Water Fast Electrolyte Calculator
General reference for daily sodium, potassium and magnesium during an extended water fast — with a clear warning that prolonged fasting needs medical supervision.
How it works
daily sodium = clamp(4000 × (0.7 + 0.3 × weight ÷ 175), 3000, 5000) mg · potassium ≈ 2500 mg/day · magnesium ≈ 350 mg/day · total = daily × days
During an extended water fast the body excretes sodium and other electrolytes rapidly, especially in the first days as insulin falls, and low electrolytes are behind many of the headaches, cramps, palpitations and dizziness fasters report. General fasting references suggest replacing roughly 3,000–5,000 mg of sodium, 1,000–3,500 mg of potassium and 300–400 mg of magnesium per day; this tool centres sodium near 4,000 mg and nudges it mildly with body weight, holding potassium and magnesium at typical mid-range figures. Read the disclaimer carefully before acting on any of this — the numbers are context, not medical clearance, and extended fasting carries genuine risks including dangerous heart-rhythm disturbances and refeeding syndrome.
Worked example
For a 3-day fast at 175 lb: sodium sits at the 4,000 mg baseline, potassium at 2,500 mg and magnesium at 350 mg per day — about 12,000 mg of sodium, 7,500 mg of potassium and 1,050 mg of magnesium supplemented across the three days.
Sources: Refeeding syndrome — overview (StatPearls / NCBI) · Mehanna et al. — refeeding syndrome (BMJ)
Frequently asked questions
Is it safe to do a multi-day water fast on my own?
No — this cannot be stressed enough. A fast of three or more days should be medically supervised. It carries real risks including cardiac arrhythmia from electrolyte imbalance and refeeding syndrome when you start eating again. Anyone with kidney or heart disease, diabetes, an eating-disorder history, or who is pregnant, breastfeeding or on medication should not follow generic electrolyte numbers and should not fast without a doctor’s direct involvement.
Why do electrolytes matter so much when fasting?
Without food you take in no sodium, potassium or magnesium, yet you keep losing them in urine — and the loss speeds up early in a fast. Because these minerals govern nerve signalling, muscle contraction and heart rhythm, letting them fall too low can cause cramps, weakness and, at the extreme, life-threatening rhythm problems. This is exactly why supervised fasts monitor blood chemistry.
What is refeeding syndrome?
It is a potentially fatal shift in fluids and electrolytes — particularly phosphate, potassium and magnesium — that can strike when someone starts eating again after prolonged fasting or starvation. It is one of the main reasons extended fasts and the return to food need clinical oversight, and it is a core reason the numbers on this page are educational reference only, not medical advice.
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IMPORTANT: Extended fasting carries real medical risks, including dangerous heart-rhythm disturbances and refeeding syndrome. This is general educational information, NOT medical advice. Anyone attempting a fast of 3 or more days should be supervised by a doctor, and people with kidney or heart conditions, diabetes, or who are pregnant or on medication must not follow generic electrolyte numbers — seek individual medical guidance first.