Calculate maintenance fluids
What the Holliday-Segar method is
In 1957, Malcolm Holliday and William Segar published a simple observation that has guided paediatric fluid prescribing ever since: the body's water needs track its energy needs. A child burning 100 kilocalories needs about 100 millilitres of water to handle the waste products of that metabolism and the water lost through skin and lungs. Because energy expenditure per kilogram falls as children grow, the formula is written in three bands rather than as a single rate.
The daily version gives 100 mL per kg for the first 10 kg of body weight, 50 mL per kg for the next 10 kg, and 20 mL per kg for everything above 20 kg. A 25 kg child therefore needs 1,000 mL for the first 10 kg, 500 mL for the next 10 kg, and 100 mL for the final 5 kg, a total of 1,600 mL over 24 hours. The hourly 4-2-1 shortcut divides the same idea into an infusion rate: 4 mL/kg/hour for the first 10 kg, 2 mL/kg/hour for the next 10 kg, and 1 mL/kg/hour above 20 kg, which for the same 25 kg child gives 65 mL per hour.
Notice that the hourly shortcut and the daily formula do not agree exactly. Sixty-five millilitres per hour over 24 hours is 1,560 mL, slightly less than the 1,600 mL from the daily bands. Both are approximations of the same physiology, and either is acceptable in practice; the daily figure is usually quoted for total volume planning and the hourly figure for setting an infusion pump.
Worked examples you can check
A 10 kg toddler. Daily: 100 × 10 = 1,000 mL per 24 hours. Hourly: 4 × 10 = 40 mL per hour. This is the most commonly quoted teaching example and the calculator reproduces it exactly.
A 20 kg child. Daily: 1,000 + (50 × 10) = 1,500 mL per 24 hours. Hourly: 40 + (2 × 10) = 60 mL per hour. Sixty millilitres per hour is the figure used in most textbook examples for a 20 kg child.
A 32 kg child. Daily: 1,500 + (20 × 12) = 1,740 mL per 24 hours. Hourly: 60 + 12 = 72 mL per hour. Above about 40 kg the daily total approaches the adult range, which is why many references cap maintenance at 2,400 mL per day; this calculator applies that cap and tells you when it has done so.
Deficit, ongoing losses and boluses
Maintenance covers a well child at rest. An ill child usually needs more, and the extra is prescribed in three separate parts so each can be adjusted independently. The deficit replaces water already lost: percent dehydration × weight in kg × 10 gives the deficit in millilitres, so a 5 percent dehydrated 18 kg child is 900 mL behind. The deficit is typically replaced over 24 hours on top of maintenance, with the first portion often given faster if the child is significantly volume depleted.
Ongoing losses from vomiting, diarrhoea, drains or fever are replaced millilitre for millilitre as they occur, usually with an isotonic fluid. Boluses are different again: a child in shock receives 20 mL per kg of isotonic crystalloid as a rapid bolus, reassessed after each one, up to 60 mL per kg in the first hour for septic shock in settings where intensive care backup exists. Bolus fluid is resuscitation, not maintenance, and it is never simply added to the daily total without reassessment.
Which fluid, and when the formula does not fit
Holliday and Segar originally described their method with hypotonic fluids, but practice has moved on. Acutely ill children given hypotonic maintenance fluids develop low sodium more often, and the American Academy of Pediatrics 2018 guidance recommends isotonic maintenance fluids (such as Hartmann's solution or normal saline with added potassium and glucose as needed) for most hospitalised children from 28 days to 18 years. This calculator gives volumes; the choice of fluid and its composition is a clinical decision.
The formula also has boundaries. Neonates in the first days of life have very different fluid physiology and need individualised prescribing, not the 4-2-1 rule. Children with heart failure, kidney failure, liver failure or brain injury retain water or cannot tolerate standard volumes, and children with diabetes insipidus or on diuretics lose far more. In all of these situations the calculated volume is a starting point for discussion with the treating team, never a prescription on its own.
Common prescribing mistakes
The most frequent error is prescribing maintenance alone for a dehydrated child and forgetting the deficit, which leaves the child dry for another day. The second is the reverse: adding the whole deficit as extra maintenance indefinitely rather than as a one-off replacement. The third is using the child's current dehydrated weight instead of a recent well weight, which underestimates every component.
Units cause the remaining mistakes. The 4-2-1 rule gives millilitres per hour; multiplying it by 24 does not exactly reproduce the daily bands, so pick one convention for the drug chart and document which you used. And always write the rate the pump will actually run at, not just the daily total, because a daily total divided wrongly across 24 hours is how tenfold errors happen.
Key takeaways
- A 10 kg child needs about 1,000 mL per day (40 mL/hour); a 20 kg child about 1,500 mL per day (60 mL/hour).
- The 4-2-1 hourly shortcut and the 100/50/20 daily bands are approximations of the same physiology and differ slightly.
- Replace dehydration deficit separately: percent dehydrated times weight in kg times 10 gives millilitres.
- Shock needs 20 mL/kg isotonic boluses with reassessment, not just faster maintenance.
- Use isotonic maintenance fluids for most acutely ill children, and individualise volumes in cardiac, renal or neurological disease.
Frequently asked questions
Why do the hourly and daily formulas give slightly different answers?
Both are approximations of the same underlying physiology, rounded for bedside convenience at different times. The daily 100/50/20 bands were the original 1957 description; the 4-2-1 hourly rule came later as a pump-friendly shortcut. For a 25 kg child the daily bands give 1,600 mL while 65 mL/hour over 24 hours gives 1,560 mL. The difference is clinically negligible, but you should document which convention the prescription uses so the next clinician does not recalculate and change the rate.
Should I use the child's current weight if they are dehydrated?
No. Use the child's most recent well weight, or an estimated well weight, for maintenance and deficit calculations. A dehydrated weight underestimates fluid needs at exactly the moment the child needs more. If no well weight is available, estimate from the dehydration percentage: well weight is approximately the current weight divided by (1 minus the dehydration fraction).
What is the maximum maintenance volume?
Most references cap maintenance at about 2,400 mL per day, which is roughly the adult requirement and is reached at around 65 to 70 kg by the Holliday-Segar bands. Beyond that weight, adult fluid management principles apply. This calculator applies the 2,400 mL cap automatically and flags when it does so.
Can I use this for a newborn baby?
No. Neonates, especially in the first week of life, have very different fluid and electrolyte physiology, including large insensible losses, immature kidneys and rapidly changing body composition. Their fluids must be prescribed individually by the neonatal team, often starting around 60 to 80 mL/kg/day on day one and advancing by clinical parameters. The Holliday-Segar method applies from about one month of age.
Which fluid should run at the calculated rate?
For most acutely ill hospitalised children, an isotonic crystalloid such as Hartmann's solution (compound sodium lactate) or normal saline, with glucose and potassium added according to blood results. The 2018 American Academy of Pediatrics guidance recommends isotonic over hypotonic maintenance fluids to reduce hospital-acquired hyponatraemia. The exact composition depends on the child's electrolytes and losses, so it remains a clinical decision.