Weight vs. Pace Calculator
Weight Loss & Running Times
This calculator is based on an idea from Daniels' Running Formula. The suggestion is that changes in weight following a break from running will result in changes in VO2 Max (or VDOT), which will affect training paces and race performance.
Many runners are intrigued by the idea that they can run faster times by losing weight. But it's not as simple as " lose weight, run faster". In fact, a study carried out on the correlation between BMI and 50 meter running speed in high school students showed that those with a greater BMI tended to run faster!
It's true that losing any non-functional weight can help improve your performance. And conversely, adding non-functional weight can see times slow as a result. However, losing weight when it's unnecessary can actual have a negative effect on your running performance. By non-functional weight we mean any weight that is not necessary for health, wellbeing and the various components of fitness.
This calculator works on the simplistic idea that, all other things being equal, changes in weight will cause changes in your VO2 Max, and therefore performance. Coach Jack Daniels suggests that such a calculation can be useful for those returning to running after a break who may have gained weight due to decreased activity. He also provides calculations to help determine how much fitness may have been lost due to inactivity.
Wikipedia has a good description of VO2 Max. Simplifying, it is a number that indicates the maximum amount of oxygen your muscles can make use of during exercise. If you have a higher VO2 Max then you can make better use of oxygen, so your muscles will work more effectively, so you'll be able to run more quickly.
VO2 Max is usually expressed as the rate of millilitres of oxygen used per minute for each kilogram of body mass:
VO2 Max = oxygen used per minute / body mass
If "body mass" is lower in the above equation, then VO2 Max will be higher. This suggests that losing weight will result in a higher VO2 Max. This conclusion comes with the rather large requirement that other things remain equal.
In reality, other things are rarely equal. For example, losing weight will mean you have less of it to carry around and therefore less work to do, but some of the loss will probably be muscle mass, so you'll end up weaker and may end up less able to transport that reduced weight. And if you're already underweight, then further weight loss will certainly weaken performance.
There are also examples of functional weight that aren't directly linked to our longer-term body composition. For example, over the short term, weight can fluctuate because of hydration levels and energy stores. Weigh yourself before and after a long run to see this in action!
One difficulty in determining the precise impact of weight gain or weight loss on running paces is that useful changes in body composition take time. And during that time various other things that directly contribute to fitness will also change. Bear in mind that training volume, frequency and intensity will generally have a far greater influence on fitness than body mass.
For example, a runner who gains 6 pounds when injured and unable to train, and who then notices a reduction in performance when returning to train, would struggle to determine the proportion of that reduction that was due to extra mass and the proportion that was due to lost training time. Each will have had an impact, but the precise contribution of each is very difficult, if not impossible, to work out.
Some suggest that it's possible to gauge the effects of extra mass by running with a weighted backpack or weighted vest, but this ignores the distribution of mass that occurs in real-life and the ratio of functional to non-functional mass.
With that all said, it may seem that the calculator below is not particularly helpful. However, it does have some uses. For example:
- A runner who is slightly above their ideal race weight, and planning to lose that weight between now and a race, could use the calculator to help predict their probable race time. They can then adjust training paces, perhaps incrementally as weight is lost, in accordance with this.
- A runner who has gained some weight without any reduction in training could use the calculator to indicate how they should adjust their training paces or how to approach a race.
Beyond a few pounds or kilos in either direction, the predictions will become less and less reliable. If you need to lose a lot of weight, then it's important to continually reassess your performance and training and racing paces.
How to use the calculator
To use the calculator simply enter your weight, choose or enter a race distance, pick a time or pace and hit "Calculate". The results table shows the predicted effects of weight loss/gain on times for your chosen distance.
Please note that the calculator doesn't make allowances for age, height and sex, so it's entirely possible that it will falsely suggest that faster times are possible at unachievable and unhealthy weights. It also assumes that you are above optimal race weight.
The estimates are also for races carried out on the flat rather than uphill or downhill. Extra weight tends to have a greater impact when running uphill and less of an impact when running downhill.
A final note
If you do need to lose weight, then the best approach is a sensible diet that brings about gradual changes. This will minimise the loss of lean body tissue and ensure you're meeting your nutritional requirements so that energy levels and running performance are not negatively affected. Another benefit of gradual weight loss is that it is far easier to assess and monitor the impact of such loss and determine exactly what your ideal running weight actually is.
Avoid crash diets and gimmicks. These may result in short-term rapid weight loss, but are no magic bullet and you are very likely to see a drop in performance if you take this approach. The key point, as mentioned above, is that losing weight will not necessarily mean you run faster times. Determining your ideal running or racing weight requires serious consideration of many factors.
We like the approach to determining and reaching your ideal race weight laid out in Matt Fitzgerald's Racing Weight.
Many runners are keen to know the best types of running sessions to lose weight. It's difficult to give a simple answer. Obviously, more intense running will burn more calories, but it's not possible to maintain a high intensity for long, so your session duration will be limited. Tougher sessions also take longer to recover from, which means your overall running volume will be less than if you opted for less-intense sessions. The best approach to lose weight by running is to focus on using your running training to improve your running performance, and then adjust your diet according to your training output.
» Daniels' Running Formula by Jack Daniels
» Racing Weight by Matt Fitzgerald
» VO2 max/Daniels VDOT calculator