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 achieve faster times by simply losing weight. Losing any non-functional weight can help improve your performance. And conversely, adding non-functional weight can see times slow as a result. 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.
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 to carry around and therefore less work to do, but some of the loss will probably be muscle mass, so you could end up less able to transport that reduced weight. And if you're already underweight, then further weight loss will actually weaken performance.
There are also examples of functional weight that aren't directly linked to our body composition. For example, over the short term, weight can fluctuate because of hydration levels and energy stores.
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 how much of that reduction was due to extra mass and how much was due to lost training time. Each will have had an impact, but the precise contribution of each is very difficult 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, again, 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 line 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 kilos or pounds 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.
A final note
If you do need to lose weight, then the best approach is a sensible diet that sees 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.
We like the approach to determining and reaching your ideal race weight laid out in Matt Fitzgerald's Racing Weight