Is it possible to calculate a predicted marathon time based on your half marathon time?
The simple answer is yes! There are lots of race time prediction calculators available on the internet. Perhaps the most famous of these is the Jack Daniels VDOT Running Calculator. But there are many others as well.
Of course, the question is how accurate are these calculators. Can you trust any of these race time prediction calculators to provide you with an accurate predicted marathon time based on your race time over another distance?
In order to answer this question, I am going to focus on whether online race prediction calculators can accurately calculate a predicted marathon time based on times achieved over the half marathon distance and the 10Km race distance. I am choosing these two predictor distances for two reasons. Firstly, they are very popular race distances. Secondly, they are close enough in distance to the marathon distance to have relevance. Most of the online race prediction calculators will predict marathon times from a whole range of other race distances, right down to one mile. Clearly, a race distance that is further removed from the marathon will be less suitable for use in predicting marathon times.
Why Bother With Predicted Marathon Times?
But why would we even want to calculate a predicted marathon time based on your half marathon time or your 10km time? Surely you can just go and run the marathon itself and find out directly just how fast you can run a marathon?
Well, there are a few reasons why you would want to have an accurate predicted marathon time.
The most important reason is that it will enable you to pace yourself appropriately on race day. Without an accurate idea of what kind of pace you are capable of sustaining for the marathon distance, you run the risk of setting off either too fast or too slow on race day. Inevitably, this will lead to you underperforming on marathon race day.
The next most important reason is that it will improve your ability to fine tune your training for race day. Key training sessions for elite marathon runners are long runs at close to target marathon pace. The goal is to gradually increase your tolerance to maintaining target marathon pace at longer and longer distances as you approach race day. Without an accurate idea of your predicted marathon time, it will be very difficult to perform this type of training to a satisfactory standard.
Aside from these major two reasons for needing an accurate predicted marathon time, there is also the curiosity factor. Even if you have zero intentions of racing a marathon, you might just be curious about what kind of marathon performance your 10Km time or half marathon time would equate to.
So let’s kick things off by comparing the outcomes from some of the more popular online race time prediction calculators.
Pat Riegel Marathon Time Predictor
The Jack Daniels VDOT Running Calculator, the Runners World Race Time Predictor, and the Pete Riegel Race Tim Predictor are perhaps the three most pre-eminent race time prediction calculators.
Interestingly, the Runners World Race Time Predictor is heavily influenced by the Pete Riegel Race Time Predictor. Despite this, Runners World predictions are more similar to the Jack Daniels VDOT predictions than they are to the Pete Riegel predictions. The take home point from this is that the Runners World calculator is probably an updated and improved version of the Pete Riegel calculator. As such, I would recommend lending greater credence to the predicted times from the Runners World Calculator and the Jack Daniels VDOT Calculator.
Jack Daniels Marathon Time Predictor
If we now compare the Jack Daniels VDOT Running Calculator to the Runners World Race Time Predictor, we can notice a particular trend.
With higher performance levels (2:30 predicted marathon time), the Jack Daniels VDOT Calculator suggests that slightly faster half marathon and 10Km times are required in comparison to the results produced by the Runners World Calculator. With lower performance levels (3:30 predicted marathon time), it is the other way round. However, with an intermediate performance level (3 hour predicted marathon time), the two calculators produce an almost identical result with respect to the given half marathon and 10Km times that would result in a predicted marathon time of 3 hours.
On the basis of this initial analysis, it would appear that both the Jack Daniels VDOT Running Calculator and the Runners World Race Time Predictor produce very accurate results when it comes to determining what race times over shorter distances would equate to a 3 hour marathon time. However, when we start to deviate from that 3 hour marathon performance level, whether that be faster or slower, it is likely that the truth lies somewhere in between the two.
Runners World Marathon Time Predictor
If we were to end the analysis at this point, that would already be very useful. We would have the following take home points:
If a runner is targeting a 3 hour marathon, then either the Runners World Race Time Predictor or the Jack Daniels VDOT Running Calculator would be suitable. The runner could use either of these online calculators to determine if they are on schedule to be able to run a 3 hour marathon on the basis of their recent half marathon or 10Km times.
If a runner is targeting either significantly faster than 3 hours for the marathon or significantly slower than 3 hours for the marathon, then it would be recommended to use both the Runners World Race Time Predictor AND the Jack Daniels VDOT Running Calculator. In this instance, the runner would input their 10Km or half marathon time into both calculators. This would result in two different predicted marathon times. The difference between the two predicted marathon times would then be split in order to produce a more accurate predicted marathon time.
However, we are going to now take this analysis one step further by looking at how the running calculators stack up in the real world.
UK National Marathon Rankings
For the above analysis, in order to look at the full spectrum of the UK national rankings for this year (2022) I decided to look at the top ranked, the 50th ranked, 100th ranked athlete etc. all the way up to the 500th ranked athlete. After that I aimed to include the 1000th, 2000th and 3000th ranked athletes. You will see that Christopher Peck is listed as being ranked 449th. Also, a number of athletes are included who are ranked 2995. This is because in these cases, there were a number of athletes equally ranked, which meant that there was not a 500th ranked athlete and there was not a 3000th ranked athlete.
The half marathon times listed for the above athletes represent the fastest half marathon time that particular athlete achieved within a three month period of their marathon ranking time. In some instances, athletes did not have a half marathon time achieved within that three month period. In those cases, they were omitted from the analysis.
Of particular note is no.1 ranked athlete, Johnny Mellor. His half marathon time was just one second slower than what the Jack Daniels VDOT Calculator predicted based upon his marathon time. That is a huge vote of confidence for the Jack Daniels calculator. In fact, with the exception of Jack Nixon, whos’ half marathon performance seems like an anomaly, both calculators appear to be able to predict times in the fast runners (2:30 or quicker) with exceptional accuracy; with the Jack Daniels VDOT Calculator appearing to have the slight edge in terms of accuracy.
At this point, it is worth looking at the faster end of the spectrum in more detail.
Top Ranked UK Marathon Runners
For this new sample of marathon runners, we have used the same inclusion criteria a previously (must have ran a half marathon within three months of the marathon ranking performance), and we have included the first 10 athletes in the rankings that met this criteria.
In the previous analysis the Jack Daniels Calculator held a slight edge. However, with this new analysis which is purely looking at elite level athletes, the pendulum has swung and the Runners World Calculator is providing more accurate predictions. The overall ability of these calculators to make accurate predictions seems slightly weaker in this elite group as well. This might not have been expected, based on the breakdown of the previous sample. But it does go to show that, whilst these prediction calculators can give us a fairly good idea, there is a limit to how accurate we can expect them to be.
It is not entirely surprising that there is a limit to the accuracy we can expect from race time prediction calculators. All athletes are individual in make up. Some athletes approach a certain event from more of a speed bias whilst others approach the same event from more of an endurance bias. Those athletes may perform to the same level as each other over a given race distance. However, their ability to take that form from one event to another event will depend on their individual make up.
Extrapolation Method of Predicted Marathon Times
Given the above, there’s another avenue we can explore to more accurately use prediction calculators.
For any athlete, we can calculate two separate predicted marathon times. One predicted time will be based on 10Km and the other will be based on half marathon performance. Given some athletes approach an event from a speed bias and others approach the event from an endurance bias, it is likely that for some athletes predicted marathon times will be quite different when based on 10Km performance compared to when based on half marathon. This could actually help us create more accurate predictions, since we can extrapolate the difference in predicted marathon times.
Here’s an example:
Martin used the Jack Daniels Calculator to predict his marathon time based on his best 10Km. The calculator predicted he would run 3:00.00. Martin then used the calculator to predict his marathon time based on his best half marathon. This time the prediction was a time of 3:05.00. At first glance, you might assume that the answer lies between the two. However, this is showing a trend for the predicted marathon time to get slower when based on performances over longer distances. As such, we may need to extrapolate that trend up to the marathon distance itself.
Since the half marathon is half as long as the marathon, and the 10Km is about half as long as the half marathon, we need to add on the same difference again between the two predicted times. In our example, Martins’ predicted time based on his half marathon is five-minutes slower than his predicted time based on his 10Km. This means his extrapolated time is 10-minutes slower than the time that was based on his 10Km performance. So for Martin, a predicted marathon time of 3:10 is likely to be more accurate.
Extrapolation Method in UK Marathon Runners
I have tested out how this theory works in reality with the same set of athletes as in the previous sample.
With this analysis, I have used each athletes best ever times over 10Km, half marathon and the marathon.
The extrapolation of predicted times appears to hold up fairly well against the athletes actual best marathon times. However, there is still a reasonable margin of error apparent here. Once again, this shouldn’t come as a huge surprise. I am sure we all have Pb’s at certain distances that are not really a true reflection of our capabilities. If this is true for any of the above athletes in any of the three distances of relevance here, then it will distort the accuracy of the predicted times.
As ever, though, it pays to have great attention to detail. If we take on board the notion that it is likely that, for at least some of the above athletes, either their best 10Km time or their best half marathon time will not be a true reflection of what they are capable of over that distance; then there may just be a work around solution….
The Marathon Solution
Indeed there IS a work around solution!
For each of the athletes above we have three different predicted marathon times:
- Predicted time based on 10Km performance
- Predicted time based on half marathon performance
- Extrapolated time based on both 10Km and half marathon performance
If we now decided that we can choose any one of those three predicted marathon times for each respective athlete, all of a sudden we are in a situation whereby eight out of the ten athletes have a predicted marathon time that is within just 33 seconds of their actual best ever marathon time. In addition to this, of the two athletes that do not fall within this bracket; Weynay Ghebresillasie has a predicted marathon time (based on his best half marathon performance) that is barely more than a minute slower than his actual marathon personal best.
This is actually quite an astonishing level of accuracy.
But how do you, as an aspiring marathon runner, actually use this information?
Self-Reflection – Do you have any ‘Weak’ Pb’s?
Well, I believe most athletes in their heart of hearts do know whether their Pb at any given distance is really a true reflection of their capabilities. There are many reasons why this might be the case. You may have very little experience racing over that particular distance. Or you might have been in shape to run much quicker than your current Pb at a certain race, but the weather that day was terrible, or perhaps it was a really hilly course. Whatever the reason might be, you need to have an honest self-reflection, and decide for yourself whether your 10Km Pb is a true reflection of what you are capable of and whether you half marathon Pb is a true reflection of what you are capable of.
Once you have done this, here’s what you need to do:
- If your half marathon Pb is a true reflection of your capabilities over that distance, but your 10Km Pb is not, then simply input your half marathon Pb into an online running calculator and use the result of this as your predicted marathon time.
- If your 10Km Pb is a true reflection of your capabilities over that distance, but your half marathon Pb is not, then simply input your 10Km Pb into an online running calculator and use the result of this as your predicted marathon time.
- If both your 10Km Pb and your half marathon Pb are true reflections of your capabilities over those distances, then use the extrapolation method outlined above in order to determine your predicted marathon time.
Accurate Predicted Marathon Time
If you go about ascertaining your predicted marathon time in this manner then you are almost certain to come up with an extremely accurate prediction. Of course, it is still not fool proof. You are not guaranteed to run very close to that predicted time on marathon race day. Just as with the shorter distances, circumstances can sometimes dictate that you do not run to your full potential on race day. Especially so, since we are talking about the marathon here. Factors such as race day fuelling, hydration, and pacing can all have a much bigger impact on the end result of a marathon.
Of course, there is always a spanner in the works to consider as well…
What if neither your half marathon time or your 10Km time are true reflections of your capabilities over those distances? Well, you could use an online running calculator to provide you with a predicted marathon time based on your performance over shorter distance, such as 5Km. But, I wouldn’t recommend that. As the race distances become further and further removed from marathon race distance, they will correlate less well with marathon performance. So my honest recommendation, for someone who feels that neither their 10Km Pb or their half marathon Pb are true reflections of their capabilities over those distances, is to first focus on getting those distances nailed down. Don’t even worry about what you might or might not be capable of over the marathon distance until you have gained some more experience at 10Km and the half marathon.
Jack Daniels Running Calculator
However, for those of you that DO have a 10Km Pb and / or a half marathon Pb that is a good reflection of your capabilities over the distance, you can follow the link to the Jack Daniels VDOT Running Calculator below. So, go ahead and input your details and see what it comes up with!
Jack Daniels VDOT Running Calculator
Tim Egerton
The EGG
Egerton’s Garage Gym
Foxwood Personal training,York