Predict your marathon time from a 5K
Scroll down to 03 — RACE TIMES. Type your 5K time into SEED TIME (or pick another seed distance — 10K, half-marathon, even a hard tempo). The table predicts your times for every common race distance from a mile up to 50K, plus the matching per-km / per-mile pace.
Behind the scenes: the Riegel formula, T₂ = T₁ × (D₂ / D₁)^k. Default mode is AUTO, which uses the Vickers-Vertosick (2016) distance-dependent exponent: k=1.06 for short-distance equivalents, climbing through 1.07 (10K), 1.08 (half), 1.10 (marathon), and 1.12 (ultras). Pete Riegel's original fixed k=1.06 was tuned around 5K↔10K and underestimates marathon fade by ~2-3 minutes for a typical sub-3 runner; the distance-aware K fixes that.
Hit MANUAL on the slider to lock k to one value across every row — useful if you know your own fade pattern (elite ultrarunners often run closer to 1.04; first-time marathoners often closer to 1.12).
Important: Riegel assumes you've trained for the longer distance. Predicting a marathon from a 5K time is mathematically valid — but you still need the mileage in your legs. Treat the predictor as your fitness ceiling, not your fitness floor.
WANT BETTER ACCURACY?
The standalone Riegel calculator above uses one fixed exponent across all distances. Our AI coach uses a richer stack: Jack Daniels' VDOT for VO₂max-equivalent times, Vickers-Vertosick (2016) distance-dependent Riegel exponents that don't underestimate marathon fall-off, Critical Speed 2-parameter fits from your recent training, and the Banister fitness/fatigue/form model from your training load. Calibrates to your real Strava history. Try the coach →
Riegel projections from common 5K times
auto K · Vickers-Vertosick exponents per target (10K→1.07, half→1.08, marathon→1.10, 50K→1.12)
5K 10K HALF MARATHON 50K
18:0037:471:25:133:08:013:57:17
20:0041:591:34:423:28:544:23:39
22:0046:111:44:103:49:484:50:01
24:0050:231:53:384:10:415:16:23
26:0054:352:03:064:31:355:42:45
28:0058:472:12:344:52:286:09:07
30:001:02:592:22:025:13:226:35:29
Values rounded; use the live predictor for precise output.
Predictor FAQ
- How accurate is the race time predictor?
- It uses the Riegel formula T₂ = T₁ × (D₂ / D₁)^k with the Vickers-Vertosick (2016) distance-dependent exponent — k=1.06 short, climbing through 1.08 (half), 1.10 (marathon), 1.12 (ultras). That fixes Pete Riegel's original fixed-k=1.06 which underestimates marathon fade. Accurate to within ~2% for trained runners on flat courses; less accurate for first-time marathoners or hilly courses (in which case nudge k toward 1.10–1.15 manually). The connected coach goes further — see /coach.
- Why does the Riegel exponent vary by distance?
- Pete Riegel's 1981 paper used a single k≈1.06 because his data was dominated by 5K↔10K equivalents, where fade is small. Vickers & Vertosick's 2016 meta-analysis showed that fade increases with race duration — marathon fits k≈1.10, ultras even higher. Our AUTO mode picks the right k for the target distance. Lock it to a single value via MANUAL if you know your own fade pattern: elite ultrarunners often run k≈1.04, first-time marathoners often k≈1.12.
- What's a negative split and why does it matter?
- A negative split means running the second half of a race faster than the first half. Research consistently shows it produces faster overall times than positive or even splits, because you stay aerobic longer and have energy when others fade. Most world records — including the marathon — were set with negative splits. Pick "NEG" in the Splits section to plan one.
- What's a good marathon pace?
- A "good" pace depends on training, age, and goals. As benchmarks: 4:00 marathon = 5:41 /km (9:09 /mi). 3:30 marathon = 4:58 /km (8:00 /mi). 3:00 marathon = 4:16 /km (6:52 /mi). Boston qualifying times for most age groups land between 3:00 and 4:00.
- What does PIN do?
- PIN tells the calculator which field is the output. The other two are inputs. The pinned field updates live as you edit the others. By default PACE is pinned — so you set distance and time, and pace is computed. Pin DISTANCE to plan "how far can I run in 45:00 at 5:00/km?". Pin TIME to plan "what time do I run 10K in at 4:30/km?".
- Can I share my calculation with someone?
- Yes. Every change updates the URL with your full state — distance, time, pace, units, split mode, predictor seed, everything. Copy the URL or hit SHARE to grab a clipboard-ready link. Sending it to a friend opens the calculator with exactly the same numbers you saw.