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.
The calculator is based on the simplistic idea that, all other things being equal, changes in weight will effect changes in your VO2 Max, and therefore performance. 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 .
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, ensure you're meeting your nutritional requirements and that energy levels and running performance are not negatively impacted.
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.