Riegel
T2 = T1 × (D2 ÷ D1)1.06
World records follow this curve for efforts of about 3.5 to 230 minutes. Outside that range the curve bends, so those predictions get an asterisk.
Race time predictor
Riegel and Cameron both stretch one recent race out to the other distances, and they don’t agree. Each bar runs from one to the other: the wider it is, the less the prediction holds.
h:mm:ss
e.g. 24:30 or 1:52:10
Why Riegel and Cameron split
Both formulas extend your result along a curve fitted to the best performances on record. They assume you’re as ready for the target race as you were for the one you ran: fair between a 5K and a 10K, often wrong for the marathon.
Cameron
v = 13.49681 − 0.048865 × d + 2.438936 ÷ d0.7905
Speed in mph, distance in miles, fitted to the best times from 400 m to 50 miles. Your race sets a personal factor that the tool applies to every other distance. The numbers match Cameron’s published chart.
The color
effort = share of VO2max you can hold
Each race is colored by the share of your VO2max you can hold for that long (Daniels-Gilbert). Under about 20 minutes, above 95%: Z5. Up to about 2:20, 83 to 95%: Z4, threshold. Longer than that: Z3, marathon effort. More on the VO2 max calculator.
A formula doesn’t know your training
A prediction holds only if you’ve trained for the target distance: long runs and weekly mileage for the marathon, faster work for the mile. Riegel assumes you’ll hold pace for the last 10K; your legs at mile 22 may disagree.
Frequently asked questions
How accurate is a race time predictor?
Between nearby distances, say 5K to 10K, both formulas usually land within a minute or two, as long as you’ve trained for the new distance. The further you stretch, the shakier it gets. A marathon predicted from a parkrun is a best case, not a pacing plan.
Can I predict my marathon time from a half marathon?
Yes, and it’s the best race to start from. Even so, in a survey of 2,303 recreational runners, Riegel predicted the marathon at least 10 minutes too fast for half of them. Weekly mileage explains most of the gap: on low mileage, you slow down more over the last 10K.
Why do Riegel and Cameron give different times?
Riegel uses one fatigue exponent for every distance. Cameron fitted his curve to the best times from 400 m to 50 miles, so his slowdown changes with distance. From a 25:00 5K, both say about 52 minutes for the 10K, but Riegel says 3:59:47 for the marathon and Cameron 4:03:59. When they disagree, plan around the slower one.
Can I use my parkrun time?
Yes, if you ran it hard on a typical course. A relaxed parkrun, a hilly or muddy course, or a crowded start makes every prediction slower than you can run.
What exponent does the Riegel formula use?
1.06, the value almost every calculator uses. Fitting world records, Riegel got 1.077 for men and 1.083 for women. A higher exponent means slower predictions over longer races.
Sources
- Riegel PS. Athletic records and human endurance. Am Sci 1981;69(3):285–290.
- Cameron DF, model description (archived page).
- Vickers AJ, Vertosick EA. BMC Sports Sci Med Rehabil 2016;8:26.
- Daniels J, Gilbert J. Oxygen Power: Performance Tables for Distance Runners. Tempe, AZ, 1979.