To predict your race time for an upcoming race you can use the Riegel formula and apply to a recently completed race. While this formula gives you a good approximation there are a few caveats.
The Riegel Formula
Tgoal = Tprev x (Dgoal / Dprev)R
- R is the Riegel constant at 1.06
- Dprev is the previously completed distance
- Tprev is the finish time for the completed race
- Dgoal is the goal race distance
- Tgoal is the predicted time
Doing the Math
I will demonstrate how to do the calculations through an example. Lets assume that you completed a 10K race in 0:55:37 hours and your goal race is a marathon.
Dprev = 10K Tprev = 1:05:37 Dgoal = marathon Tgoal = ?
To simplify the calculations I prefer to convert time to seconds and distance to meters.
Dprev = 10000 m Tprev = 1 x 3600 + 5 x 60 + 37 = 3937 s Dgoal = 42195 m Tgoal = ? s
And now plug all these numbers into the formula.
Tgoal = 3937 x (42195 / 10000)1.06 = 3937 x 4.21951.06 = 3937 x 4.6 = 18110 s
And convert this back to hours, minutes and seconds.
18110 / 3600 = 5 (hours) remainder 110 110 / 60 = 1 (minutes) remainder 50 (seconds)
Therefore the predicted time is 5:01:50 hours.
Issues and Corrections
The main issue with the Riegel formula is that it was developed by analyzing the finishing times of elite runners. That means it works very well for super-fast runners, but for the slower ones it will be far from punctual and underestimated the finishing time for the goal race.
To combat this problem all you have to do is use a different Riegel constant. The slower you are the higher this value should be.
- 1.06 for elite
- 1.07 – 1.10 for faster than average
- 1.11 – 1.15 for average
- 1.16 – 1.20 for slower than average
Training for the Target Race
The most important thing in the whole formula is that you properly train for your target race.
No matter how fast you ran your 5K race if you never run more than 10K in your life, your marathon will be a disaster and predicting a finishing time for it is useless.
When to Run the Test Race
To do the prediction you need to complete a test race. Timing this well is the key to a good prediction.
If you run it too far ahead of your target race it won’t be a good candidate for prediction as your fitness level can increase or decrease significantly until you reach yoru goal race. On the other hand, if you run it too close to your goal race you may not have enough time to recover from it.
Also running a race will impact your training, so you need to take that into account and change your training plan accordingly.
Therefore your goal should be to run your test race as close to your target race as possible without significantly impacting your training.
While elite runners can complete a shorter race 4-5 weeks before their target race, for most everyday runners that won’t give enough time for recovery, especially if your target race is a marathon.
In my opinion the best timing is somewhere 4-8 weeks before your target race, the longer your test and target races are the further away.
E.g. half-marathon 8 weeks before a marathon; 10K 6 weeks before a marathon; 5K 4 weeks before a 10K.
As you can see predicting a finishing time for a running race is not as simple as putting numbers into a magic formula and looking at the result.
Training properly would be your best bet to finish a race as fast as possible and running one or a few shorter races beforehand can help you to measure your fitness level.
To help you with the math there is a calculator using the Riegel formula that you can use to predict your race time.