100-50-20 is the daily Holliday-Segar maintenance method. The familiar 4-2-1 rule is a simplified, rounded hourly shortcut based on the same weight bands. Because the hourly values are rounded, multiplying a 4-2-1 result by 24 does not always reproduce the 100-50-20 daily result exactly.
Need the number rather than the explanation?
Use the calculator for the arithmetic. This Guide is for understanding the relationship between the methods, their history and their limits.
4-2-1 vs 100-50-20: the short answer
4-2-1 — hourly estimate
- First 10 kg: 4 mL/kg/hr
- Next 10 kg: 2 mL/kg/hr
- Each kg above 20 kg: 1 mL/kg/hr
100-50-20 — daily Holliday-Segar method
- First 10 kg: 100 mL/kg/day
- Next 10 kg: 50 mL/kg/day
- Each kg above 20 kg: 20 mL/kg/day
Why 4-2-1 × 24 does not always equal 100-50-20
If the daily coefficients are divided by 24, they are about 4.17, 2.08 and 0.83 mL/kg/hr. The bedside 4-2-1 rule rounds those bands to 4, 2 and 1. That makes 4-2-1 convenient as an hourly shortcut, but it is not an exact unit conversion of the daily method.
Example: 25 kg
4-2-1 hourly result = 40 + 20 + 5 = 65 mL/hr
65 mL/hr × 24 = 1,560 mL/day
100-50-20 daily result = 1,000 + 500 + 100 = 1,600 mL/day
Both calculations can be correct for their own convention. Do not force the two totals to match.
Where Holliday-Segar comes from
Malcolm A. Holliday and William E. Segar published The maintenance need for water in parenteral fluid therapy in Pediatrics in 1957. Their work linked maintenance water needs to caloric expenditure and became the historical basis of the stepped daily 100-50-20 mL/kg/day method.
If you want to understand why the daily method uses 100, 50 and 20, see the 100-50-20 rule explained. If you want the hourly shortcut itself broken down, see the 4-2-1 rule explained.
The exact 4-2-1 shorthand is a later rounded hourly convention. MedMaths therefore credits Holliday and Segar for the historical daily maintenance-water method without claiming that their 1957 paper published the later 4-2-1 wording verbatim.
What the full-maintenance number means
The formula estimates routine maintenance water needs. It does not automatically tell you the child's total fluid requirement or complete IV-fluid prescription. Existing dehydration deficit, ongoing abnormal losses, resuscitation, fluid composition and condition-specific adjustments are separate clinical questions.
Use the calculation-weight basis specified by the applicable guideline or clinical plan. Actual measured weight is common, but selected children may require an ideal, dry or body-surface-area approach. MedMaths can verify the kilogram arithmetic, but it does not decide which clinical weight basis is appropriate.
Full maintenance is not automatically the prescribed rate
Use the arithmetic as a starting estimate, then follow the clinical guideline that applies.
Royal Children's Hospital Melbourne advises that the majority of unwell children require less than full maintenance, commonly using two-thirds maintenance unless dehydrated. NICE advises considering 50–80% of routine maintenance when there is a risk of water retention from non-osmotic ADH secretion. Those are clinical adjustments, not changes to the underlying formula.
Fluid composition is a separate decision
A volume formula does not choose the IV solution. The AAP gives a strong Grade A recommendation for isotonic maintenance IV fluids with appropriate potassium chloride and dextrose in patients aged 28 days to 18 years within its studied scope because this reduces hospital-acquired hyponatraemia. Important exclusions include neurosurgical, cardiac, hepatic and renal disease, cancer, diabetes insipidus, voluminous watery diarrhoea and severe burns. NICE also recommends isotonic crystalloids for routine maintenance in children and young people. Follow the current local guideline and patient-specific clinical plan rather than treating a volume result as a fluid prescription.
An updated systematic review and meta-analysis of 33 randomised trials involving 5,049 children likewise found isotonic maintenance fluid reduced mild hyponatraemia compared with hypotonic fluid. That evidence supports tonicity selection within the appropriate population; it does not remove the need for monitoring or condition-specific adjustments.
When separate guidance is needed
- Neonates under 28 days: use age- and gestation-specific neonatal guidance rather than the general child calculator.
- Patients older than 18 years: use adult fluid guidance rather than extending the paediatric formula beyond its supported age scope.
- Resuscitation: shock or significant hypovolaemia is a separate emergency-fluid problem, not a maintenance calculation.
- Deficit and ongoing abnormal losses: assess and manage these separately from routine maintenance.
- Important comorbidity or specialist situations: renal, cardiac, hepatic, neurosurgical, burns and other high-risk conditions can require condition-specific fluid management.
- Larger children and adolescents: check the applicable guideline for adult-sized maximums or alternative approaches.
Monitoring and fluid creep matter
The maintenance bag is not the only source of fluid. NICE recommends repeated assessment of weight, fluid input/output and balance, electrolytes, glucose and the ongoing need for IV therapy. ESPNIC further recommends counting IV medicines, bolus-drug volumes, line flushes, blood products and enteral intake when considering total daily maintenance-fluid exposure in acutely and critically ill children. This helps reduce fluid creep and cumulative positive fluid balance.
Large-child maximums are guideline dependent
MedMaths leaves the Holliday-Segar arithmetic uncapped. RCH Melbourne and Children's Health Queensland use a normal maximum of 2,400 mL/day or 100 mL/hr in larger children, while other guidance uses different adult-sized limits. There is no single universal cap that should be silently applied to every patient, so the applicable local guideline still decides the maximum.
Why different calculators can show different maintenance totals
One calculator may show 4-2-1 only, another may use the daily 100-50-20 method, and a hospital tool may also apply a local restriction or maximum. Before comparing two answers, check the method, age range, weight rule, restriction and maximum used by each tool.
Original publication: Holliday MA, Segar WE. The maintenance need for water in parenteral fluid therapy. Pediatrics. 1957;19(5):823–832. PMID 13431307. DOI 10.1542/peds.19.5.823.
NICE guidance: NICE NG29 — Intravenous fluid therapy in children and young people in hospital.
Royal Children's Hospital Melbourne guidance: Royal Children's Hospital Melbourne — Intravenous fluids.
American Academy of Pediatrics guidance: American Academy of Pediatrics — Maintenance Intravenous Fluids in Children.
Pediatric weight-safety guidance: American Academy of Pediatrics — Pediatric Readiness in the Emergency Department: Technical Report (2026). Recommends weighing and recording children in kilograms.
Critical-care maintenance-fluid guidance: ESPNIC clinical practice guidelines: intravenous maintenance fluid therapy in acute and critically ill children. Intensive Care Med. 2022;48:1691–1708.
Systematic review: Isotonic versus hypotonic intravenous maintenance fluids in hospitalized children. 33 RCTs; 5,049 patients.
Queensland guidance: Children's Health Queensland — Intravenous maintenance fluids skill sheet.