How this instrument works
Published in 1957 by Holliday and Segar, the formula ties fluid need to metabolic rate rather than weight alone: 100 mL per kilogram for the first 10 kg, 50 mL per kilogram for the next 10 (11-20 kg), and 20 mL per kilogram for every kilogram beyond that. Smaller bodies burn energy, and lose water through breathing and skin, at a proportionally higher rate per kilogram than larger ones do — a flat single rate would underfeed a small infant or overfeed an older child, so the formula steps the rate down in three tiers to approximate that curve with straight lines.
The same three tiers get taught as the '4-2-1 rule' when expressed per hour instead of per day: 4 mL/kg/hr for the first 10 kg, 2 mL/kg/hr for the next 10, and 1 mL/kg/hr after that. Multiply any hourly total by 24 and it lands on the exact daily figure, since both versions describe the identical relationship at different timescales — the daily form suits writing a fluid order, the hourly form suits setting a pump rate on rounds.
This is a baseline for a well child with ordinary ongoing losses, nothing more. It doesn't build in a dehydration deficit, doesn't add extra for abnormal losses such as vomiting, diarrhea, fever, or burns, and doesn't apply in situations calling for fluid restriction. Every one of those needs separate clinical judgment layered on top of this starting number, not folded into the arithmetic itself.
- Enter Weight in kilograms (or pounds — the unit converts automatically).
- Read Maintenance fluid rate in mL/day.
- Divide by 24 for an hourly pump rate, or compare against the '4-2-1' tiers directly.
- Add separately for any dehydration deficit or abnormal ongoing losses — this baseline doesn't include either.
Worked example — 8 kg, 15 kg, and 30 kg
An 8 kg infant sits entirely in the first tier: 8 × 100 = 800 mL/day, or roughly 33 mL/hour spread evenly around the clock.
A 15 kg child fills the first tier completely — 10 × 100 = 1000 mL — then adds the second tier for the remaining 5 kg: 5 × 50 = 250 mL. Total: 1000 + 250 = 1250 mL/day.
A 30 kg child fills both of the first two tiers — 1000 + (10 × 50) = 1500 mL — then adds the third tier for the 10 kg above 20: 10 × 20 = 200 mL. Total: 1500 + 200 = 1700 mL/day, or roughly 71 mL/hour.
Questions
Why is the rate per kilogram lower for bigger children?
Metabolic rate and water turnover scale with body surface area, which grows more slowly than weight as a child gets larger — a small infant has far more surface area, and burns far more energy, per kilogram than an older child or an adult. Stepping the mL/kg rate down in tiers approximates that curve without needing a full surface-area calculation for every patient.
What is the '4-2-1 rule' and how does it relate to this formula?
It's the identical three tiers expressed as an hourly rate instead of a daily total: 4 mL/kg/hr for the first 10 kg, 2 mL/kg/hr for the next 10, 1 mL/kg/hr beyond that. Multiply any hourly figure by 24 and it reconciles exactly with the daily version here, since both describe the same relationship at a different timescale.
Does this account for a child who is already dehydrated?
No — this is a maintenance estimate for ongoing needs only. Replacing an existing deficit from dehydration is a separate calculation added on top, typically the estimated volume lost replaced over a set number of hours based on the degree of dehydration found on clinical exam.
What about fever, vomiting, or diarrhea?
Those count as abnormal losses on top of this baseline, not built into it. Ongoing diarrhea or vomiting volume is often replaced roughly milliliter-for-milliliter, and fever typically raises maintenance needs by a percentage for every degree above normal body temperature — both are added under clinical guidance rather than assumed by the formula itself.
Does Holliday-Segar apply to newborns?
Generally not in the first days of life. Term newborns follow their own rapidly changing daily fluid schedule tied to birth weight and day of life, and preterm infants have entirely separate protocols. Holliday-Segar describes established pediatric maintenance beyond the newborn period, not day-one fluid orders.
References
- Holliday MA, Segar WE. 1957, Pediatrics — original formula (PubMed)
- Feld LG et al. 2018, AAP Clinical Practice Guideline, Pediatrics (PubMed)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.