The math behind your plan.

Units and standard distances

We use exactly 1.609344 kilometers per international mile. Standard race presets are 5 km, 10 km, 21.0975 km for a half marathon, and 42.195 km for a marathon.

Pace, speed and time

Pace in seconds per kilometer = time in seconds ÷ kilometers. Seconds per mile = seconds per kilometer × 1.609344. Speed in km/h = distance in kilometers ÷ time in seconds × 3600.

Split distribution

For even pacing, elapsed time at a marker is total time × marker distance ÷ full distance. The 51/49 scenario assigns 51% of total time to the first half of the distance and 49% to the second. The 49/51 scenario reverses those shares. Split times are differences between consecutive cumulative times.

Run-walk estimates

Distance per cycle = run seconds ÷ run pace + walk seconds ÷ walk pace, using one consistent distance unit. Complete cycles are followed by only the part of the next run or walk needed to reach the finish. Each cycle starts with the running phase.

Rounding and limits

Time is displayed to the nearest second. Distances and speeds are rounded only for display. Independently rounded splits can sum to a slightly different number than the displayed total. Race calculations accept distances up to 200 km and times up to 48 hours.

Reference definitions

• NIST: unit conversion factors
• World Athletics: half marathon distance
• World Athletics: marathon distance

Definitions checked September 24, 2026. Site calculations and explanations are independently implemented.