How many calories does my run burn?

I know how far my run is and how long it takes, but how can I calculate the number of calories I’ve burned in every run without using a gadget?
Jeff Holmes, by email

Matt Hart answers: Unfortunately even a heart rate monitor can only estimate the calories you burn – to calculate calorie output without one would require the mathematical genius of Einstein. Your own biomechanics and bodyweight, wind speed, terrain and altitude all affect your calorie output, so you’ll never get an accurate answer. But you can maximise your calorie burn by progressing your run every time you do it. Get out of your comfort zone and add distance or aim to reduce the time, every time, even if it’s only by a few seconds.