You just ran a half marathon in 1:50 and felt strong. Now the group chat is asking the question every runner eventually faces: “so what could you run a marathon in?” Guessing is easy. Predicting it accurately takes a bit of math, and an honest look at what the math cannot see.

Marathon prediction is one of the oldest problems in running science, and it matters more than ever because so much rides on the answer: your training paces, your race-day plan, whether that Boston qualifying time is realistic this year or next. This guide explains the formula behind every serious marathon predictor, shows real conversion tables from 5K, 10K, and half marathon times, and covers the three situations where the prediction will lie to you.

How Marathon Prediction Works: The Riegel Formula

In 1977, engineer and runner Pete Riegel published a deceptively simple equation that is still the backbone of most race time predictors today:

T2 = T1 × (D2 / D1)^1.06

In plain English: take your known time (T1) over a known distance (D1), multiply it by the ratio of the two distances, raised to the power of 1.06. That exponent, sometimes called the fatigue factor, is the whole insight. If humans did not slow down with distance, you could just multiply pace by 42.195 and be done. The 1.06 corrects for the reality that everyone fades, and it was fitted from real world-class and recreational race results.

A concrete example. You ran a 10K in 50:00. Divide the marathon distance by 10K: 42.195 / 10 = 4.2195. Raise that to 1.06: roughly 4.6. Multiply by your time: 50 × 4.6 = 230 minutes, or about 3:50. That is your predicted marathon time, assuming your endurance training matches your 10K fitness.

Marathon Prediction Tables: 5K, 10K, and Half Marathon

Nobody wants to do exponent math on their phone after a race. Here are the Riegel conversions for the most common benchmark performances. Find the row closest to your recent race time.

Two patterns worth noticing. First, the rows are internally consistent: a 25:00 5K, a 52:05 10K, and a 1:54:55 half all describe the same underlying fitness, so they predict the same marathon. Second, the marathon column moves fast. Every minute you shave off your 5K is worth roughly nine to ten minutes over the full distance. That is the compounding effect of the fatigue factor working in reverse.

If your benchmark race is not in the table, the fastest way to get your number is to type it directly: convert your half marathon time to a marathon prediction and you will see every equivalent distance at once, from the mile up to the marathon.

Why Your Predicted Marathon Time Might Be Wrong

The Riegel formula assumes one crucial thing: that your fitness is balanced. It assumes you have the endurance to match your speed. That assumption breaks down in three very common situations.

1. You are speed-rich but endurance-poor

This is the classic trap. A runner who mostly does short, fast workouts can post a flashy 5K time, and the formula happily extrapolates it to a marathon they cannot currently finish at that pace. If your longest run is 12 kilometres, a prediction built on your 5K is a fantasy. The fatigue factor of 1.06 describes well-trained runners; for undertrained runners the real exponent is closer to 1.10 or higher, which can add 15 to 25 minutes to the prediction. The fix is not a different formula, it is more long runs.

2. The marathon course is harder than your benchmark course

Riegel assumes equivalent conditions. A prediction earned on a flat, cool 10K does not survive contact with a hilly marathon in 24°C heat. As a rough rule, above about 15°C you lose around 1.5 seconds per kilometre per degree, which compounds to several minutes over a marathon. Altitude has a similar effect above roughly 1,000 metres. You can adjust your marathon pace for race-day heat before you lock in a goal.

3. You are converting from too short a distance

The further apart the two races, the noisier the prediction. Mile to marathon is a guess. Half marathon to marathon is genuinely useful. If you want one number to trust, run a half marathon four to eight weeks out, all-out, and convert that. Coaches consider it the single best predictor precisely because the fatigue profiles of the two distances are so similar.

From Predicted Time to Marathon Pace

A goal time only becomes a race plan when you translate it into pace. The math is simple division: a 3:50 marathon means 230 minutes across 42.195 kilometres, which is 5:27 per kilometre (8:46 per mile). A sub-4 attempt needs 5:41/km. A Boston-qualifying 3:00 needs 4:16/km, and that number should feel uncomfortably precise, because it is.

Knowing the pace is half the battle; holding it is the other half. The evidence from mass-participation race data is remarkably consistent: the fastest finishers run nearly even splits, and the biggest blowups come from banking time in the first half. Elite marathoners run within a percent or two of even pace almost without exception. Your predicted time is an even-pace prediction. Treat it that way on race day: lock into goal pace from the gun, and only press in the final 10K if the legs still answer.

If you want to rehearse that discipline, build the workout before race day. Typing 5:27/km splits 8x1km turns your goal pace into a concrete interval session you can pin to the fridge.

What Your Prediction Says About Your VO2Max

Every race prediction has a physiological fingerprint. Jack Daniels’ VDOT system, built from decades of lab testing, maps any race performance to an estimate of your VO2Max, the maximum rate at which your body can use oxygen. A 25:00 5K corresponds to a VO2Max of roughly 38; a 20:00 5K lands near 50. Because the marathon prediction and the VO2Max estimate describe the same fitness, they move together: raise one and the other follows.

This is why a single recent race is such a powerful input. One honest performance gives you a predicted marathon time, a goal pace, and an estimate of your aerobic engine, all from the same number. You can estimate your VO2Max from your latest race and watch it shift as training progresses. It is a far more sensitive progress marker than the scale or the mirror.

Putting the Prediction to Work

A marathon prediction is not a promise. It is a planning tool, and it earns its keep in three places:

1. Choosing a goal. If the prediction says 3:52, racing for 3:35 is not ambition, it is a planned blowup. Race the fitness you have, train toward the fitness you want.
2. Setting training paces. Tempo runs, marathon-pace segments, and long-run targets all key off your current fitness. A prediction that updates as you race keeps every workout honest.
3. Writing the race plan. Even pacing from a realistic prediction is the single highest-leverage decision most marathoners make. The prediction gives you the number; discipline gives you the result.

The runners who nail their marathon debut are rarely the most talented. They are the ones who predicted honestly, trained at the right paces, and respected the distance for 42 kilometres instead of 30. Start with your most recent race, get your number, and build from there.