Splits Calculator

How the splits get adjusted

A naive split calculator divides goal time by aid stations and gives you a flat pace per segment. That's wrong for ultras. Two things actually happen:

• You fade. Pace gets worse as miles accumulate (Riegel formula, exponent 1.07 for trained ultrarunners). Your last 30-mile segment will be 25–35% slower than your first 30-mile segment.
• Vert costs disproportionately. Naismith-style 12 minutes per 1,000 ft of climb. Distributed across the course, but weighted to where the climb actually happens.

This calculator distributes vert evenly across aid stations (a simplifying assumption — your actual race may have one or two big climbs), then applies a faster-early/slower-late pacing curve based on the Riegel exponent.

How to use the output

1. Print the splits card. Tape it inside your vest's chest pocket or have your crew hold it.
2. Treat early splits as ceilings, not targets. If you're 5 minutes ahead of pace at mile 20, that's fine — but don't try to bank more time. The race punishes runners who go out hot.
3. Late splits are floors. If you fall behind by hour 18, the goal becomes not falling further behind, not catching up.

The honest caveat

A splits card is a hypothesis, not a plan. Mountain races have weird middle sections that destroy any flat math. Use this for goal-setting and the first half of pacing; throw it away after mile 60 and run by feel.