How To Predict Your Race Time


I use this formula to predict my race time. It's quite accurate. Google "Pete Riegel" for more information.


Example 1:
If your 5k race time is 20:00. Your 10k race time will be:
20:00 * (10 / 5) ^ 1.06 = 41:42

Example 2:
If your 21.1k race time is 1:51:13. Your 10k race time will be:
1:51:13 * (10 / 21.1) ^ 1.06 = 50:24

This formula can apply under most circumstances for the race time prediction between 5km to 42km and if you do not require very accurate timing. However, if you want to be very precise then you might need to read the following sections.


Everyone is different
The value of 1.06 given for the exponent is quite unachievable for most of the people. It's different for everyone. You will have to experience and adjust the exponent value yourself. For example: mine is somewhere around 1.07~1.08 for most of the time. If you are a sprinter type of runner, then it's maybe somewhere around 1.09~1.10. If you are a gifted endurance runner, then it's maybe 1.05~1.06. You will also have to change according to the various conditions. There are a few factors affecting its accuracy.

The accuracy of the distance
How's the distance measured? Is your 5km a true 5km? Is it measured by GPS, Google Map or by any tools? Is it an AIMS/IAAF certified route? How are the slopes distance measured? Are they point to point? Are the inclinations considered?

If they are AIMS/IAAF certified route then the 42.195km will be exactly 42.195km regardless what your GPS device tells you. If the route is not certified, I will collect different GPS distances measured by different runners, remove those inaccurate measurements. Then we take the mean. I only familiar with Garmin watch so for example if I want to pick, I would pick FR910xt and FR310xt measurements. Load their GPS map and see whether the measured points taken are accurate or not. If they jump around and not inline with the route then they are not accurate. Some GPS devices take the slope distance into consideration like Garmin Fenix and Tactic so they tend to give longer distance.

The other way to measure the route is to use the online distance measurement tool like Google Map, Mapmyrun or Wikimapia. They give more accurate distance than the GPS devices.

Also, the way you run the route also have to be considered. For example, did you cut the corner? Did you run the shortest distance?

The more accurate your distance is the more accurate your prediction is.

Is the route hilly?
A hilly course is going to be slower than a flat course, all else being equal. But how much does a hill slow you down? According to Jack Daniels, the author of Daniels Running Formula, there is a “rule of thumb” states that every percent gradient of incline (uphill) will slow you by 7.5-9.3 seconds per km, and every percent gradient of decline (downhill) will aid you by 5 seconds per km. I'm not sure how accurate it is because the uphill and downhill difference should be different according to your running pace. It shouldn't be a static number. You will have to figure out yourself.

It’s surprisingly easy to figure out the incline of a particular hill, either by using a GPS device or an online tool like Mapmyrun. For example, 600m distance and rises 27m, that’s a gradient of 4.5%, and over 600m, we would expect that segment to be 20-25 seconds slower.

Uphills would slow a runner much more than the downhills would help a runner so even if it's back to the same sea level, if there are a lot of uphills and downhills on your course then you would have to expect your race time to be slower.

Same sea level?
Is the two races that you are trying to compare at the same sea level? You can find out more on this site: There is one very interesting paragraph:
Going Up:Every 1% upgrade slows your pace 3.3% (1/30th)Every 100 feet of elevation gain slows you 6.6% of your average one mile pace (2% grade/mile).Example: A race that climbs 300 feet would slow an 8-minute miler (3 x .066 x 8 x 60 seconds) = 94 seconds slower at the finish
Going Down:Every 1% downgrade speeds your pace 55% of 3.3% = 1.8%Every 100 feet of elevation descent speeds you 3.6% of your average one mile pace (2% grade/mile).Example: A race that descends 300 feet would speed an 8-minute miler (3 x .036 x 8 x 60 seconds) = 55 seconds faster at the finish
Running HighEvery 1,000 feet of altitude above sea level slows you 1% (up to 8,000 feet, then all bets are off)Example: A race at 3,000 feet would slow an 8-minute miler (3 x .01x 8 x 60) = 14.4 seconds per mile, or 6:20 total in the marathon.
You will have to take this into consideration if you are running at the different elevation.

The weather
Is the weather of the 2 races that you are comparing the same? If you are predicting your 10k summer race with a 5k winter race time then the prediction will not be accurate. The cooler your race is the faster your race time is.

How much temperature affects your run, you can refer to this calculator:

Also running into headwind will slow a runner much more than the tailwind would help a runner.

Your race day body condition
After all, it's down to your race day condition. Whether your body is fit enough on race day or not. Do you have enough sleep? Do you consume any caffeine drink or food before the race? Do you wear cushioning shoes or racing flat? What time is your race? Do you stand a lot or travel on foot a lot pre-race? Do you have pre-race stress or living stress? Did you just recover from sick or injury? Did you lose or put on weight after your last race? Did you have sex pre-race? Did you taper correctly? All these factors will more or less affect your performance.

Your race day strategy
Do you use the right strategy? Do you hydrate enough during your marathon? Do you consume refill properly during the run? Do you easily affected by other runners? For example, when someone over take you, will you be easily be provoked and the pace is affected by the runner? Do you have to run zig-zag to overtake the starting pack? All these factors will affect your performance too.

Is your training sufficient
OK, you have used the calculator to convert your half marathon race time to your equivalent marathon race time. So your half marathon race time is 1:30 and your projected marathon race time is 3:08. You have consider the hills, the weather, the elevation again, your body weight, your body condition, your food intake, your lifestyle, etc. Now, you think your training that successfully made you complete your half marathon is sufficient to make you to complete the marathon too then I would guarantee that you will hit the wall very hard and not only you can't hit 3:08 and maybe you will end up doing 4 hours ish.

Why? The reason is simple. This formula only tell you your projected race time. It's a conversion of your running activities in the "endurance range", namely lasting between 3.5 minutes to 230 minutes (this is according to Pete Riegel, for most of the amateur runners it seems only accurate in the range between 15 minutes to 180 minutes). Theoretically converting a longer run to a shorter run is always accurate and most of the time is achievable but converting a shorter run to a longer run, you will have to do the distance specific training that sufficient for your long run in order to achieve your projected race time. For the above example, your half marathon training will not be sufficient for you to run your project race time marathon.

A lot of people complaining that when converting to marathon or a race that last for more than 180 minutes it's seems unachievable. Most of the time it is because of lack of distance specific training or maybe you just uncovered your other weaknesses that only expose when your race time is longer. Eg. calf overused or hamstring overused.

There are many factors playing when you want to get a very accurate prediction. There is no such things as very accurate. So, I would just screw the "advance" part, use the formula describe at the top of the page. Simple as that! :)