Methodology
Internal units are meters and seconds
Every calculator converts your inputs into two canonical units — meters for distance and seconds for time — before any arithmetic runs. Display units (km, mi, km/h, mph, min/km, min/mile) are presentation only. Displayed values are never fed back into the math, so rounding on screen can never compound into the result.
Exact constants
One mile is exactly 1609.344 meters, per the international yard and pound agreement of 1959. One kilometer is exactly 1000 meters. Pace conversions use the exact ratio 1.609344 (seconds per km × 1.609344 = seconds per mile); speed conversions use the same factor in division.
Formulas
- Pace: pace = time ÷ distance. For a 10 km run in 3000 s, pace is 300 s per km.
- Speed: speed (km/h) = 3600 ÷ pace in seconds per km.
- Even splits: cumulative time at distance x is T × x ÷ D, where T is the target time and D the race distance, both exact. Boundaries are placed at exact multiples of the interval, with a small numeric tolerance so an exact multiple never spawns a micro-tail row.
- Two-pace strategy: with average pace p0 = T ÷ (D ÷ 1000) and half difference q, first-half pace is p0 + q/2 and second-half pace is p0 − q/2. The difference is capped at |q| ≤ 120 seconds per km and both half paces must remain positive; the total always equals T.
Rounding policy
Internal math never rounds. Cumulative times are rounded to the whole second only for display, with overflow carried correctly (59.6 s never renders as 0:60). Each displayed segment time is the difference between two rounded cumulative times, so displayed segments always sum exactly to the finish time; a 1-second difference between similar segments comes from display rounding, not from the model. The final cumulative always equals the rounded target. Distances display to 3 decimals and speeds to 2; residuals on split tables show exact meters with up to 3 decimals.
Product range limits
The calculators accept distances from 100 m to 100 km and durations from 1 second to 48 hours. These are product scope limits — chosen so tables stay readable and inputs stay sane — not safety limits or statements about what distances humans can run. Computed results that fall outside the range (for example, a time over 48 hours) are refused with a message rather than displayed truncated.
What this tool does not do
RunPaceKit calculates arithmetic scenarios from the numbers you enter. It does not predict your fitness, forecast race outcomes, or judge whether a target is achievable. It provides no training, medical or nutritional guidance. Decisions about training and health belong to you and, where relevant, to qualified professionals. The full disclaimer appears on every calculator page, and the terms cover the warranty position.
Where the math runs
All calculations execute in your browser. No input you type is transmitted to any server; the privacy policy documents the local storage keys the site uses.