Kettles boiled by my workout
Your body turns energy into heat when you exercise. See how many kettles of water your workout could boil.
Your details
Running, 6 mph (10 minute miles), as kettles
4.4 kettles
1.0 litres from 15 degrees to boiling
- Energy your body used376 kcal, 1,572 kJ
- One kettle356 kJ (4.186 kJ per kg per degree)
- Cups of tea (250 ml each)18
Muscles turn only about a fifth of their energy into movement; the rest comes out as heat, which is why you warm up. This ignores heat lost from the kettle, so a real kettle needs a bit more.
Stop and get help if you have chest pain, feel faint or are unusually breathless, and call 999 if chest pain does not go away. If you have a heart, lung or joint condition, check with your GP before stepping up.
The working
30 minutes running at 6 mph, 73 kg
The run uses about 376 kcal in total, 1,572 kJ. Heating 1 litre from 15 to 100 degrees takes 4.186 x 1 x 85 = 356 kJ, so that is 4.4 kettles, or 18 cups of tea.
Where the energy goes
Muscles and heat
Muscles turn only about a fifth of the energy they use into movement. The rest becomes heat, which is why you get hot and sweaty, and why a crowded gym warms up so fast.
Kettle facts
For tea lovers
- Boil only the water you need: it saves energy and time
- A full 1.7 litre kettle needs about 600 kJ from cold
- Tap water is colder in winter, so kettles take longer
Read with care
What this cannot tell you
- Count heat lossesReal kettles lose some heat, so need a bit more.
- Measure your burnThis is an estimate from corrected METs.
- Make teaStill a manual job.
Questions
Common questions
How much energy boils a kettle?
About 356 kJ for 1 litre from 15 degrees, using water's specific heat of 4.186 kJ per kg per degree.
How much heat does exercise make?
Roughly 80% of the energy your muscles use comes out as heat.
Why do I get so hot running?
Running uses energy fast, and most becomes heat that you shed by sweating.
How many calories is a kettle?
About 85 kcal to boil a litre, roughly what a gentle 20 minute walk uses in total.
Sources
- Specific heat capacity of water: about 4.2 kJ per kg per degree (BBC Bitesize rounds 4,186 J to 4,200)
- Herrmann SD et al. 2024 Adult Compendium of Physical Activities. J Sport Health Sci 2024;13:6-12 (codes noted beside each value; two walking and running values are from the 2011 Compendium)
- ACSM's Guidelines for Exercise Testing and Prescription: metabolic equations for walking (VO2 = 0.1 S + 1.8 S G + 3.5) and running (VO2 = 0.2 S + 0.9 S G + 3.5)
Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.
Stop and get help
Stop and get help if you have chest pain, feel faint or are unusually breathless, and call 999 if chest pain does not go away. Check with your GP before stepping up if you have a heart, lung or joint condition.
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