Fasting Electrolyte Calculator
Estimate conservative adult sodium, potassium, and magnesium reference targets during a planned fast, with hydration pacing and extended-fast warning bands.
Last updated: 2026-03-27
Fasting electrolyte calculator
Enter your values
Estimate adult electrolyte reference targets and hydration pacing during a planned fasting window.
Reference Sodium Target
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Enter sex, body weight, fast duration, and activity level to see conservative adult electrolyte reference targets.
Calculation History(0)
Example calculations
Tap an example to prefill the calculator with sample values.
36-hour light-activity fast
165 lb adult with light activity
A moderate fasting window where baseline hydration and electrolyte awareness usually matter more than aggressive dosing.
Result: The output stays in reference-target territory with a moderate monitoring warning rather than a supplement prescription
72-hour heavier-bodyweight fast
210 lb adult with sweaty activity
A stress case showing why longer fasts and higher sweat loss quickly move into clinician-supervision territory.
Result: Hydration pacing rises, but the page leans even harder on monitoring and medical-supervision warnings
How the fasting electrolyte reference works
The calculator starts with adult sodium, potassium, and magnesium reference targets from DRI-style and NIH ODS sources, then spreads those targets across a simple body-weight-aware hydration baseline.
That keeps the output useful as context while staying conservative. Clinical guidance for prolonged fasting and refeeding focuses on monitoring and medical supervision rather than one-size-fits-all supplement dosing.
Fasting electrolyte FAQs
Use the page as an educational reference only, especially for longer fasting windows.
Is this prescribing supplements or packets?
No. The calculator gives reference intake context and hydration pacing only. It is intentionally not a supplement-dosing protocol because prolonged fasting safety depends on personal medical context.
Why is sodium the headline metric?
Sodium shifts are a common early-fasting issue, especially when fluid intake stays high while food intake falls to zero. The page still shows potassium and magnesium because they matter too, but sodium is the cleanest top-line reference.
What changes after 72 hours?
The main issue is not that a universal electrolyte dose suddenly changes. The bigger concern is that longer fasts and the refeeding period deserve more medical caution, symptom monitoring, and often clinician involvement.
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</script> Related tools
Pair fasting context with neighboring hydration and nutrition calculators.
Water Intake Calculator
Use a broader hydration-planning calculator when fasting is not the primary context.
Micronutrient Calculator
Compare fasting electrolyte context with a broader vitamin and mineral reference profile.
Calorie Calculator
Switch to an energy-intake planning tool when the question shifts away from fasting windows.
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Medical Disclaimer
This calculator is educational only and does not prescribe fasting supplements, electrolyte packets, or refeeding plans.
Longer fasts, kidney disease, heart failure, diabetes medications, eating disorders, pregnancy, lactation, and prior electrolyte issues require clinician supervision.