Pace is the currency of running. Every workout, race, and race prediction runs on it. Get comfortable converting between min/km and min/mile, computing splits, and predicting race times from training paces, and you unlock the entire structured-training playbook — Jack Daniels' VDOT, McMillan pace zones, Riegel's race predictor, 80/20 polarized training.

The math is simple but the units are annoying. Pace = time ÷ distance. A 25:00 5K is 25 ÷ 5 = 5:00/km or 25 ÷ 3.107 = 8:03/mile. Converting between min/km and min/mile: min/mile = min/km × 1.60934. The calculator handles the seconds cleanly (5:32/km, not 5.53/km) which is where mental math falls apart.

Riegel's race time formula — T2 = T1 × (D2/D1)^1.06 — predicts race times across distances assuming similar training. A 20:00 5K predicts ~41:26 10K, ~1:31:58 half marathon, ~3:12 marathon. Accuracy is ±3–5% when the target distance is close to your training weekly volume. Marathon predictions from 5K times are notoriously optimistic if you skip the long runs and glycogen depletion practice.

Training pace zones follow VDOT (a VO2max-derived running fitness score from Jack Daniels). Easy runs are 60–90 seconds slower than 5K pace, marathon pace is ~15 sec/mi slower than half-marathon pace, threshold is roughly 10K pace to 10K pace + 10 sec/mi, interval is 3K–5K pace, repetition is mile pace. Running everything at 'medium' pace is the classic amateur mistake — 80/20 (80% easy, 20% hard) is the elite pattern.

For the marathon specifically, the calculator's pace band feature saves races. Print a wristband with target splits, wear it, and stick to the plan. Positive splits (running the second half slower) end in the wall; negative splits (running second half faster) are how nearly every PR is set. Start 5–10 seconds per mile slower than target average pace for the first 3 miles, hold planned pace through mile 18, and let effort take over in the closing 8 miles. That single tactic outperforms most training changes.