Maintenance — Holliday-Segar (1957). Expressed per hour as the 4-2-1 rule: 4 mL/kg/h for the first 10 kg, 2 mL/kg/h for the next 10 kg, 1 mL/kg/h for every kg above 20. Expressed per day as 100 / 50 / 20 mL per kg for the same bands. The two are not identical: 100 mL/kg over 24 hours is 4.17 mL/kg/h, not 4, so the daily form gives slightly more. At 25 kg the hourly rule gives 65 mL/h = 1560 mL/24 h while the daily rule gives 1600 mL — a 2.5 % difference that does not matter clinically, but this tool shows both rather than pretending they are the same number.

Deficit replacement. Deficit in mL = % dehydration × weight in kg × 10 (because 1 % of body weight is roughly 10 mL/kg of water). A 10 kg child at 5 % has a 500 mL deficit. Standard practice replaces half the deficit over the first 8 hours alongside maintenance, and the remaining half plus maintenance over the next 16 hours. This tool adds the proportional share of maintenance to each window, so 500 mL deficit + 1000 mL/day maintenance gives 250 + 333 = 583 mL over 8 hours = 72.9 mL/h, then 250 + 667 = 917 mL over 16 hours = 57.3 mL/h. Shock is treated separately with boluses, not by the deficit calculation.

Burn resuscitation. Volume in the first 24 hours = multiplier × weight × %TBSA, half from the time of the burn over 8 hours and half over the following 16. The multiplier is not one number: the American Burn Association and ATLS 10th edition start adults at 2 mL/kg/%TBSA for flame and scald, use 3 mL/kg/%TBSA in children, and 4 mL/kg/%TBSA for electrical injury; the classic Parkland formula is 4 mL/kg/%TBSA and is still widely quoted. All three are offered here rather than one being presented as "the" formula. The 8-hour clock runs from the moment of injury, not from arrival: enter the hours already elapsed and the tool divides the first half by the time actually left. A 70 kg adult with 40 %TBSA on the 2 mL formula needs 5600 mL in 24 hours, 2800 mL in the first 8 hours at 350 mL/h — but if 3 hours have already gone, the same 2800 mL has to run in 5 hours at 560 mL/h.

What these formulas miss. Every one of them is a starting rate, not a prescription. Maintenance is derived from healthy children's caloric expenditure and is a poor guide to a septic, febrile, ventilated or post-operative patient; adult maintenance in UK practice is usually written as 25–30 mL/kg/day with sodium 1 mmol/kg/day, not as 4-2-1. Deficit percentages are a clinical estimate with wide error bars — weight change, capillary refill and urine output beat the formula. Burn formulas under-predict in inhalation injury, deep burns and delayed presentation, and over-predict if you count erythema; the endpoint is urine output (0.5 mL/kg/h in adults, 1 mL/kg/h in children, higher for myoglobinuria), titrated hourly, and "fluid creep" from blindly following the calculated rate is a recognised harm. The Rule of Nines is inaccurate in children under about 15 — use a Lund-Browder chart. Ongoing losses, insensible loss, third-spacing, cardiac and renal function are all outside these three calculations.

Disclaimer. Arithmetic for qualified clinicians — not medical advice and not a resuscitation protocol. Follow your local guideline and titrate to the patient. Sources: Holliday & Segar, Pediatrics 1957 · American Burn Association burn shock resuscitation guidance · ATLS 10th edition · NICE NG29 intravenous fluid therapy. Last reviewed: 2026-09-21.