Water Fasting Hydration and Electrolyte Requirement Calculator
Almost every unpleasant symptom attributed to fasting — headache, dizziness, cramps, heart palpitations, crushing fatigue — is an electrolyte problem, not a calorie problem. When insulin falls, the kidneys switch from retaining sodium to excreting it rapidly, and water follows. Replacing sodium, potassium and magnesium turns a miserable fast into an uneventful one.
DIY mix per litre: 3.22 tsp salt, a pinch of potassium chloride and 100 mg magnesium.
| Nutrient | Per day | Total over 2.0 days | Practical source |
|---|---|---|---|
| Water | 2.8 L | 5.6 L | Sip steadily, not in boluses |
| Sodium | 3,500 mg | 7,000 mg | 9.0 tsp unrefined salt |
| Potassium | 2,140 mg | 4,280 mg | Lite salt / KCl, divided doses |
| Magnesium | 350 mg | 700 mg | Glycinate at night |
The natriuresis of fasting
Within twenty-four hours of stopping food, insulin drops far enough that renal sodium reabsorption falls sharply. Sodium losses of three to five grams a day are typical in the first seventy-two hours. Drinking plain water in that state dilutes the remaining electrolytes further and can produce genuine hyponatraemia — which is why plain-water-only fasting protocols cause the worst symptoms.
Matching the symptom to the mineral
Headache, light-headedness on standing and profound fatigue usually indicate sodium depletion. Muscle cramps and palpitations point to potassium or magnesium. Constipation, poor sleep and eye twitching are classic magnesium signals. Correcting the specific mineral resolves the symptom within a few hours in most cases.
Safety limits and who should not fast
Potassium supplementation carries real cardiac risk at high doses and should stay well within standard dietary amounts, spread through the day. Anyone taking diuretics, ACE inhibitors, SGLT2 inhibitors, lithium or insulin, and anyone with kidney or heart disease, pregnancy or a history of eating disorders should not fast without medical supervision.
Electrolyte targets during fasting
| Electrolyte | Daily target | Deficiency symptoms | Common source | Caution |
|---|---|---|---|---|
| Sodium | 3,000-5,000 mg | Headache, dizziness, fatigue | Unrefined salt in water | Raise slowly if hypertensive |
| Potassium | 1,000-3,500 mg | Cramps, palpitations, weakness | Potassium chloride / lite salt | Never bolus large doses |
| Magnesium | 300-500 mg | Cramps, poor sleep, twitching | Glycinate or citrate | Citrate is laxative at high dose |
| Calcium | Usually adequate | Rare in short fasts | — | Monitor in extended fasts |
| Water | 30-40 ml per kg | Dark urine, dry mouth | Sipped steadily | Excess plain water dilutes sodium |
People also ask
How much salt should I take during a water fast?
Roughly three to five grams of sodium a day — about one and a half to two teaspoons of salt — divided across the day and dissolved in water rather than taken at once.
Can I drink too much water while fasting?
Yes. Without food-derived sodium, excessive plain water can dilute blood sodium and cause hyponatraemia. Sip to thirst and always include electrolytes.
What causes keto flu and fasting headaches?
Predominantly sodium loss driven by falling insulin, compounded by magnesium and potassium depletion. Replacing all three usually resolves symptoms within hours.
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: body weight 62.4 kg, climate / activity "Cool and sedentary"
On the lower / more conservative end. DIY mix per litre: 4.13 tsp salt, a pinch of potassium chloride and 100 mg magnesium.
Example 2: body weight 80 kg, climate / activity "Temperate, light activity"
A typical middle-of-the-road setup. DIY mix per litre: 3.22 tsp salt, a pinch of potassium chloride and 100 mg magnesium.
Example 3: body weight 104 kg, climate / activity "Warm or active"
On the higher / more demanding end. DIY mix per litre: 2.48 tsp salt, a pinch of potassium chloride and 100 mg magnesium.
Quick answers about the Fasting Water & Electrolyte Needs
What exactly does the Fasting Water & Electrolyte Needs work out?
Almost every unpleasant symptom attributed to fasting — headache, dizziness, cramps, heart palpitations, crushing fatigue — is an electrolyte problem, not a calorie problem. You enter body weight, fasting duration, climate / activity and sauna or heavy sweating and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 4 fields: body weight, fasting duration, climate / activity and sauna or heavy sweating. Nothing else is needed and nothing is stored.
How is it calculated — the natriuresis of fasting?
Within twenty-four hours of stopping food, insulin drops far enough that renal sodium reabsorption falls sharply. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for fasting Water & Electrolyte Needs?
Headache, light-headedness on standing and profound fatigue usually indicate sodium depletion. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "Electrolyte targets during fasting" table on this page tell me?
It is the reference range this tool works against — 5 rows from "Sodium" (3,000-5,000 mg) up to "Water" (30-40 ml per kg). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Which climate / activity should I pick?
The dropdown offers 4 choices — Cool and sedentary, Temperate, light activity, Warm or active and Hot climate or training. Pick the one that matches your real situation rather than the one you would like to be true; climate / activity usually moves the final figure more than any other single input, so it is worth running it twice with the option above and below your guess.
Do I have to press a button or reload the page to see the result?
No. Fasting Water & Electrolyte Needs runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into Fasting Water & Electrolyte Needs leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
Roughly three to five grams of sodium a day — about one and a half to two teaspoons of salt — divided across the day and dissolved in water rather than taken at once. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
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