Marathon Predictor
Free marathon time predictor and race time predictor. Predict marathon time from a recent 5K, 10K, or half marathon using Riegel, Cameron, and adjusted formulas. Predicts every distance from 1 mile to 50K.
Target Time Locked In. Train for It on a Live City Map.
You have your predicted marathon time. Now build the mileage that gets you there. Motera makes every training run competitive: capture territory in your neighborhood, climb your local leaderboard, and watch the Fog of War lift as you log the miles.
MoteraFrequently Asked Questions
How accurate is the Riegel formula for predicting race times?
The Riegel formula is remarkably accurate for distances between 5K and marathon, especially for trained runners. It tends to be most accurate when predicting times for distances close to your input race. For example, predicting a half marathon from a 10K result is generally more reliable than predicting a marathon from a 5K. Most runners find predictions within 2 to 5 percent of their actual times when properly trained.
What is the difference between the Riegel and Cameron formulas?
The Riegel formula uses a simple power law relationship (T2 = T1 * (D2/D1)^1.06) and works well for most recreational runners. The Cameron formula uses a more complex fatigue curve that accounts for how fatigue increases at different rates for different distances. Cameron tends to give slightly faster predictions for shorter races and slightly slower ones for ultra distances.
Can I predict a marathon time from a 5K result?
Yes, but with caveats. The larger the gap between your input distance and predicted distance, the less accurate the prediction. A 5K tests your speed and VO2max, while a marathon tests endurance and fuel management. Someone who is fast at 5K but has not done long run training may find their actual marathon time is slower than predicted. For best results, use a 10K or half marathon time to predict a marathon.
Why do the three formulas give different predictions?
Each formula models the relationship between distance and fatigue differently. The original Riegel uses an exponent of 1.06, the adjusted version uses 1.07 (slightly more conservative for longer distances), and Cameron uses a curve-based approach. The range between them gives you a realistic window of what to expect on race day.
Should I use my training pace or race pace for predictions?
Always use your best recent race time or a genuine all-out time trial effort. Training paces are typically 30 to 60 seconds per mile slower than race pace. Using a training pace will give you predictions that are too slow. If you have not raced recently, do a solo time trial at a local track or measured course and give maximum effort.
How recent should my input race time be?
Ideally within the last 3 months. Fitness changes over time, so an old PR may not reflect your current ability. If you have been training consistently and improving, your current fitness may actually be better than your most recent race suggests. If you have taken time off, your predictions may be optimistic.
What does the performance level rating mean?
The performance level gives you context for how your time compares to other runners. Elite means you are in the top 1 to 2 percent of all runners. Advanced puts you in the top 10 to 15 percent. Intermediate is the middle of the pack, and Beginner means you are in the early stages of your running journey. Every level is an achievement and a starting point for improvement.
Do these formulas work for ultramarathons?
The formulas become less reliable beyond marathon distance because ultra running introduces variables like elevation, terrain, sleep deprivation, and nutrition strategy that shorter races do not have. The 50K prediction shown here should be treated as a rough estimate. For distances beyond 50K, specialized ultra prediction models that account for elevation and aid station stops are more appropriate.
How can I improve my predicted times?
The fastest way to improve race predictions is to actually train for your target distance. Include a mix of easy runs, tempo runs, intervals, and long runs in your training. Consistency matters more than intensity. Most recreational runners see significant improvements in their first 1 to 2 years of structured training, with 5K times dropping by several minutes and marathon potential improving dramatically.
How do I predict my marathon time from a half marathon?
The simplest method is to double your half marathon time and add 10 to 20 minutes. A 1:45 half typically becomes a 3:40 to 3:50 marathon. The Riegel formula gives a more precise prediction: marathon time = half time x (42.195/21.0975)^1.06. Predictions from the half are the most reliable because the distance and physiological demands are closest to the full marathon.
How do I predict my marathon time from a 10K?
Multiply your 10K time by approximately 4.66 to estimate your marathon time. So a 50:00 10K predicts roughly a 3:53 marathon. The Riegel formula gives a more accurate prediction. Be aware that 10K predictions assume you have done the long-run training needed for the marathon distance. Without weekly long runs, your actual marathon will likely be slower than predicted.
What is a marathon prediction chart?
A marathon prediction chart is a reference table showing predicted marathon times based on common 5K, 10K, and half marathon results. The chart on this page covers 5K times from 18:00 to 35:00. Use it as a quick lookup, then use the calculator above for your exact result and three formula predictions.
Why does my marathon time predictor show a slower marathon than I expected?
The Riegel and Cameron formulas account for the natural slowdown over longer distances. If your training has been short and fast (lots of 5K work, no long runs), the predictor still applies the standard slowdown factor, which may match reality. If you have been doing 18 to 22 mile long runs, your actual marathon time may beat the prediction by a few minutes.
How It Works
Enter a recent race time
Pick a distance you have raced recently and enter your finish time. The more recent and accurate, the better your predictions will be.
Three formulas crunch the numbers
The Riegel, adjusted Riegel, and Cameron formulas each calculate your predicted finish time using different mathematical models of running fatigue.
Get predictions for every distance
See your predicted finish times for 1 mile through 50K, with pace per kilometer and per mile. The range between formulas gives you a realistic target window.
How the Riegel Formula Actually Works
The formula behind most race predictors, including the Riegel calculation this tool runs, comes from a 1981 paper by engineer and runner Pete Riegel titled "Athletic Records and Human Endurance," published in American Scientist. Riegel studied world record times across distances from sprints to ultramarathons and found that finish time does not scale in a straight line as distance increases. Instead it follows a predictable curve, because fatigue accumulates faster than distance does.
The equation is T2 = T1 x (D2 / D1) ^ 1.06, where T1 is your known time, D1 is that known distance, D2 is the distance you want to predict, and T2 is the predicted time. The exponent 1.06 is the part that captures fatigue. If running were purely linear, the exponent would be exactly 1.0 and doubling your distance would exactly double your time. Because the exponent is slightly above 1, predicted times grow a bit faster than distance, which matches how real races actually feel.
Worked example: 10K to marathon
Say you just ran a 10K in 45:00 (2700 seconds) and want to predict your marathon time. The marathon is 42.195 km, so D2 / D1 = 42.195 / 10 = 4.2195.
Raise that ratio to the power of 1.06: 4.2195 ^ 1.06 is approximately 4.53. Multiply by your known time: 2700 seconds x 4.53 is approximately 12,231 seconds, which converts to about 3 hours 23 minutes.
That is the same math the calculator above runs instantly, alongside the Cameron formula, which adjusts the fatigue curve based on the specific distance pair instead of using one fixed exponent for every distance.
Riegel's original paper: Riegel, P.S. (1981). "Athletic Records and Human Endurance." American Scientist, 69(3), 285 to 290, indexed on PubMed as PMID 7235349.
Tips for Accurate Predictions
Use a recent race for best accuracy
A race from the last 3 months gives the most reliable predictions. Old PRs may not reflect your current fitness. If you have not raced recently, do a timed effort at a local parkrun or track.
Predict up, not just down
Most runners use this to predict marathon times from shorter races. But it works both ways. If you have a marathon result, you can see what your 5K potential is and train specifically for that distance.
Train for the distance you want to race
Predictions assume you are properly trained for the target distance. A runner with a fast 5K but no long runs will likely run slower than their predicted marathon time. The formulas work best when your training matches your goal.
Use the range as your target window
The three formulas give slightly different results. Use the fastest as your A goal, the average as your B goal, and the slowest as your safe starting pace. This helps you race smart without going out too fast.
Watch: How Race Time Prediction Works
Prefer to see the reasoning explained out loud before trusting a number on a screen. This video walks through how coaches and runners use a recent race to set a realistic goal for a longer distance, and where predictions tend to break down.
Marathon Prediction Chart
This chart predicts marathon finish times from common 5K, 10K, and half marathon results using the Riegel formula. Use it as a rough guide. The calculator above gives you the exact prediction with three formulas, plus pace per km and per mile.
Predictions assume comparable training across distances. A runner with strong 5K speed but no long runs will likely run slower than the predicted marathon time. If your race sits well above sea level, use our altitude pace calculator to adjust your goal pace for elevation.
How to Predict Half Marathon Time from a 5K or 10K
The Riegel formula works just as well for half marathon prediction. Plug your 5K or 10K result into the calculator above and the half marathon prediction appears alongside the full marathon. As a quick mental shortcut, double your 10K time and add 7 to 9 minutes for a rough half marathon estimate. A 50-minute 10K usually translates to about a 1:50 half.
Predicting downward (long to short) also works. If you have a recent half marathon, the calculator will tell you what 5K and 10K times you should be capable of. This is useful for setting goals at shorter races and noticing if your 5K speed has fallen behind your endurance.
Why Race Predictions Are Sometimes Wrong
Mileage gap
A predictor cannot tell whether you ran 20 mile long runs in training. A runner with a strong 10K but only 30 weekly miles will almost always run slower than predicted at the marathon distance because the body has not adapted to the demand.
Race day conditions
Heat, humidity, hills, and wind can add 5 to 30 seconds per mile to your pace. The prediction assumes ideal conditions on a flat certified course. Adjust your goal pace down for hot or hilly races.
Race specificity
A pure speed athlete will outperform predictions at the 5K and underperform at the marathon. A pure ultra runner is the opposite. The Riegel formula works best when your training matches the target distance.
Old race data
A 5K from two years ago is not a useful input. Fitness changes month to month. Use a race result from the last 8 to 12 weeks for the most accurate prediction. If you have not raced recently, use a parkrun or a track time trial.
The Most Accurate Race Predictor: Tanda vs Riegel vs Cameron
Three formulas dominate race prediction. Riegel (1977) uses T2 = T1 × (D2 / D1) ^ 1.06. Cameron adjusts the exponent based on race distance, which makes it more accurate for marathons. Tanda uses weekly mileage plus average pace, which is why the Tanda race predictor is often cited as the most accurate race predictor for the marathon specifically. Our calculator runs Riegel and Cameron side by side so you see the range.
For a pure marathon race time predictor, the Cameron output is usually the safer goal. For a half marathon predictor or 10K prediction, Riegel is the gold standard. For first-time marathoners, take the predicted marathon time and add 2 to 3 percent to account for the back half slow down.
Cameron built his formula in the late 1990s by fitting a curve to elite race results across distances from 400 meters to 50 miles, using a fatigue function f(x) = 13.49681 - 0.000030363x + 835.7114 / x ^ 0.7905 that varies the exponent by distance instead of holding it fixed at 1.06 like Riegel does. That variable exponent is why Cameron tends to track closer to actual marathon results, where fatigue compounds more than a single fixed constant can capture.
Sources and Further Reading
This predictor is built on published, peer-reviewed math rather than a proprietary black box. If you want to read the original research or dig into the Cameron formula in more depth, these are the sources this page draws from.
Riegel, P.S. (1981). Athletic Records and Human Endurance. American Scientist, 69(3), 285 to 290
PubMed PMID 7235349, the original paper behind the Riegel exponent used in this calculator.
Peter Riegel
Background on the engineer and runner who developed the formula this calculator uses.
Cameron Race Time Predictor Formula
A worked explanation of the Cameron fatigue function used alongside Riegel in this calculator.
Half Marathon to Marathon Time and Other Conversions
Marathon time calculator from half marathon
Multiply your half marathon time by 2.11. A 1:45 half predicts a 3:41:45 marathon under good training. Use the calculator above for the adjusted Riegel and Cameron values.
Predict half marathon time from 5K
A 25:00 5K predicts roughly a 1:55 half marathon. The prediction assumes you have done at least four long runs of 10+ miles in the last 8 weeks.
Estimate marathon time from 10K
Multiply your 10K by 4.67. A 50:00 10K predicts a 3:53 marathon. Subtract 5 to 10 minutes if your weekly mileage is above 50.
Half marathon time to pace
Divide your half marathon time by 13.11 to get pace per mile, or by 21.0975 for pace per kilometer. A 2:00 half = 9:09 per mile or 5:41 per km.
How to Use Your Predicted Time to Train
A predicted time is only useful if it changes what you actually do in training. Once you have a number, turn it into training paces with our training pace calculator, which converts a goal time into easy, tempo, and interval zones rather than a single number to chase every run.
If your predicted marathon time feels ambitious, pick a training plan by goal time that matches it, rather than one built around a race you have not yet proven you can run. Log each workout and race in a PR tracker so you can see whether your times are trending toward the prediction or away from it, and adjust the goal race pace accordingly.
It also helps to sanity check a prediction against your aerobic fitness rather than just your last race result. Our VO2 max estimator gives a second, independent read on your current fitness level, so you are not relying on a single race performance that could have been an unusually good or bad day.
