The 4-2-1 rule is a quick way to estimate a child's hourly maintenance fluid rate from body weight. The numbers are not random. They are rounded hourly versions of the daily Holliday-Segar 100-50-20 method, which is why 4-2-1 gives an answer in mL/hr.
Need the calculation rather than the explanation?
Use the calculator for the arithmetic. This Guide explains where 4, 2 and 1 come from, how the weight bands work and why the hourly shortcut does not always match the daily method exactly.
What is the 4-2-1 rule?
The rule divides body weight into three bands. Each band contributes a different amount to the hourly maintenance estimate:
First 10 kg
4 mL/kg/hr
Next 10 kg
2 mL/kg/hr
Above 20 kg
1 mL/kg/hr
Remember
100-50-20 is the daily method. 4-2-1 is the rounded hourly shortcut.
Why are the numbers 4, 2 and 1?
The easiest way to understand 4-2-1 is to start with the daily 100-50-20 rule. That method uses 100, 50 and 20 mL/kg/day across the same three weight bands.
To turn a daily amount into an hourly amount, divide by 24 hours:
| Daily value | ÷ 24 hours | Rounded shortcut |
|---|---|---|
| 100 mL/kg/day | 4.17 mL/kg/hr | 4 mL/kg/hr |
| 50 mL/kg/day | 2.08 mL/kg/hr | 2 mL/kg/hr |
| 20 mL/kg/day | 0.83 mL/kg/hr | 1 mL/kg/hr |
That is the key idea: 4-2-1 is not a completely separate physiological formula. It is a practical hourly shortcut based on the daily Holliday-Segar weight bands. A modern review of paediatric parenteral fluids describes the 4-2-1 method this way: divide the daily Holliday-Segar components by 24, then use the simplified hourly values.
The simple version
Daily numbers divided by 24 give roughly 4, 2 and 1. The shortcut rounds them so the hourly rate is easy to calculate at the bedside.
Why does 4-2-1 × 24 not always equal 100-50-20?
Because the hourly numbers have been rounded. For example, 100 ÷ 24 is about 4.17, not exactly 4. Likewise, 20 ÷ 24 is about 0.83, not exactly 1.
So if you calculate a 4-2-1 hourly rate and multiply that result by 24, the answer can be slightly different from a separate 100-50-20 daily calculation.
Remember
Related does not mean identical. 4-2-1 is rounded for convenience, so the two methods can give slightly different 24-hour totals.
How the three weight bands work
The rule is cumulative. You do not look at the child's final weight band and apply that number to the whole weight. You fill each band in order.
Example 1: an 8 kg child
All 8 kg sits in the first band:
8 kg × 4 mL/kg/hr = 32 mL/hr
Example 2: a 15 kg child
The first 10 kg uses 4 mL/kg/hr. Only the remaining 5 kg uses 2 mL/kg/hr.
First 10 kg: 10 × 4 = 40 mL/hr
Remaining 5 kg: 5 × 2 = 10 mL/hr
Total = 50 mL/hr
Example 3: a 28 kg child
All three bands are needed:
First 10 kg: 10 × 4 = 40 mL/hr
Next 10 kg: 10 × 2 = 20 mL/hr
Remaining 8 kg: 8 × 1 = 8 mL/hr
Total = 68 mL/hr
Common mistake: 28 × 1 = 28 mL/hr is wrong. The child's whole weight does not move into the final band when the weight passes 20 kg.
Memory rule
First 10, next 10, then what is left.
What does mL/kg/hr actually mean?
The units tell you what the number represents. 4 mL/kg/hr means 4 millilitres for each kilogram of body weight, each hour.
kg × mL/kg/hr = mL/hr
The kg units cancel, leaving the hourly volume rate that you are trying to calculate.
This is a useful check. If the question asks for an hourly maintenance rate, your final unit should normally be mL/hr, not mL/day.
4-2-1 vs 100-50-20
They use the same weight bands but answer the question in different time units:
| 4-2-1 | 100-50-20 |
|---|---|
| Hourly shortcut | Daily Holliday-Segar method |
| Answer in mL/hr | Answer in mL/day |
| Rounded 4, 2 and 1 coefficients | 100, 50 and 20 coefficients |
This Guide focuses on why the hourly rule is 4-2-1. If you want a full comparison of when the two methods differ and how to interpret the two results together, use the dedicated comparison Guide.
Compare 4-2-1 vs 100-50-20 →What does the calculated rate actually tell you?
The 4-2-1 result estimates a routine maintenance fluid rate. It does not automatically tell you the full fluid plan for an unwell child.
The calculation does not, by itself, determine:
- Resuscitation: fluid used to treat shock or significant hypovolaemia is a separate clinical problem.
- Existing deficit: dehydration and routine maintenance are not the same calculation.
- Ongoing abnormal losses: vomiting, diarrhoea, drains and other losses may need separate assessment and replacement.
- Fluid composition: 4-2-1 estimates a rate; it does not select the IV fluid solution.
- Patient-specific adjustment: illness, fluid balance and current clinical guidance can change the prescribed rate.
Full maintenance is not always the prescribed rate
Treat the calculation as an estimate, not an automatic prescription.
Royal Children's Hospital Melbourne notes that the majority of unwell children retain water and often require less than full maintenance. NICE also separates routine maintenance from resuscitation, deficit replacement and ongoing losses, and recommends patient-specific reassessment and adjustment.
This is why it helps to keep the maths and the clinical decision separate. First understand what the formula estimates. Then follow the current prescription, local paediatric guidance and clinical assessment.
Current practice boundary: volume is only one part of the plan
A correct 4-2-1 rate is still only a baseline maintenance-volume estimate.
In patients aged 28 days to 18 years who are within the AAP guideline population and require maintenance IV fluid, AAP guidance recommends isotonic solutions with appropriate potassium chloride and dextrose because this reduces hospital-acquired hyponatraemia. NICE also recommends isotonic crystalloid for routine paediatric maintenance. The 4-2-1 arithmetic itself does not select a bag, potassium content or dextrose plan.
- Route: the formula does not decide whether IV therapy is needed. Oral or enteral hydration should be used or considered when clinically appropriate and tolerated under the applicable guidance.
- Weight basis: use the current authorised kilogram weight basis. If the clinical plan specifies ideal weight, dry weight, BSA or another approach, use that method rather than letting the calculator choose.
- Reduced maintenance: acute illness can increase ADH and water retention. RCH commonly describes two-thirds maintenance for many unwell children, NICE gives a 50–80% example when non-osmotic ADH creates water-retention risk, and ESPNIC describes condition-specific restriction ranges. These are clinical adjustments, not a universal MedMaths percentage.
- Large-child maximums: the formula can continue mathematically above a local clinical maximum. RCH and Children's Health Queensland use 2,400 mL/day or 100 mL/hr as a normal maximum, while NICE uses different adult-sized limits. Do not silently impose one universal ceiling.
- Monitoring and fluid creep: reassess the need for IV maintenance and the child's fluid balance, weight, electrolytes and glucose according to the applicable guideline. Count other fluid sources such as IV medicines, medication carriers/boluses, line flushes, blood products and enteral intake.
- Precision: a displayed decimal is presentation, not a universal prescribing or pump-programming rule. Final documented/programmed precision follows the authorised order, local policy and device capability.
What about newborns?
Do not treat 4-2-1 as a universal newborn-fluid rule. Neonatal fluid requirements change rapidly after birth and need age-, gestation- and condition-specific guidance. NICE, for example, gives separate daily maintenance guidance for term neonates rather than applying the child and young-person Holliday-Segar recommendation from birth.
Common mistakes
1. Multiplying the whole weight by the final band
A 28 kg child is not 28 × 1. Each weight band keeps its own rate.
2. Giving the answer in mL/day
4-2-1 is an hourly shortcut, so the result is in mL/hr.
3. Assuming 4-2-1 × 24 must equal 100-50-20
The hourly coefficients are rounded, so the methods are related but not exact 24-hour conversions.
4. Using maintenance as a resuscitation or deficit calculation
Maintenance, deficit replacement, resuscitation and ongoing losses answer different questions.
5. Treating the calculated rate as an automatic prescription
The prescribed fluid plan still depends on the child's condition, monitoring and current guidance.
4-2-1 in one minute
First 10 kg → 4 mL/kg/hr
Next 10 kg → 2 mL/kg/hr
Above 20 kg → 1 mL/kg/hr
Memory rule: 100-50-20 is daily. 4-2-1 is the rounded hourly shortcut.
Sources and references
US clinical guidance: Feld LG, Neuspiel DR, Foster BA, et al. Clinical Practice Guideline: Maintenance Intravenous Fluids in Children. Pediatrics. 2018;142(6):e20183083. Strong Grade A recommendation for isotonic maintenance IV fluid in the guideline population, with important exclusions.
Systematic-review evidence: Amer BE et al. Efficacy and safety of isotonic versus hypotonic intravenous maintenance fluids in hospitalized children. Pediatr Nephrol. 2024;39:57–84. Updated meta-analysis of 33 randomised trials / 5,049 children.
Australian clinical guidance: Royal Children's Hospital Melbourne. Clinical Practice Guideline: Intravenous fluids. Covers maintenance rates, restriction, monitoring, enteral preference and guideline-dependent maximums.
Queensland paediatric guidance: Children's Health Queensland. Intravenous maintenance fluids. v3 April 2026. Includes 4-2-1 calculation, current weight workflow, fluid composition and local upper-limit context.
UK clinical guidance: NICE NG29. Intravenous fluid therapy in children and young people in hospital: recommendations. Covers routine maintenance, isotonic fluid, monitoring, restriction and neonatal-specific recommendations.
International critical-care guidance: ESPNIC clinical practice guidelines: intravenous maintenance fluid therapy in acute and critically ill children. Supports reassessment, restriction in selected patients and accounting for total fluid exposure / fluid creep.
Original maintenance-water paper: Holliday MA, Segar WE. The Maintenance Need for Water in Parenteral Fluid Therapy. Pediatrics. 1957;19(5):823-832. DOI: 10.1542/peds.19.5.823. Historical basis of the daily maintenance-water method; the later rounded 4-2-1 shorthand is not attributed verbatim to this paper.
Modern explanation of the hourly shortcut: Management of Pediatric Parenteral Fluids. Describes 4-2-1 as a simplified hourly shortcut related to the Holliday-Segar daily method.