Paediatric Fluid & Dose Calculator
The 4-2-1 rule and Holliday-Segar are taught as the same rule in different units. They are not — they cross at exactly 35 kg.
A study aid, not a prescribing tool. The milligram-per-kilogram figures here are illustrative teaching examples, not a formulary — real dosing is drug, indication and route specific, and nothing should be dosed from a generic figure without the actual reference. Maintenance rules were derived in healthy children and do not describe sick ones. Nothing here knows anything about a patient.
The two rules across the weight range
Where each tier disagrees
Multiply the hourly rule by 24 and compare with the daily one. All three tiers differ, and the third differs in the opposite direction.
Where weight-based dosing hits the adult ceiling
The 4-2-1 rule is not Holliday-Segar in different units
They are routinely taught as the same rule expressed hourly rather than daily, and they are not. Multiply the hourly tiers by 24 and you get 96, 48 and 24 mL per kilogram per day against Holliday-Segar's 100, 50 and 20 — so the first two tiers run four per cent low and the third runs twenty per cent high per kilogram. The discrepancy therefore changes sign, and it crosses at exactly 35 kilograms. That is algebra rather than a sampled figure: setting 24(60 + k − 20) equal to 1500 + 20(k − 20) gives 4k = 140.
Below 35 kg the hourly rule delivers less — a flat four per cent less at any weight under 20, since both tiers are proportional there — and above it the hourly rule delivers more, reaching about seven per cent by 80 kg. Neither difference matters over an hour and both matter over days, which is the honest summary. The point is not that one rule is right, but that switching between them without noticing is a systematic four to seven per cent error in a quantity that gets titrated far more tightly than that.
Weight-based dosing has the same shape of trap. It is a floor-to-ceiling rule rather than a ratio, and the ceiling arrives earlier than people expect: at 25 mg/kg against an 875 mg adult dose the ceiling is 35 kg, which is a large ten-year-old. A 70 kg adolescent dosed purely by weight would receive exactly twice the adult dose. And the underlying reason all of this matters more in children is turnover — a neonate moves ten per cent of its own body weight per day in maintenance fluid against 3.6 per cent for an adult, so the same proportional error is nearly three times the physiological insult with far less reserve to absorb it.
How to use
- Enter a weight to see both maintenance rules side by side.
- Note which way they disagree — the sign flips at 35 kg.
- Check the ceiling weight before dosing an adolescent by kilogram.
- Use the actual drug reference for any real dose; these examples are illustrative.
Frequently asked questions
What is the 4-2-1 rule?
A bedside shortcut for maintenance fluid: 4 millilitres per kilogram per hour for the first 10 kilograms of body weight, 2 for the next 10, and 1 for every kilogram above 20. So a 25 kilogram child needs 40 plus 20 plus 5, which is 65 millilitres an hour. It exists because it can be done in your head.
Is the 4-2-1 rule the same as Holliday-Segar?
No, though it is almost always taught as if it were the same rule in hourly rather than daily units. Multiply the hourly tiers by 24 and you get 96, 48 and 24 millilitres per kilogram per day against Holliday-Segar 100, 50 and 20. All three tiers differ, and the third differs in the opposite direction from the first two.
At what weight do the two rules agree?
Exactly 35 kilograms, which falls out of the algebra rather than being an approximation — setting 24 times (60 plus k minus 20) equal to 1500 plus 20 times (k minus 20) gives 4k equals 140. Below 35 kilograms the hourly rule delivers less than the daily one, and above it the hourly rule delivers more.
How big is the disagreement between them?
A flat four per cent below 20 kilograms, because both rules are proportional in that range, narrowing to nothing at 35 kilograms and then reversing to about seven per cent by 80. Neither difference matters over an hour and both matter over days, which is the honest summary of it.
Does it matter which maintenance rule I use?
Less than it matters to know they differ. The point is not that one is right but that switching between them without noticing produces a systematic four to seven per cent error in a quantity that gets titrated far more tightly than that. Pick the one your unit uses and stay with it.
Why do these rules over-hydrate sick children?
Because they were derived by Holliday and Segar in 1957 from healthy metabolic data, and illness changes the requirement in both directions. Fever, burns and ongoing losses raise it, while the antidiuretic hormone response to surgery, pain, nausea and respiratory illness substantially reduces the ability to excrete free water.
Why is maintenance fluid now isotonic?
Because it used to be hypotonic, and giving hypotonic fluid at maintenance rates to children with a raised antidiuretic hormone response caused hyponatraemia, seizures and deaths. Most guidelines now specify isotonic maintenance fluid. The volume was only half the question and the composition turned out to be the half that killed people.
What is a ceiling weight in weight-based dosing?
The weight at which a milligram-per-kilogram dose first reaches the adult maximum. Above it, dosing purely by weight gives more than an adult would receive, which is why the calculation is a floor-to-ceiling rule rather than a ratio. The cap is not a rounding convenience; it is where the weight-based reasoning stops applying.
When does a mg/kg dose exceed the adult dose?
Earlier than most people expect. At 25 milligrams per kilogram against an 875 milligram adult dose the ceiling is 35 kilograms, which is a large ten-year-old. At 10 mg/kg against 400 mg it is 40 kilograms. A 70 kilogram adolescent dosed purely by weight can easily receive twice what an adult would.
Why do fluid errors matter more in small children?
Because of turnover. A neonate moves about ten per cent of its own body weight per day in maintenance fluid, against 3.6 per cent for a 70 kilogram adult — so the same proportional error is nearly three times the physiological insult, and there is far less reserve to absorb it before it shows.
Should I use actual or ideal body weight for children?
For maintenance fluid, actual weight is conventional, and for most drugs in children of normal body composition it makes little difference. In obese children it can, and several drugs specify a different weight basis — which is one more reason to work from the actual drug reference rather than a generic figure.
Can I use this to prescribe?
No. This is a study and teaching aid for people learning these rules. The milligram-per-kilogram figures are illustrative teaching examples rather than a formulary, real dosing is drug, indication and route specific, and nothing here knows anything about a patient. It never replaces protocol or pharmacy.
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