Marathon Time Predictor

This tool uses Riegel's Formula, published by exercise physiologist Peter Riegel in 1977, to project a marathon and half marathon time from a recent race result. Below is exactly how the math works, a worked example you can check by hand, and where a prediction like this stops being reliable.

How the Calculation Works

This calculator takes the distance and time you enter and, if you selected miles, first converts the distance to kilometers by multiplying by 1.60934. It then applies Riegel's Formula twice — once with a target distance of 42.195 km for the marathon prediction, and once with 21.0975 km for the half marathon:

T2 = T1 × (D2 ÷ D1)^1.06

T1 and D1 are the time and distance you enter, D2 is each target distance, and 1.06 is the fatigue exponent — a value greater than 1 that models how pace naturally slows as distance increases, even for a well-trained, evenly-paced effort.

Worked Example

Enter a 10K (10 km) run in 45:00 (2,700 seconds):

• Marathon: 2,700 × (42.195 ÷ 10)^1.06 = 2,700 × 4.600 = 12,420.5 seconds → 3:27:00
• Half Marathon: 2,700 × (21.0975 ÷ 10)^1.06 = 2,700 × 2.206 = 5,957.3 seconds → 1:39:17

If you'd entered that same effort in miles instead — 6.21 miles in 45:00 — the calculator would first convert 6.21 × 1.60934 ≈ 10.0 km before running the identical formula, landing on the same predicted times. If a 45:00 10K sounds close to your current fitness, our average 10K time by age page shows how that compares to other runners your age.

Accuracy & Best Input Distance

Riegel's formula holds up best when the input and target distances are relatively close together, which is why a half marathon is a far more reliable input for a marathon prediction than a 5K. A half marathon draws on much of the same aerobic endurance and fueling strategy a marathon demands, while a 5K is run at a pace and energy system that has little in common with three-plus hours of marathon effort — predicting a marathon from a 5K asks the formula to extrapolate across a gap it has no way to fully account for. Some coaches use an even simpler rule of thumb for this same reason: roughly double your half marathon time and add 10 to 15 minutes as a ballpark marathon goal.

Using Your Predicted Time

A predicted finish time becomes actionable once it's turned into a pacing plan. Our race split calculator can convert either predicted time above into a full kilometer-by-kilometer or mile-by-mile checkpoint chart, and our pace calculator converts a target finish time into the per-km or per-mile number you'll watch on race day. If you're building toward your first marathon or half marathon off the back of this prediction, our guides on running your first half marathon and a beginner 10K training plan are useful starting points, and our average marathon time page is a good sanity check on whether a predicted goal looks realistic for your age group.

Limitations

This calculator has no way to know your training history — if you haven't put in the long runs a marathon demands, the predicted time represents more of a fitness ceiling than a time you're ready to run. It also can't see hills, weather, or fueling strategy on race day, and it assumes you'll pace the target distance evenly and disciplined, which is where most blown-up predictions actually go wrong. Treat the marathon number especially as an optimistic upper bound rather than a guaranteed finish time, and lean more heavily on the half marathon prediction if you have both.

Frequently Asked Questions

What pace is a 4 hour marathon?

A 4:00:00 marathon over the standard 42.195 km distance works out to an average pace of 14,400 ÷ 42.195 ≈ 341 seconds per km, which is 5:41/km, or roughly 9:09 per mile.

How accurate is a marathon time predictor from a 5K?

Less accurate than one built from a longer input distance. A 5K tests only a few minutes of speed, while a marathon demands hours of pacing, fueling, and specific endurance the formula has no way to see from a single short race — treat a 5K-based marathon prediction as an optimistic ceiling rather than an expected outcome.

Can I predict my marathon time from a 10K?

Yes — enter your 10K distance and time and this calculator runs the same Riegel formula to project both a marathon and a half marathon time. A 10K sits roughly in the middle in terms of reliability: closer to marathon-relevant endurance than a 5K, but still well short of marathon-specific fatigue.

What is Riegel's formula and what does the 1.06 exponent mean?

Riegel's Formula is T2 = T1 × (D2 ÷ D1)^1.06, published by exercise physiologist Peter Riegel in 1977. T1 and D1 are your entered time and distance, D2 is the target distance, and the 1.06 exponent models how pace naturally slows as distance increases — a number just above 1 that makes doubling the distance take more than double the time.

Why is a half marathon a better predictor of marathon time than a 5K?

Because it's closer in both duration and physiological demand — a half marathon relies much more heavily on the same aerobic endurance and fueling strategy a marathon requires, while a 5K is run at a pace and energy system that has little in common with marathon effort. Some coaches use an even simpler rule of thumb: roughly double your half marathon time and add 10 to 15 minutes as a ballpark marathon goal.

Does this calculator work with miles or only kilometers?

Both — select 'miles' in the unit dropdown and the calculator converts your entered distance to kilometers (multiplying by 1.60934) before running the prediction, so you can enter a 5-mile or 10-mile time just as easily as a 5K or 10K.

How many long runs do I need before trusting this marathon prediction?

This calculator has no way to know your training history, so it can't answer that directly — but the predicted time assumes you've built the endurance for the full distance, typically through a training block that includes several long runs. Without that base, the predicted time is more of a fitness ceiling than a realistic race-day target.

What if my predicted marathon time seems unrealistic?

That usually means the input distance was too short relative to the marathon to extrapolate reliably. Try entering a longer recent race, like a half marathon, if you have one — the prediction tends to get more trustworthy as the gap between your input distance and 42.195 km shrinks.