The 100-50-20 rule estimates a child's routine daily maintenance fluid requirement from body weight. The numbers are not random: they come from the historical Holliday-Segar approach, which linked estimated energy use to maintenance water needs and used smaller fluid amounts for each additional kilogram as body weight increased.
Need the calculation rather than the explanation?
Use the calculator for the arithmetic. This Guide explains what 100-50-20 means, why the three numbers are different and where the rule came from.
What is the 100-50-20 rule?
The rule divides body weight into three bands. Each band contributes a different amount of fluid to the daily maintenance estimate:
First 10 kg
100 mL/kg/day
Next 10 kg
50 mL/kg/day
Above 20 kg
20 mL/kg/day
Remember
100-50-20 gives a daily volume in mL/day. It is a stepped calculation, so you fill each weight band in order.
Why are the numbers 100, 50 and 20?
This is the part that makes the rule easier to remember. In their original model, Malcolm A. Holliday and William E. Segar estimated that energy use per kilogram becomes lower as body weight increases. They linked that estimated energy use to the amount of water needed for maintenance.
So the first kilograms contribute the most fluid per kilogram, the next kilograms contribute less, and kilograms above 20 kg contribute less again.
First 10 kg
Highest estimated energy use per kilogram in the model → 100 mL/kg/day.
Next 10 kg
Lower estimated energy use for each additional kilogram → 50 mL/kg/day.
Each kilogram above 20 kg
Lower again in the historical model → 20 mL/kg/day.
The child gets heavier, but each extra kilogram contributes less to the maintenance estimate.
Where did the rule come from?
Holliday and Segar published The Maintenance Need for Water in Parenteral Fluid Therapy in Pediatrics in 1957. They were trying to estimate how much water would normally need to be supplied when a patient could not meet that need in the usual way.
Their paper linked maintenance water requirements to estimated caloric expenditure. In the model, estimated caloric expenditure followed the same stepped pattern that later became familiar as 100-50-20: 100 calories/kg/day for the first 10 kg, 50 calories/kg/day for the next 10 kg and 20 calories/kg/day above 20 kg.
They also considered water lost through the skin and breathing, water lost through the kidneys, and water produced by metabolism. After accounting for these components, the model arrived at about 100 mL of externally supplied water for each 100 calories of estimated expenditure. That is why the caloric pattern and the water-maintenance pattern use the same numbers.
The simple version
Holliday and Segar estimated energy use from weight, then linked that energy use to water need. That is the idea behind 100-50-20.
How the three weight bands work
The most common student mistake is to look at the child's final weight band and multiply the whole weight by that number. That is not how the rule works. The bands are cumulative.
Example 1: an 8 kg child
All 8 kg fits inside the first band:
8 kg × 100 mL/kg/day = 800 mL/day
Example 2: a 15 kg child
The first 10 kg is calculated at 100 mL/kg/day. Only the remaining 5 kg is calculated at 50 mL/kg/day.
First 10 kg: 10 × 100 = 1,000 mL/day
Remaining 5 kg: 5 × 50 = 250 mL/day
Total = 1,250 mL/day
Common mistake: 15 × 50 is wrong because the 50 mL/kg/day rate applies only to the kilograms between 10 and 20 kg.
Example 3: a 28 kg child
Now all three bands are used:
First 10 kg: 10 × 100 = 1,000 mL/day
Next 10 kg: 10 × 50 = 500 mL/day
Remaining 8 kg: 8 × 20 = 160 mL/day
Total = 1,660 mL/day
Remember
Fill the first band, then the second, then calculate what is left.
Why does each extra kilogram get less?
A small child and a much larger child do not have the same estimated energy requirement for every kilogram of body weight. The historical Holliday-Segar model was designed to account for that changing relationship rather than using one flat mL/kg value at every weight.
You do not need to memorise the full physiology to use the rule. The important idea is that 100-50-20 is stepped because the original model assumed energy use per kilogram falls as weight increases.
What does the calculated number actually mean?
A result such as 1,660 mL/day is an estimate of routine maintenance volume. It is not automatically the child's complete fluid prescription.
The calculation does not, by itself, account for:
- Resuscitation: fluid given to treat shock or significant hypovolaemia is a separate problem.
- Existing deficit: dehydration is not the same thing as routine maintenance.
- Ongoing abnormal losses: vomiting, diarrhoea, drains and other losses may need separate assessment and replacement.
- Fluid composition: the formula estimates volume; it does not choose the IV solution.
- Clinical restriction or adjustment: illness can change how much water a child should actually receive.
Full maintenance is not always the prescribed amount
The calculation is a starting estimate, not an automatic prescription.
Royal Children's Hospital Melbourne notes that many unwell children retain water and may require less than full maintenance, for example two-thirds maintenance in many situations. The actual plan depends on the child's condition, hydration status, losses, electrolytes and local clinical guidance.
This is why it is useful to separate the maths from the clinical decision: first understand what the formula estimates, then follow the current patient-specific prescription and local guideline.
Current practice boundary: daily volume is only one part of the plan
A correct 100-50-20 total 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 100-50-20 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 whole-mL/day value 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 100-50-20 as a universal newborn-fluid formula. Neonatal fluid requirements change rapidly after birth and neonatal guidance uses age-, gestation- and condition-specific approaches. The MedMaths paediatric maintenance-fluid pathway is not a substitute for neonatal-specific guidance.
How is 100-50-20 related to the 4-2-1 rule?
The 100-50-20 method gives a daily maintenance estimate. The familiar 4-2-1 rule is a rounded hourly shortcut based on the same weight bands. Because the hourly values are rounded, multiplying 4-2-1 by 24 does not always give exactly the same result as the separate daily calculation.
To see exactly why the hourly shortcut uses 4, 2 and 1, read the 4-2-1 rule explained. If your question is about the difference between the two methods, use the comparison Guide rather than treating them as interchangeable formulas.
Compare 4-2-1 vs 100-50-20 →Common mistakes
1. Using only the final weight band
A 28 kg child is not 28 × 20. The first 20 kg still fills the first two bands.
2. Forgetting the units
100-50-20 produces mL/day, not mL/hr.
3. Treating maintenance as a dehydration calculation
Maintenance, deficit replacement and ongoing losses answer different questions.
4. Assuming the calculated volume must be prescribed
The child's clinical condition can change the actual fluid plan.
5. Applying the rule indiscriminately to neonates
Use neonatal-specific guidance when caring for newborns.
100-50-20 in one minute
First 10 kg → 100 mL/kg/day
Next 10 kg → 50 mL/kg/day
Above 20 kg → 20 mL/kg/day
Memory rule: It is a stepped daily estimate. Fill each weight band in order.
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 100-50-20 and 4-2-1 maintenance rates, restriction, monitoring, enteral preference and guideline-dependent maximums.
Queensland paediatric guidance: Children's Health Queensland. Intravenous maintenance fluids. v3 April 2026. Provides current paediatric maintenance-fluid practice and local upper-limit context.
UK clinical guidance: NICE NG29. Intravenous fluid therapy in children and young people in hospital: recommendations. Uses Holliday-Segar for routine maintenance and covers 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 formula 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.