Pediatric Fluid Calculator

The Holliday-Segar method

Published in 1957, Holliday and Segar related fluid requirements to metabolic rate and produced the calculation still used today, commonly called the 4-2-1 rule.

For maintenance: 100 millilitres per kilogram per day for the first 10 kilograms, 50 for the next 10, and 20 for each kilogram above 20. As an hourly rate that is 4 millilitres per kilogram per hour for the first 10 kilograms, 2 for the next 10, and 1 thereafter.

A 24 kilogram child therefore needs 40 plus 20 plus 4, which is 64 millilitres an hour, or roughly 1,540 millilitres a day.

Which fluid, and the change in practice

The composition matters as much as the volume, and practice has changed substantially. Hypotonic maintenance fluids were standard for decades and were associated with hospital-acquired hyponatraemia, in some cases fatal.

Following safety alerts, including from the National Patient Safety Agency in the UK, guidelines now recommend isotonic fluids for maintenance in most children.

NICE guidance specifies isotonic crystalloids containing sodium in the range of 131 to 154 millimoles per litre for routine maintenance, with glucose added as appropriate. This is one of the clearer examples of a long-standing practice being changed by accumulated harm data.

Deficit and ongoing losses

Maintenance is only one component. A dehydrated child also needs replacement of the existing deficit and of ongoing losses.

Deficit is estimated from the percentage of body weight lost, assessed clinically from signs including capillary refill, mucous membranes, skin turgor and mental state. Five per cent dehydration in a 12 kilogram child is a 600 millilitre deficit.

Ongoing losses from vomiting, diarrhoea, drains or fever are added and reassessed. Fever increases insensible losses by roughly 10% to 12% for each degree above normal.

Resuscitation is a separate calculation

A shocked child needs fluid boluses rather than maintenance arithmetic. Current UK guidance is 10 millilitres per kilogram of isotonic crystalloid over less than ten minutes, reassessed after each bolus.

That figure has changed: 20 millilitres per kilogram was standard and has been revised downward in several guidelines following evidence including the FEAST trial, which found increased mortality with fluid boluses in a specific African paediatric population.

Boluses are given and reassessed rather than calculated in advance, and the endpoint is clinical response. This is emergency management and it belongs entirely to the team at the bedside.

The limits of any calculation here

Paediatric fluid prescribing is high-risk. Errors have caused death, both from inadequate resuscitation and from inappropriate fluid composition causing hyponatraemia.

The calculation is checked, the prescription is checked, the electrolytes are monitored, and the plan is revised against the child's response. None of that is available to a web page.

This exists to check arithmetic against the standard formula for anyone who already knows what they are prescribing and why. It is not a prescribing aid, it does not know the child, and anyone caring for an unwell child should be with a clinician rather than with a calculator.

One arithmetic detail that trips people up: the Holliday-Segar figures are based on weight, and in an obese child using actual weight substantially overestimates requirements, because the calculation was derived from metabolic rate rather than from mass as such. Several approaches exist, including using ideal body weight or capping the calculated volume, and local guidance should specify which. As with the fluid composition question, this is an area where a plausible calculation performed without knowing the local protocol can produce a number that is confidently wrong.