Hot weather hydration calculator

How the number is worked out

It starts from a base for the work: about 250 ml an hour for light work, 400 for moderate and 600 for heavy, which sits inside the widely published guidance for working in heat of roughly a cup (200-250 ml) every 15-20 minutes. That is scaled by temperature band - heat adds up to 50% - and nudged by bodyweight, because bigger people sweat more. The result is capped at 1 litre an hour: the stomach cannot absorb much more, and drinking well beyond sweat losses over a long day can drive blood sodium dangerously low. It is general guidance for healthy adults, not a prescription.

What the HSE says about heat

There is no legal maximum working temperature in the UK, but employers have a duty to keep workplaces at a reasonable temperature and to assess the risk of heat stress where people work hard in the heat or in protective clothing. In practice that means water on site, shade for breaks, rotating the heaviest jobs, and looking out for each other. If you are the one running the site, this calculator is a reasonable starting point for how much water to have available per person.

Signs of heat illness

Heat exhaustion: headache, dizziness, feeling sick, heavy sweating, pale and clammy skin, cramps in arms, legs or stomach, fast breathing or pulse, very thirsty, temperature rising. Get into shade, lie down with the feet raised, drink water or a sports drink, cool the skin with water and a fan. The person should feel better within about 30 minutes.

Heatstroke - call 999: confusion or not making sense, still unwell after 30 minutes of cooling and rest, very hot skin, not sweating despite the heat, fits, loss of consciousness, temperature over 40°C. This is an emergency. Do not leave them alone.

When to stop

Stop and get into shade if you feel dizzy, sick or confused, if you have stopped sweating, or if you have cramps that are getting worse. Stop the job, not just yourself, if a colleague is confused or drowsy in the heat - that is the sign that separates a bad afternoon from a hospital trip. The water intake calculator covers the normal-day version of this.

FAQs

How much water should I drink on a hot day at work?

For moderate to heavy work in heat, general guidance is roughly 500 to 1,000 ml an hour - about a cup every 15 to 20 minutes - and not more than about a litre an hour. Over an 8-hour shift that is 4 to 8 litres. Drink to a schedule rather than waiting to feel thirsty, and eat normally so you replace salt as well as water.

Is there a maximum working temperature in the UK?

No. UK law does not set a maximum. The HSE expects employers to keep the workplace at a reasonable temperature and to manage heat stress - water, shade, breaks, rotating heavy work - where people work hard in hot conditions or in protective clothing.

Can you drink too much water?

Yes. Drinking well beyond what you sweat out, over hours, can dilute blood sodium (hyponatraemia), which is dangerous. Stay around or under a litre an hour, spread it out, and get salt from food or an electrolyte drink on long hot days.

What are the first signs of heat exhaustion?

Headache, dizziness, nausea, heavy sweating, clammy skin, cramps, fast pulse and intense thirst. Move into shade, cool down with water and drink. If someone is confused, has stopped sweating, or is not better after 30 minutes, call 999 - that is heatstroke.