What is the 4-2-1 rule?

The 4-2-1 rule gives the hourly maintenance fluid rate: 4 mL/kg/h for the first 10 kg of body weight, 2 mL/kg/h for the next 10 kg, and 1 mL/kg/h for every kilogram above 20 kg. The three parts are added together.

How does the 4-2-1 rule relate to the daily 100-50-20 method?

They are the same method expressed differently. The daily version uses 100 mL/kg for the first 10 kg, 50 mL/kg for the next 10 kg, and 20 mL/kg beyond 20 kg. Dividing the daily total by 24 gives approximately the 4-2-1 hourly rate.

Does this account for ongoing losses or dehydration?

No. Holliday-Segar estimates baseline maintenance only. Deficits from dehydration, ongoing losses (vomiting, diarrhoea, drains), and increased needs from fever must be calculated and added separately.

Is there an upper weight limit?

The formula is most validated in children. In larger adults the calculated rate is often capped (commonly around 100–120 mL/h) and individualised, because pure weight scaling can overestimate needs in heavy adults.

Is my data stored anywhere?

No. The calculation runs entirely in your browser and the weight you enter is never transmitted or saved on any server.

Holliday-Segar Fluid Maintenance Calculator

The Holliday-Segar Fluid Maintenance Calculator estimates how much intravenous fluid a patient needs over 24 hours simply to replace normal ongoing losses (urine, sweat, breathing, and stool). It uses the classic Holliday-Segar method, most commonly remembered as the 4-2-1 rule for hourly rates.

How it works

The method scales fluid to body weight in three tiers. The hourly (4-2-1) form:

4 mL/kg/h for the first 10 kg
2 mL/kg/h for the next 10 kg (from 10 to 20 kg)
1 mL/kg/h for each kg above 20 kg

The three contributions are summed. The equivalent daily (100-50-20) form uses 100 mL/kg, 50 mL/kg, and 20 mL/kg over the same weight tiers; dividing the daily total by 24 reproduces the hourly figure.

For a 25 kg child: (10 × 4) + (10 × 2) + (5 × 1) = 40 + 20 + 5 = 65 mL/h, or about 1560 mL/day.

Tips and notes

This figure is a maintenance baseline only. Add separately for replacement of any existing deficit (dehydration) and for ongoing abnormal losses such as vomiting, diarrhoea, or surgical drains. Fever raises insensible losses by roughly 10–12% per degree Celsius above normal.

In larger adults the weight-scaled rate is usually capped (often near 100–120 mL/h) and individualised, because linear scaling overestimates needs at high body weights. Always reassess fluid status clinically and with electrolytes rather than relying on the formula alone.