level your aerobic system can supply indefinitely.

The binding constraint changes too. A 5K is limited mostly by how much oxygen you can process. A marathon is limited by fuel — you carry roughly 90 minutes to two hours of glycogen at race intensity — and by the accumulating mechanical damage of tens of thousands of foot strikes.

This is why the marathon is not simply a long 10K. Around the 20-mile mark most runners approach glycogen depletion, which is the physiological event behind "hitting the wall". No amount of pacing arithmetic avoids it; only training and fuelling do.

Riegel's exponent captures the average of all this empirically. It does not model any single mechanism — it just fits the observed relationship across a large body of race results.

Predictions are most reliable when the two distances are close. A 10K predicted from a 5K is usually within a couple of percent for a trained runner. A marathon predicted from a 5K is a much longer extrapolation and should be treated with corresponding caution.

Some coaches prefer an exponent of 1.07 or 1.08 for marathon predictions specifically, on the basis that Riegel is slightly optimistic at that end. If your marathon results have consistently come in behind the prediction, that is the adjustment to make.

An even split — the same pace throughout — is close to optimal for most distances and most runners, and it is far harder to execute than it sounds because everyone feels good in the first mile.

A slight negative split, running the second half a little faster, is the pattern behind a large share of personal bests. It costs almost nothing if it fails and protects against the far more common failure of starting too fast.

The classic marathon error is banking time: running the first half a few minutes quicker than target in the belief it can be held in reserve. It cannot. Glycogen spent early is not recoverable, and the second half loses far more than the first half gained.

Course profile and heat both shift the calculation. Hills cost more going up than they return coming down, and performance falls measurably above roughly 15°C, with the effect growing sharply through the marathon distance.

Even split —

the same pace throughout; the default plan and usually the right one.

Negative split —

second half faster than the first; the pattern most personal bests follow.

Positive split —

slowing in the second half; almost always the result of starting too fast rather than a chosen strategy.

Threshold pace —

roughly the pace you could hold for an hour — close to 10K pace for many runners, and the anchor most training paces are derived from.

Easy pace —

conversational. Most weekly mileage should be here, and running easy days too hard is the most common training mistake there is.

Race distances are metric almost everywhere, while a great many runners think in minutes per mile. Both are shown here to avoid the conversion becoming its own source of error.

The relationship: a mile is 1.609344 km, so a pace per mile is the pace per km multiplied by 1.609. A 5:00/km pace is 8:03/mile.

Useful anchors: 5:00/km is a 25-minute 5K and a 3:30 marathon under straight multiplication — but a 3:58 marathon under Riegel, which is the number to plan around. A 4:00/km pace is a 20-minute 5K. A 6:00/km pace is a 30-minute 5K.

One quiet source of confusion on race day: your watch will almost always record a longer distance than the official course. Courses are measured along the shortest legal route, and nobody runs that line exactly. Finishing a marathon with 42.5 km on the watch is normal, and it means your actual pace was slightly faster than the watch suggested.