What this result means
The general predictor will already give you a marathon row. This page exists because “what marathon does my half suggest?” is a search of its own, and because the copy can spend all of its scepticism on the marathon: heat, fuelling, and the long-run problem.
Pick 5K, 10K, or half, enter the time, get one marathon estimate. Compare with VDOT equivalents if you want a second model.
A 46:20 10K predicts about 3:33:09 for the marathon under Riegel 1.06 on this site. A 1:45:00 half predicts a marathon slower than 3:30 even if doubling 1:45 and adding ten minutes is a folk formula people quote.
A 20:00 5K maps to an ambitious marathon. If you have not run longer than 12 km, treat that number as a ceiling, then train.
Calculation method
T_marathon = T_known × (42.195 / D_known)^1.06. Same engine as the race time predictor. Cite Riegel 1981.
Deterministic estimate. Not a qualifying standard.
Undertrained marathons miss Riegel by a lot. Heat, hills, and no long runs are the usual reasons. Inverse required-performance tells you the half you need for a marathon goal.
Frequently asked questions
What marathon does a 1:45 half predict?
Riegel with 1.06, not “double plus ten.” Enter half and 1:45:00 on this page for the number this site uses.
Why not just use the race time predictor?
You can. This page locks the target to the marathon and writes marathon-specific limits.
Is this my Boston qualifier?
No. Qualifying times are published by race organisers. This is a formula.
Should I compare with VDOT as well?
Yes if you want a second model. This page is Riegel only, locked to 42.195 km. The race equivalent calculator inverts Daniels VDOT across distances.