Brick Workout Run Pacing Calculator
Estimate your off-the-bike run pace and split for a brick workout, allowing for the first-kilometre "jelly legs" slowdown that fades as you settle.
How it works
Brick time = 1 km × pace×(1 + first-km slow) + remaining km × pace×(1 + settled slow)
Running straight off the bike feels awful because blood is pooled in the cycling muscles and your stride mechanics have to switch over — the classic "jelly legs" that cost the most in the opening kilometre. This model applies a larger penalty to that first kilometre and a smaller, lasting penalty once you settle, so you can set a realistic target split instead of blowing up chasing your fresh pace. Regular brick training shrinks both penalties over a season.
Worked example
A runner whose fresh 5 km pace is 5:00/km, with a 12% first-km penalty and a 4% settled penalty, runs the opening km at 5:36 and the remaining four at 5:12 — a total near 26:24, about 1:24 slower than the 25:00 they would run fresh.
Frequently asked questions
Why do my legs feel so heavy off the bike?
Cycling and running load the legs differently and keep blood pooled in the cycling muscles, so for the first several minutes your running muscles are under-perfused and your stride is inefficient. As blood redistributes and your cadence normalises, the pace comes back — usually within the first kilometre or two.
How do I reduce the brick slowdown?
Do brick workouts regularly so the transition becomes familiar, spin the final minutes on the bike at a higher cadence to pre-load your run stride, and start the run slightly conservatively rather than sprinting off the mount line. Over weeks the first-km penalty typically shrinks noticeably.
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This calculator is for educational purposes. Double-check important results before acting on them.