Pace, finish time, or distance — solve any of the three, with race predictions and splits
| Race | Predicted time (Riegel) | Required pace |
|---|
Every runner's three variables — pace, time, distance — lock together, and this calculator solves the triangle from any two, then does the two genuinely useful extensions: race-time prediction across distances via Riegel's endurance formula, and the pace conversions (mph for treadmills, min/km for metric races) that trip everyone at the gym or abroad.
| Pace (min/mi) | 5K | 10K | Half | Marathon | mph |
|---|---|---|---|---|---|
| 12:00 | 37:17 | 1:14:34 | 2:37 | 5:14 | 5.0 |
| 10:00 | 31:04 | 1:02:08 | 2:11 | 4:22 | 6.0 |
| 9:00 | 27:58 | 55:55 | 1:58 | 3:56 | 6.7 |
| 8:00 | 24:51 | 49:43 | 1:45 | 3:30 | 7.5 |
| 7:00 | 21:45 | 43:30 | 1:32 | 3:04 | 8.6 |
Endurance decays predictably: double the distance and pace slows ~6%. A 25:00 5K predicts a ~52:00 10K, a 1:55 half, and a ~3:59 marathon — if the training matches the distance. The formula's known bias: it flatters marathon predictions for runners without 18+ mile long runs in their legs (glycogen, not fitness, is the marathon's gatekeeper). Predicting up one distance level is reliable; predicting a marathon from a 5K is an aspiration with math attached.
Within 2-3% for trained runners predicting adjacent distances off a recent race. Accuracy decays predicting far up the ladder (5K → marathon) or off stale efforts. Best input: a race from the last 6-8 weeks.
None — run by feel (conversational) and let pace emerge. Beginners who chase pace targets get injured; the couch-to-5K progression is time-based for good reason. The calculator becomes useful once you have a race to plug in.
No wind resistance and a moving belt make treadmills slightly 'easier' at the same setting (1% incline approximates road effort), but perceived effort varies with heat and boredom. Match effort, not just numbers.
Median finisher times: ~30-34 min (5K), ~1:00-1:05 (10K) — but the honest answer is 'faster than your last one.' Age-graded calculators exist for comparing across ages; personal progression beats population comparison.
The calculator's pace per mile IS your split table — write key splits on your arm or set watch alerts. The rule that wins races: first mile 5-10 seconds SLOWER than goal pace; the bank-time strategy has ruined more marathons than walls have.
Race distances are certified; GPS wanders (tangents, tunnels, buildings) and typically reads 0.5-2% long — which makes watch pace read faster than true pace. In races, trust the mile markers and this math over the wrist.
Yes — everything computes locally in your browser.
Plug in your latest race, respect the easy-day offsets, and pace the next one even. The triangle is simple; the discipline it prescribes is the entire sport.