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Caffeine vs Alcohol Hydration Calculator

Enter your fluid intake, caffeine, and alcohol to estimate net hydration.

How it is calculated

The tool subtracts an assumed fluid loss from the fluid you drank. Caffeine only counts above 300 mg: caffeine loss = max(0, caffeine − 300) × 1.2 mL, with caffeine in mg. Alcohol costs 10 mL per gram of pure alcohol: alcohol loss = alcohol × 10 mL. Then diuretic loss = caffeine loss + alcohol loss and net = fluid intake − diuretic loss, all rounded to whole mL. The verdict is Hydrated when the net is zero or above and Deficit when it is negative. The 300 mg threshold and the two factors are fixed assumptions of the tool, not individual measurements, and body size, heat, exercise and habitual caffeine use are not included.

Worked example

You drank 2,000 mL of fluids in total, with 500 mg of caffeine and 20 g of alcohol. Caffeine above the 300 mg threshold: 500 − 300 = 200 mg, times 1.2 = 240 mL. Alcohol: 20 × 10 = 200 mL. Diuretic loss: 240 + 200 = 440 mL. Net balance: 2,000 − 440 = 1,560 mL, and the verdict is Hydrated.

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Common mistakes

  • Entering caffeine in mL or cups instead of milligrams. The field is mg of caffeine, and anything at or below 300 mg adds no loss at all in this tool.
  • Entering the volume of the alcoholic drink instead of grams of pure alcohol. The field is grams of alcohol, so a drink has to be converted first; typing 330 for a bottle of beer would give a loss of 3,300 mL.
  • Leaving out the water in coffee or beer when entering the fluid intake. The intake should include the full volume of every drink, since the tool subtracts the loss afterwards.
  • Reading Hydrated as proof you are well hydrated. It only says the estimated net is not negative; thirst, urine colour and activity level matter as well.

About this calculator

Both caffeine and alcohol have a mild diuretic effect, increasing urine output and offsetting some of the fluid you take in from the same drink. This calculator estimates your net fluid balance by subtracting an estimated diuretic fluid loss from your total fluid intake across all beverages. You enter the fluid in mL, the caffeine in mg and the alcohol in grams; the loss is counted as 1.2 mL for every mg of caffeine above 300 mg (smaller amounts are ignored, since research shows the diuretic effect is modest and largely absent in habitual consumers) plus 10 mL for every gram of alcohol. Those factors are rough working assumptions, not measurements of any one person.

The old idea that “coffee and beer count against your hydration” is largely outdated — studies of moderate coffee drinking have found the net fluid contribution is still positive — but athletes, people in hot climates, and anyone monitoring fluid intake for a medical reason still find it useful to see a rough net number rather than assume every ounce of coffee, tea, or beer counts fully toward daily hydration, especially at higher intake levels where the diuretic effect becomes more noticeable.

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Comments (1)

  • Editorial team

    The alcohol field wants grams of pure alcohol, which you can get from the label: volume in mL times the strength as a decimal times 0.789. A 330 mL beer at 5 percent holds about 13 g, so the tool subtracts roughly 130 mL. Caffeine under 300 mg changes nothing here, so alcohol usually drives the result. Which drinks would you like to see worked out as examples?

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