Bridie, 30°C in the city
70 kg, 30°C, 2 hours outdoors
About 3.9 litres (6.8 pints), including 800 ml extra for the heat and time outside.
Heatwave Hydration
A heatwave hydration figure is a warmer-day fluid sketch. We start from a simple water guide based on your weight, then add millilitres for temperature above 20°C and hours outside. It is not a medical fluid prescription and not a diagnosis of dehydration.
Your details
Enter weight, daytime temperature and hours outdoors.
What is this calculator?
A heatwave hydration figure is a warmer-day fluid sketch. We start from a simple water guide based on your weight, then add millilitres for temperature above 20°C and hours outside. It is not a medical fluid prescription and not a diagnosis of dehydration.
How it works
- Enter weight in stone and pounds or kilograms.
- Enter the daytime temperature and hours you will be outside.
- Read litres and pints. Drink to thirst as well, and follow any fluid limit from a clinician.
Common questions
What is the heatwave hydration calculator?
A warmer-day fluid guide from weight, temperature and time outside. It is a planning figure, not a hospital chart.
How much should I drink in a heatwave?
NHS materials still talk about regular drinks and not waiting until you are very thirsty. This page only adds a number on top of a basic weight-based sketch.
Do tea and squash count?
Fluid counts. Alcohol does not help in heat. The water intake calculator is the cooler-day sister page.
Who should ignore a high litre target?
Anyone on a fluid restriction, or with heart or kidney advice from a clinician. Follow that advice.
Example scenarios
Parvez, long garden afternoon
82 kg, 33°C, 4 hours outdoors
About 5.7 litres (10 pints), including 2,080 ml extra. That is a high sketch. Sip, do not gulp.
Maeve, milder 25°C walk
58 kg, 25°C, 1 hour outdoors
About 2.7 litres (4.8 pints), including 200 ml extra.
Understanding your results
If the litre figure looks huge, treat it as a ceiling to sip toward, not a challenge. Dizziness, confusion or no urine are reasons to get help, not to refresh this page.
About this result
Guidance only. Calculations run in your browser. Numbers are estimates, not a diagnosis.
How we calculate
About 35 ml per kg, raised for higher activity, plus 40 ml for each degree above 20°C multiplied by hours outdoors.
Sources
Limitations
- Not a medical fluid prescription.
- Illness, exercise and alcohol change need.
- Fluid-restriction plans override this number.
When to seek help
Call NHS 111 or 999 if you feel confused, faint, or cannot keep fluid down in the heat.
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