Maintenance Fluids Calculator (4-2-1)

How it is calculated

The 4-2-1 rule estimates hourly maintenance fluid from body weight in kilograms. Each weight band has its own rate: 4 mL/h for each of the first 10 kg, 2 mL/h for each kg from 10 to 20 kg, and 1 mL/h for each kg above 20 kg. The three parts add up, so above 20 kg mL/h = 60 + (weight − 20). The daily figure is the hourly rate times 24. Both are rounded to whole millilitres, and weight must be above zero. This is a starting estimate for maintenance only; it does not account for deficits, ongoing losses, fever, kidney or heart disease, and a clinician sets the actual order.

Common mistakes

• Entering weight in pounds; the rule needs kilograms.
• Applying 4 mL/h to the whole body weight instead of only to the first 10 kg.
• Treating the maintenance figure as the full fluid order when the patient is dehydrated or losing fluid.
• Mixing up mL/h with mL/day, which differ by a factor of 24.