Fluid, Sodium, and Calorie Calculations

The maintenance fluids calculator applies the classic weight-based 100/50/20 rule — 100 mL/kg for the first 10 kg, 50 mL/kg for the next 10 kg, and 20 mL/kg for each kilogram thereafter — originally described by Holliday and Segar for parenteral fluid therapy in children.

The fluid requirement calculator takes a simpler fixed 35 mL/kg/day estimate for adults. The two calculators answer the same broad question with different conventions, and the difference between their outputs is a reminder that these are approximations, not measurements. Adult intravenous fluid therapy guidance, such as NICE Guideline CG174, emphasizes prescribing fluids as a drug with explicit daily goals and reassessment.

Three calculators address sodium disorders, and each addresses a different step. In hyperglycemia, the corrected sodium calculator adjusts the measured serum sodium for the glucose level so that the sodium status is not misinterpreted as hyponatremia. When a true sodium disorder requires correction, the sodium deficit calculator estimates the sodium needed to raise a low sodium toward a target, and the free water deficit calculator estimates the volume of free water needed to lower a high sodium toward a target.

All three produce estimates that support a management plan. Hyponatremia and hypernatremia corrections carry the risk of over-correction (and of correction occurring too quickly in hyponatremia), so the numbers from these calculators must always be paired with rapid clinical reassessment rather than followed blindly.

The calorie requirement calculator estimates total daily calorie need by multiplying an estimated basal metabolic rate by an activity factor, following the FAO/WHO/UNU Human Energy Requirements approach and the Dietary Reference Intakes framework. The result is a starting estimate for nutrition planning and is sensitive both to the accuracy of the underlying weight and to the activity factor chosen.

The shared theme across these calculators is that each converts a weight and a few inputs into a daily number. The number is a reasonable starting estimate, and it becomes unsafe when treated as precise. Fluid status is best tracked by serial clinical assessment, intake and output, and laboratory or urine measures; the correct volume or rate is the one that works for the individual patient under re-evaluation.