How the predictor works
Your race is turned into a VDOT — one number for your running fitness, from the Daniels–Gilbert formulas. Those formulas relate the oxygen cost of running at a speed to the share of your maximum you can sustain for a given time. The predictor then solves them the other way: for each distance, the time at which the same VDOT comes out.
Because the share you can sustain falls as a race gets longer, the model slows you down for longer races by the right amount without a separate rule for each distance. It is built for road races from the mile to the marathon. It is not valid for ultras, and it knows nothing about hills, heat or trails.
Using the paces in training
- Threshold pace is for tempo runs and cruise intervals: 20 to 40 minutes of work in total.
- 5K and 10K pace are for intervals of 2 to 5 minutes with short recoveries.
- Marathon and half marathon pace are for the steady sections of long runs.
- Easy runs are slower than all of them: one to two minutes per mile slower than marathon pace.
The realistic marathon time
The marathon is where every fitness-based predictor is too kind. When Andrew Vickers and Emily Vertosick collected the race times and training of 2,303 recreational runners, the standard formula was ten minutes or more too fast for about half of them. What explained the gap was weekly mileage. Enter your typical week and the predictor adds their published model beside the headline: the marathon time run by people with your race result and your mileage. It needs a race between 5K and the half marathon and a week of 5 to 100 miles.
Why a prediction can be wrong
The model assumes you are equally prepared for every distance. A 5K specialist will miss a marathon prediction; a high-mileage runner will beat a 5K one. An old result predicts your old fitness, a hilly or hot race understates it, and a time from a GPS watch on an unmeasured course can be off by a minute or more.
Common questions
How accurate is a race time predictor?
It is accurate when the two races are close in length and you have trained for both. A 10K predicts a half marathon well. A 5K predicts a marathon poorly unless your long runs and weekly volume are already marathon-sized, because the prediction assumes equal fitness at every distance.
Why is my marathon prediction faster than I can run?
The marathon is limited by endurance and fuelling more than by speed. The prediction is the time your aerobic fitness supports; you only reach it with the long runs and the weekly volume to back it up. In a study of 2,303 recreational runners, a fitness-only prediction was ten minutes or more too fast for about half of them. Enter your weekly mileage and the predictor shows the time runners like you actually run.
What is VDOT?
VDOT is a single fitness number worked out from a race result, from the running formulas of Jack Daniels and Jimmy Gilbert. Two runners with the same VDOT are expected to run the same times at every distance. A 20-minute 5K is a VDOT of about 50.
What is threshold pace?
Threshold pace is the fastest pace you could hold for about an hour of racing. It is comfortably hard: you can say a few words, not sentences. It is the pace for tempo runs and cruise intervals.
How fast should my easy runs be?
Slower than most runners expect: usually one to two minutes per mile (about 40 to 75 seconds per kilometre) slower than your marathon pace. Easy means you can hold a full conversation.
How does Fit PA predict my race times?
Fit PA uses the same formulas as this page, but it finds the race for you: it reads the best efforts inside every run your watch has recorded over the last 12 months, checks them against each other so a GPS glitch cannot set your fitness, and updates the predictions as new runs arrive.
More free tools
- Heart Rate Zone Calculator — Your five training zones from max heart rate, heart rate reserve (Karvonen) or threshold heart rate, and what each zone is for.