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The gym bunny’s equation is simple enough: calories in versus calories out. After you’ve worked up a sweat in the gym, you should have gained the licence to treat yourself to a snack afterwards.

In practice, the maths is difficult to get right: it’s all too easy to overestimate how much we’ve burnt in a session, and underestimate the calories in a snack. The depressing result is that many people (up to 68%, in one study) hoping to shed pounds actually put on weight during their exercise regime.

Fortunately, BBC Future has scoured Harvard Medical School’s comprehensive table of the calories burned during everything from sex to long-distance cycling. Using this information, we equated exactly what it would take to burn off your favourite snacks. (The exact figures will vary from person to person – all the data here assumes an 11-stone, or 70kg, frame.)

You may be surprised by just how little you burn during seemingly energetic activities, such as sex

Compared to sleeping (which itself burns some calories), even something as simple as sitting at a computer, chewing gum or reading a book is equivalent to eating some modest nibbles. You may be surprised, however, by just how little you have earned during seemingly energetic everyday activities, such as sex – or how far you have to travel before you have burnt off a burger and chips. Exercise offers many benefits besides weight loss, of course. But if you are aspiring to a trimmer, more toned figure (or simply want to remain a stable weight), it’s worth knowing the facts before you hit the gym or raid the pantry.

That's 772 calories, based on measurements of the butterfly stroke and the front crawl (Credit: Olivia Howitt)

At more than 700 calories over the hour, exercise bikes offer a more intense workout than rowing machines or treadmills (Credit: Olivia Howitt)

According to the New England Journal of Medicine, if you chewed gum every hour you were awake, you would lose 5 kilograms of fat a year (Credit: Olivia Howitt)

Experienced runners can burn more than 1000 calories an hour as they work up a speed of 10mph (Olivia Howitt)

You can shake off nearly 400 calories an hour in disco and ballroom dancing (Credit: Olivia Howitt)

Standing rather than sitting would burn off an additional 11 calories (Credit: Olivia Howitt)

Another reason to be a bookworm: you burn off 28 more calories an hour reading compared with watching TV (Credit: Olivia Howitt)

Some are more energetic, however: one participant in the PLoS One study burnt three times as much as the group average (Credit: Olivia Howitt)

David Robson is BBC Future’s feature writer. He is @d_a_robson on Twitter.

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