# Running Pace & Splits

> Pace, time or distance in km or miles, splits, race predictions, treadmill speed, laps. Enter two of distance, time and pace to get the third, in kilometres or miles. Get an even, negative or positive split table, race time predictions with the Riegel formula, the treadmill speed in km/h and mph, and track lap times.

Part of usefulweb.site (https://usefulweb.site/). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://runningpacecalculator.usefulweb.site/ (interactive tool at https://runningpacecalculator.usefulweb.site/tool)
- JSON API: POST https://runningpacecalculator.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://runningpacecalculator.usefulweb.site/api/v1/tool)
- OpenAPI: https://runningpacecalculator.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://runningpacecalculator.usefulweb.site/mcp - tool `running_pace_calculator`

## Inputs
- solveFor (select): Calculate
- preset (select): Race distance
- distance (number): Custom distance - Used when the race distance is "Custom distance".
- distanceUnit (select): Distance unit
- time (text): Time - h:mm:ss or mm:ss, e.g. 1:45:00 or 24:30.
- pace (text): Pace - mm:ss per km or per mile, e.g. 5:30.
- paceUnit (select): Pace per
- splitUnit (select): Splits every
- splitStrategy (select): Split strategy
- splitPercent (number): Pace change start → finish - For negative/positive splits: how much the pace changes from the start to the finish, spread evenly.
- lapMeters (number): Track lap length

## Method
- Pace = time ÷ distance; time = pace × distance; distance = time ÷ pace. 1 mile = 1609.344 m exactly; a half marathon is 21.0975 km and a marathon 42.195 km.
- Speed (treadmill) = 3600 ÷ pace in seconds per km (km/h) or per mile (mph).
- Splits: for an even pace every split takes pace × split length. For a negative (positive) split the pace changes linearly along the course by the chosen percentage from start to finish, so the split times still add up to the target time.
- Race predictions use Riegel's formula T2 = T1 × (D2 ÷ D1)^1.06, which he described for efforts of roughly 3.5 to 230 minutes. Predictions far from the input distance, and marathon times for runners without long-run training, are usually too optimistic.
- Track laps: lap time = pace × lap length. Lane 1 of a standard track is 400 m; outer lanes are longer.