Hydration Check Calculator

This tool reads your hydration from urine color, the most reliable real-time indicator, then adds how much extra water to drink for your activity level and the heat.

Urine color scale (1 to 8), and what each one is worth in millilitres:

• 1 Clear to 3 Light yellow: well hydrated, nothing to make up. Clear all day can mean slightly over-hydrated.
• 4 Yellow: mildly dehydrated. About 350 mL, a glass and a half, over the next couple of hours.
• 5 Dark yellow: behind. About 600 mL, spread out rather than downed.
• 6 to 7 Amber to dark amber: significantly to severely dehydrated. About 800 mL with electrolytes, and stop exercising.
• 8 Brown: not a hydration reading at all. Brown urine can mean rhabdomyolysis (muscle breakdown), a liver problem, or blood, and drinking more water does not treat any of those. Call a clinician.

Sip, do not gulp. The figures above are deliberately spread over two hours. Healthy kidneys clear roughly 800 to 1,000 mL an hour, and drinking faster than that dilutes blood sodium instead of rehydrating you. Exercise-associated hyponatremia has killed marathon runners who were drinking too much, not too little, so a dehydration calculator that told you to put two litres away inside an hour would be giving you the wrong risk.

One honest caveat about the colour method. B vitamins turn urine bright yellow whatever your hydration, beetroot and some laxatives turn it pink or red, and rifampin turns it orange. Diuretics and a lot of coffee pale it out while you are still behind on fluid. Read the colour first thing in the morning, before supplements, and it is a much better signal.

Extra water for activity and heat (added to your normal intake):

• Lightly active: +500 mL
• Moderately active: +750 mL
• Intense exercise: +1,000 mL
• Moderate heat (70–85°F): +250 mL
• Hot (above 85°F): +500 mL

These are round planning figures for about an hour of activity, not measurements. Adult sweat rates run from 0.5 to 4.0 litres an hour (NATA, the National Athletic Trainers’ Association, 2017), so a long or hard session can need several times the allowance. The Exercise Hydration Calculator linked below sizes a session from your weight, duration and intensity.

Electrolytes:

For long sessions in the heat, sodium is the one that matters. Sweat carries anywhere from 10 to 100 mmol of sodium a litre, roughly 230 to 2,300 mg, so two people doing the same run can lose ten times as much salt. A sports drink or salty food covers most people. Sweat carries far less potassium, and very little magnesium.

Reference: total daily water from the Institute of Medicine (now the National Academies):

• Men: about 3.7 litres (125 oz) a day, counting all drinks and the water in food
• Women: about 2.7 litres (91 oz) a day
• Pregnancy: about 3.0 litres, roughly 10 cups of it from drinks
• Breastfeeding: about 3.8 litres, roughly 13 cups of it from drinks
• Roughly 20% of daily water comes from food

Those figures are the median intake of Americans in a national survey, not a minimum anyone must reach. The European Food Safety Authority puts adequate intake lower, at 2.5 litres for men and 2.0 for women.

For a weight-based daily target, use the Water Intake Calculator linked below. This page is a quick status check, not a full intake plan.