Hydration Calculator

Background.

The most authoritative answer to "how much water should I drink?" is looser than most people expect. When the US Institute of Medicine reviewed the evidence in 2004, it found no single intake that could be called a requirement. Instead it set Adequate Intakes from what healthy people actually consume: about 3.7 litres of total water a day for men and 2.7 litres for women, counting every drink and the water in food. It also concluded that most healthy people meet their needs simply by drinking when thirsty.

Those averages say nothing about a 55 kg office worker on a cool day versus a 90 kg labourer in July, or about the day you run for an hour. That is the gap this estimate tries to fill. It starts from a body-weight baseline of 33 millilitres per kilogram, adds an allowance for sweat during exercise — 0.6 litres an hour at moderate effort, 1.0 litre an hour when vigorous — and adds half a litre for a hot or humid day. Each part is shown separately, so you can see what drives the total.

Use the figure as a planning target for training days and hot weather, not as a quota to force down. Sweat rates differ between people by a factor of several, and the only way to know yours is to weigh yourself before and after exercise, as described below. Drinking far beyond your losses carries its own risk, exercise-associated hyponatremia, in long endurance events.

The estimate is meant for healthy adults. It does not cover children, pregnancy or breastfeeding, illness with fever, vomiting or diarrhoea, or anyone whose clinician has set a fluid limit for heart failure, kidney disease or liver disease. In those cases the clinician's figure applies, not this one.

What is hydration calculator?

A daily water need is the intake that balances the day's losses. Water leaves the body as urine, as sweat, as vapour in exhaled breath and through the skin, and in faeces; it comes back from drinks, from the water in food, and a small amount from metabolism itself. Total body water makes up roughly half to 60% of an adult's weight, less in people with more body fat. Because sweat is the most variable of the losses, exercise and heat change daily needs far more than anything else does, which is why they are the two adjustments made here. A rough day-to-day check needs no scale: pale, straw-coloured urine generally means intake is keeping up, while dark yellow urine suggests a deficit. Riboflavin in multivitamins can turn urine bright yellow regardless, so colour is a guide rather than a measurement.

How to use this calculator.

1. 01Enter your body weight in kilograms, between 40 and 150. To convert from pounds, divide by 2.205: 154 lb is about 70 kg.
2. 02Enter the minutes of exercise you plan for the day, up to 180, counting only time spent actually moving.
3. 03Choose the intensity by how much you sweat. Light adds nothing, however long it lasts; moderate assumes 0.6 litres of sweat an hour and vigorous 1.0 litre.
4. 04Choose hot/humid if the day is spent in summer heat or tropical conditions; it adds a flat half litre whether or not you exercise.
5. 05Read the total alongside its three parts, and spread the drinking through the day, with some before and after exercise rather than all at once.
6. 06If you have measured your own sweat rate, compare it with the 0.6 or 1.0 litre-an-hour allowance and adjust your plan by the difference; a measured 1.4 L an hour on vigorous days means about 0.4 L more per hour than shown.

The formula.

Total water (L/day) = 0.033 × W + f × t + c, where W is body weight in kilograms, t is minutes of exercise, f is a sweat factor and c is the climate allowance.

The first term is a baseline of 33 mL per kilogram, a widely used rule of thumb rather than an official IOM figure. It scales in a straight line with weight, from 1.32 L at 40 kg to 4.95 L at 150 kg. For a 70 kg adult it gives 2.31 L, close to the drinking part of the IOM's women's Adequate Intake (about 80% of 2.7 L, or roughly 2.2 L) and below the men's (about 3.0 L).

The second term converts an assumed hourly sweat rate into litres per minute: f = 0 for light activity, 0.01 L/min for moderate (0.6 L/h) and 0.0167 L/min for vigorous (just over 1.0 L/h). These are typical values, not measurements of you. The ACSM position stand on fluid replacement stresses that sweat rates vary widely with body size, fitness, heat acclimatisation, clothing and weather, and recommends measuring your own by weighing.

The third term, c, is 0.5 L on a hot or humid day and 0 otherwise. It is a single flat allowance, so it cannot distinguish a warm afternoon from a day of heavy work in 40 °C heat. The formula also counts water only. Sweat carries salt with it, on average around 35 millimoles of sodium per litre according to the ACSM, roughly 0.8 g, with a wide range between people; on long sessions in the heat a drink containing sodium replaces some of that and helps guard against hyponatremia. Nothing else is adjusted: altitude, illness, pregnancy, breastfeeding and very salty or very water-rich diets all fall outside the formula.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Sawka, M.N., Burke, L.M., Eichner, E.R., Maughan, R.J., Montain, S.J., & Stachenfeld, N.S. (2007). "American College of Sports Medicine position stand: Exercise and fluid replacement." Medicine & Science in Sports & Exercise 39(2):377-390.
2. [2]Institute of Medicine (2004). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press.
3. [3]National Academies of Sciences, Engineering, and Medicine (2019). Dietary Reference Intakes for Sodium and Potassium. National Academies Press.
4. [4]Popkin, B.M., D'Anci, K.E., & Rosenberg, I.H. (2010). "Water, hydration, and health." Nutrition Reviews 68(8):439-458. doi:10.1111/j.1753-4887.2010.00304.x
5. [5]American Heart Association (2023). "Staying Hydrated — Staying Healthy."