Estimate Marathon Time From Half Marathon

Your half marathon is the closest thing to a marathon you have raced, so it is the best starting point the formula can be given — and it is still the prediction most likely to leave you standing on the wrong side of the finish clock. Enter your half:

Worked example: 1:45 for the half

T₂ = 1:45:00 × (42.195 ÷ 21.0975)^1.06 = 3:38:55, a pace of 5:11/km (8:21/mile). The half itself was run at 4:59/km, so the formula asks for only twelve seconds per kilometer back over twice the distance.

Half marathon	Predicted marathon	Predicted marathon pace
1:30:00	3:07:39	4:27/km
1:45:00	3:38:55	5:11/km
2:00:00	4:10:12	5:56/km

Twelve seconds per kilometer is a small toll for doubling a race, and that is precisely the complaint against the formula at this distance.

What the data says about this specific prediction

Vickers and Vertosick surveyed 2,303 recreational runners and compared their actual marathons with what the formula predicted from a shorter race. The result is unusually blunt for sports science:

• median error 10:09 too fast;
• about a quarter of runners were more than 20 minutes too fast;
• the slowest 5% missed their predicted time by more than 36 minutes;
• mean squared error 380.7, against 227.6 and 208.3 for purpose-built marathon models.

The same paper found no such miscalibration at 10K or the half. So the failure is specific to the marathon, not a general flaw in the arithmetic. Nudging the exponent does not rescue it — with 1.08 instead of 1.07, marathon speed was still overestimated for roughly 75% of runners.

Why the marathon is different

Everything the formula cannot see becomes decisive after about 30 km: how much carbohydrate you took on and whether your gut tolerated it, how hot the day was, and whether your longest training run ever approached three hours. A half marathon can be bluffed on fitness alone. A marathon audits your preparation, and it does the audit in the last quarter of the race.

How to use the number anyway

Take the predicted time as an optimistic floor and adjust upward when any of these apply:

• your longest run stayed well under 30 km;
• it is your first marathon, so you have never met the wall;
• you have no fueling strategy rehearsed at race pace.

A reasonable working target is the prediction plus five to ten minutes for a first marathon, and a first 10K run at that adjusted pace rather than the raw one. For the full picture of where the equation holds and where it does not, see how accurate the Riegel formula is, and use the race time equivalency calculator to see every distance side by side.