Water Intake Calculator

The base is 35 ml of water per kilogram of body weight a day, then multiplied for activity and climate, plus 15 percent while fasting. About a fifth of that arrives in food, so the tool splits the total into what to drink and what you will eat. It is a rule of thumb rather than a guideline figure, and the method below says why.

Body Weight

Activity Level

Climate

Fasting today?

Pregnant or breastfeeding?

Eating Window (optional, hours)

How this is calculated

The whole calculation is one line and four adjustments, and it is worth being plain about what it is: a rule of thumb, not a guideline figure. The base is 35 ml of total water per kilogram of body weight a day. That is multiplied by your activity factor, then by your climate factor, then by 1.15 if you are fasting, and 300 ml is added on top if you are pregnant.

No official reference value is expressed per kilogram — the published ones are fixed daily totals for adults — so the per-kilogram form is a convenience for scaling, not something a guideline says. The 1.15 uplift for fasting is this site's own judgement rather than a figure anybody has published: insulin falls during a fast, the kidneys hold on to less sodium and water as a result, and people fasting report more headaches and cramps than people eating. It is a nudge in a direction the physiology supports, at a size nobody has measured.

The split below the total is the standard estimate that roughly a fifth of daily water arrives in food rather than in a glass, so the tool shows 80 percent as water to drink and 20 percent as water you will eat. Glasses are counted at 300 ml each and rounded up. Everything here is a starting point; thirst and the colour of your urine are better instruments than any calculator, and anyone on medication that affects fluid balance, or with a heart or kidney condition, should take a target from their clinician rather than from a web page.

Nothing on this site is medically reviewed — it cites its sources so you can check them. Read our editorial policy. Method last checked 21 August 2026.

Proper hydration is one of the most overlooked aspects of successful intermittent fasting. Since you are not getting water from food during your fasting window, your fluid needs change and many common fasting complaints like headaches and fatigue are actually signs of dehydration.

Knowing your personal water target — adjusted for your size, activity level, and climate — helps you fast more comfortably and safely. Staying ahead of thirst during fasting hours makes the entire experience significantly easier.

Frequently Asked Questions

Does drinking water break a fast?

No. Plain water contains zero calories and does not trigger an insulin response, so it does not break your fast. In fact, staying well-hydrated during fasting is essential for health, energy, and appetite management. Black coffee and plain tea are also generally accepted.

How much extra water should I drink during fasting?

During fasting, you miss out on water from food (which typically provides 20-30% of daily intake). Aim to drink an additional 2-4 cups beyond your normal target during fasting hours to compensate. Sip consistently throughout your fast rather than drinking large amounts at once.

What are the signs of dehydration during fasting?

Common signs include headaches, dizziness, dark urine, dry mouth, fatigue, and difficulty concentrating. Many symptoms people attribute to fasting hunger are actually dehydration. If you feel unwell during a fast, try drinking a large glass of water before deciding to break it.

What about electrolytes during fasting?

Electrolytes (sodium, potassium, magnesium) are important during extended fasts. For fasts under 24 hours, a balanced diet during your eating window usually suffices. For longer fasts, consider adding a pinch of salt to water or using an electrolyte supplement that contains no calories or sweeteners.

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