📊 Race Time Predictor

Marathon Time
Predictor

Q: How accurate is the Riegel marathon prediction formula?

The Riegel formula is accurate within ±3-5% when you have an adequate aerobic base and have done long runs of 30+ km. It under-predicts for fast-twitch runners (5K times look too good) and over-predicts when long-run endurance is missing.

Q: Which race is most predictive for marathon time?

A recent half marathon (run in the last 6 weeks) is the single best predictor. The aerobic demand is closest to marathon physiology — same fuel system, same pacing strategy, just half the duration.

Q: Can I predict my marathon time from a 5K?

You can, but the prediction is wider — typical accuracy is ±10%. 5K is mostly anaerobic; marathon is fully aerobic. A 20-minute 5K predicts roughly a 3:15 marathon by Riegel, but real-world results land anywhere between 3:10 and 3:30 depending on long-run base.

Predict your marathon finish from a recent 5K, 10K or half marathon. Riegel formula, conversion tables, accuracy ranges and the long-run caveats nobody mentions.

Open the calculator →

The math

The Riegel formula

One equation runs almost every race time predictor on the internet — including the calculator on our homepage.

Pete Riegel, an American runner and engineer, published the formula in 1977 in the journal Runner's World. It models the empirical observation that pace slows down predictably as distance increases, governed by a single exponent — usually 1.06 for trained runners.

T₂ = T₁ × (D₂ / D₁)^1.06

Where T₁ is your known race time, D₁ is its distance, D₂ is the marathon distance (42.195 km), and T₂ is the predicted marathon time.

Worked example: a 1:45 half marathon. T₁ = 6300 seconds, D₁ = 21.0975 km, D₂ = 42.195 km. T₂ = 6300 × 2^1.06 = 6300 × 2.085 = 13136 sec ≈ 3:38:56 marathon.

The exponent 1.06 is the magic number. A pure flat line (1.00) would mean perfect pace maintenance — impossible. A higher exponent (1.10+) models a runner who fades hard with distance. Elite marathoners run very close to 1.06; recreational runners with weak long-run mileage run closer to 1.08 or 1.10.

Prediction tables

Predicted marathon times

From every common race distance — pre-calculated so you don't have to.

Half marathon → Marathon

The strongest predictor. Run a tune-up half 4-6 weeks before your marathon and use this table.

Half marathon time	Marathon (Riegel)	Marathon pace (min/km)
1:20	2:46:40	3:57
1:25	2:57:00	4:12
1:30	3:07:24	4:26
1:35	3:17:48	4:41
1:40	3:28:12	4:56
1:45	3:38:36	5:11
1:50	3:49:00	5:26
2:00	4:09:48	5:56
2:15	4:41:00	6:39
2:30	5:12:12	7:23

10K → Marathon

Less reliable than half marathon — the 10K aerobic load is below marathon-specific demand. Use as a sanity check, not a target.

10K time	Marathon (Riegel)	Marathon pace (min/km)
35:00	2:42:38	3:51
38:00	2:56:31	4:11
40:00	3:05:48	4:24
42:30	3:17:24	4:41
45:00	3:29:00	4:57
48:00	3:42:54	5:17
50:00	3:52:09	5:30
55:00	4:15:24	6:03
60:00	4:38:38	6:36

5K → Marathon

Riegel's largest error band sits here. Treat 5K predictions as "best case if you have full marathon conditioning". Most runners need to add 10-20 minutes for realistic targets.

5K time	Marathon (Riegel)	Marathon pace (min/km)
17:00	2:43:46	3:53
18:00	2:53:33	4:07
19:00	3:03:14	4:21
20:00	3:12:55	4:34
22:00	3:32:14	5:02
25:00	4:01:11	5:43
28:00	4:30:07	6:24
30:00	4:49:23	6:51

Accuracy

Why Riegel breaks for many runners

The formula assumes you've prepared aerobically for the longer distance. In practice, three factors blow that assumption out of the water:

1. Insufficient long-run base

If your weekly mileage is under 50 km and your longest run is under 30 km, the formula will over-predict. Riegel was built from data of competitive runners with 100+ km/week. The "marathon wall" — glycogen depletion at km 30-32 — adds 5-15 minutes of slowdown that Riegel can't see.

2. Fast-twitch dominance

Some runners are naturally good at 5K but disproportionately slower at marathon. Their 5K time looks elite (sub-19), but their marathon physiology can't sustain that aerobic load. Riegel will under-predict their marathon, often by 20+ minutes.

3. Heat, hills, fueling

Marathon-day conditions add another 5-15% variability. A flat Berlin marathon in 12°C is the Riegel scenario. A hilly New York in 22°C will run 10-15 minutes slower for the same fitness. Always race-correct the prediction for the specific event.

Rule of thumb: For your first marathon, take the Riegel prediction and add 10-15 minutes as a realistic goal. For your second marathon, the formula is usually within ±5 minutes.

FAQ

Frequently asked questions

How accurate is the Riegel marathon prediction formula?

Within ±3-5% for trained runners with an adequate aerobic base and 30+ km long runs. Wider for beginners (±10%) and fast-twitch dominant runners (under-predicts by 5-10%). The prediction is most accurate when the input race was run in the last 6 weeks.

Which race is most predictive for marathon time?

A recent half marathon, run in the last 6 weeks at race effort, is the single best predictor. The aerobic demand is closest to marathon physiology — same fuel system, same pacing strategy, just half the duration. 10K is second-best, 5K third.

Can I predict my marathon time from a 5K?

You can, but the error band is widest. 5K is mostly anaerobic; marathon is fully aerobic. A 20-minute 5K predicts roughly a 3:13 marathon by Riegel, but real-world results land anywhere between 3:10 and 3:35 depending on long-run base. Use 5K as a sanity check, not the primary input.

Does the Riegel formula work for beginners?

Less reliably. Beginners with low weekly mileage often blow up in the last 10 km — the formula assumes marathon-specific aerobic conditioning. For first-time marathoners, take the Riegel prediction and add 10-15 minutes as a realistic goal.

What exponent does the formula use?

1.06 for trained runners. Sub-elite runners hit 1.05; recreational runners with weak long runs are closer to 1.08-1.10. Some calculators let you adjust the exponent based on your training profile.

Want a marathon plan
built around your prediction?

WattRun takes your goal time, builds an AI training plan calibrated to your current fitness, and adapts after every run. Free.

Start for free →