What maintenance fluid actually replaces
Maintenance fluid covers the water a patient loses simply by being alive over 24 hours: urine, stool, and the insensible losses from skin and breathing. It does not replace a deficit already present from vomiting or diarrhoea, and it does not cover ongoing abnormal losses from a drain, an ostomy or a fever. Those are three separate calculations added on top, and confusing them is the commonest error in a fluid order.
Malcolm Holliday and William Segar published the method in Pediatrics in 1957. Their insight was that water requirement tracks energy expenditure almost exactly — roughly 1 mL of water per kilocalorie metabolised — and energy expenditure per kilogram falls as a child grows. A 5 kg infant burns far more calories per kilogram than a 50 kg adolescent, so it needs far more water per kilogram. That is why the allowance steps down from 100 mL/kg/day in the first 10 kg to 50 in the second and 20 thereafter, rather than staying flat.
The 4-2-1 rule is the same calculation divided by 24 and rounded to whole numbers for bedside use: 100 ÷ 24 = 4.17 becomes 4, 50 ÷ 24 = 2.08 becomes 2, and 20 ÷ 24 = 0.83 becomes 1. The rounding is why the two methods do not agree exactly, and knowing which way they differ at a given weight is a useful check on your own arithmetic.
The two rules and why they disagree
The daily rule (100-50-20) is the original. Allow 100 mL/kg/day for each of the first 10 kg, 50 mL/kg/day for each of the next 10 kg, and 20 mL/kg/day for every kilogram beyond 20. A 15 kg child gets 100 × 10 + 50 × 5 = 1,000 + 250 = 1,250 mL/day.
The hourly rule (4-2-1) gives 4 mL/kg/hr for the first 10 kg, 2 mL/kg/hr for the next 10, and 1 mL/kg/hr thereafter. The same 15 kg child gets 4 × 10 + 2 × 5 = 40 + 10 = 50 mL/hr.
They differ, and predictably. Fifty mL/hr over 24 hours is 1,200 mL, against 1,250 mL from the daily rule — 4% less. The gap comes from the rounding: 4 is below 4.17 and 2 is below 2.08, so both of the first two tiers round down, while the third tier rounds up from 0.83 to 1. Below 35 kg the hourly rule therefore gives slightly less than the daily rule. The two cross at exactly 35 kg — 75 mL/hr × 24 = 1,800 mL, and 1,500 + 20 × 15 = 1,800 mL — and above that the third tier's upward rounding dominates, so at 70 kg the hourly rule gives 110 × 24 = 2,640 mL/day against 2,500 mL/day from the daily rule. Both differences are inside the precision anyone would order to, but you should know which direction your method errs in.
The adult ceiling. Most protocols stop scaling maintenance with weight around the adult requirement, commonly quoted as 100 mL/hr or about 2,400 mL/day. The 4-2-1 rate reaches 100 mL/hr at exactly 60 kg (40 + 20 + 40), while the 100-50-20 volume reaches 2,400 mL/day at 65 kg (1,000 + 500 + 20 × 45). Those two thresholds are not the same weight, which is why this calculator applies each cap to its own method rather than deriving one from the other.
Restricted maintenance. Orders for two-thirds or three-quarters maintenance simply multiply the result. Two-thirds of 50 mL/hr is 33.3 mL/hr; three-quarters is 37.5 mL/hr. The restriction is applied for a clinical reason — suspected SIADH, heart failure, raised intracranial pressure, oliguric renal failure — and should carry a stated review interval, because restricted fluid is as capable of harm as excess fluid.
Worked example: a 30 kg child on full maintenance
A 30 kg seven-year-old is admitted for appendicitis and is nil by mouth. You need the maintenance rate for the pump and the 24-hour volume for the fluid balance chart.
- Split the weight into tiers. First 10 kg: 10 kg. Second 10 kg: 10 kg. Above 20 kg: 30 − 20 = 10 kg.
- First tier, hourly. 10 kg × 4 mL/kg/hr = 40 mL/hr.
- Second tier, hourly. 10 kg × 2 mL/kg/hr = 20 mL/hr.
- Third tier, hourly. 10 kg × 1 mL/kg/hr = 10 mL/hr.
- Maintenance rate. 40 + 20 + 10 = 70 mL/hr.
- Daily volume by 100-50-20. 10 × 100 + 10 × 50 + 10 × 20 = 1,000 + 500 + 200 = 1,700 mL/day.
- Cross-check. 70 mL/hr × 24 = 1,680 mL/day, 20 mL below the daily rule's 1,700 — a 1.2% difference, in the expected direction for a child of this size.
- Per kilogram. 1,700 ÷ 30 = 56.7 mL/kg/day, which sits sensibly between the 100 mL/kg/day of an infant and the 30-odd mL/kg/day of an adult.
If the order had read two-thirds maintenance, the rate would be 70 × 2 ÷ 3 = 46.7 mL/hr, which on most pumps would be programmed as 47 mL/hr, and the daily volume 1,700 × 2 ÷ 3 = 1,133 mL.
What the number does and does not cover
Maintenance is only one of three components. A dehydrated child needs maintenance plus a deficit plus ongoing losses. The deficit is estimated as percentage dehydration × weight × 10 mL per kg per percent — a 5% dehydrated 15 kg child is down 750 mL — and is typically replaced over 24 hours alongside maintenance, with any resuscitation boluses already given subtracted. Ongoing losses from diarrhoea, an ileostomy or a nasogastric drain are measured and replaced separately, usually with a fluid matched to their electrolyte content.
Fever raises the requirement. A commonly taught adjustment is roughly 10–12% more maintenance fluid for each degree Celsius of sustained fever above normal, because insensible loss rises with temperature and respiratory rate.
Some patients need less, not more. Post-operative patients, those with meningitis or head injury, and anyone with a condition that stimulates antidiuretic hormone will retain free water. Full maintenance in those settings is a recognised route to hyponatraemia.
The volume does not choose the fluid. This is the single most important change in practice since Holliday and Segar wrote. Their paper described the water and electrolyte requirement, and for decades hypotonic solutions such as 0.45% saline or 0.2% saline in dextrose were used to deliver it. Repeated reports of hospital-acquired hyponatraemia led the American Academy of Pediatrics to issue a 2018 clinical practice guideline recommending isotonic fluid with appropriate potassium chloride and dextrose for maintenance in patients aged 28 days to 18 years. Compute the volume here; take the composition from your unit's protocol.
Reassess rather than set and forget. Weigh daily, follow the input-output chart, and check serum sodium within 24 hours of starting maintenance fluid in any child who is unwell. A maintenance calculation is a starting point that the patient's own numbers should quickly replace.
Maintenance rate and volume by weight
| Weight (kg) | Rate (mL/hr) | Volume (mL/day) | mL/kg/day |
|---|---|---|---|
| 3.5 | 14 | 350 | 100.0 |
| 5 | 20 | 500 | 100.0 |
| 8 | 32 | 800 | 100.0 |
| 10 | 40 | 1,000 | 100.0 |
| 12 | 44 | 1,100 | 91.7 |
| 15 | 50 | 1,250 | 83.3 |
| 18 | 56 | 1,400 | 77.8 |
| 20 | 60 | 1,500 | 75.0 |
| 25 | 65 | 1,600 | 64.0 |
| 30 | 70 | 1,700 | 56.7 |
| 40 | 80 | 1,900 | 47.5 |
| 50 | 90 | 2,100 | 42.0 |
| 60 | 100 | 2,300 | 38.3 |
| 70 | 100 (capped from 110) | 2,400 (capped from 2,500) | 34.3 |
The mL/kg/day column falls steadily with size, which is the whole point of the tiered rule: water requirement follows metabolic rate, and metabolic rate per kilogram falls as a child grows.
Volume is not the same as fluid choice
The AAP 2018 clinical practice guideline recommends isotonic maintenance fluid containing appropriate potassium chloride and dextrose for patients aged 28 days to 18 years requiring maintenance IV fluids, on the evidence that hypotonic solutions were associated with hospital-acquired hyponatraemia. This calculator gives volume and rate only. Take the composition, the dextrose concentration and the potassium from the order and your local protocol, and check serum sodium in any child who remains on IV maintenance for more than a day.
Errors that produce the wrong fluid order
- Using pounds as kilograms. A 33 lb child is 15 kg, not 33 kg, and the difference is 50 mL/hr against 73 mL/hr. Use the unit selector.
- Applying 4 mL/kg/hr to the whole weight. For a 30 kg child that gives 120 mL/hr instead of 70 — a 71% overdose. The tiers are cumulative bands, not a lookup by total weight.
- Confusing maintenance with resuscitation. A shock bolus of 10–20 mL/kg is given over minutes; maintenance runs over 24 hours. They are different orders with different indications.
- Adding the deficit into the maintenance figure and then also ordering maintenance. Deficit, maintenance and ongoing losses are three separate quantities that are summed once.
- Forgetting to subtract enteral intake. A child taking oral or nasogastric feeds needs the IV rate reduced by what is going in by mouth, or the total will exceed maintenance.
- Running full maintenance in a patient at risk of SIADH. Post-operative, meningitis, head injury and bronchiolitis all raise antidiuretic hormone; restriction and isotonic fluid are the standard responses.
- Applying Holliday-Segar to a neonate. The method was derived beyond the newborn period. Neonatal fluid is prescribed in mL/kg/day and advanced daily over the first week of life.
Related calculations in the same order set
A complete fluid plan for a sick child usually needs four numbers. The bolus for resuscitation is 10–20 mL/kg of isotonic crystalloid given rapidly and repeated on reassessment. The deficit comes from estimated percentage dehydration times weight. The maintenance is what this page computes. Ongoing losses are measured and matched. Only the last three are usually combined into one infusion rate.
Once you have a rate in mL/hr, the pump takes it directly, but a gravity set does not — converting to drops per minute needs the set's drop factor, which is what an IV drip rate conversion handles, and the same rate expressed for a syringe driver or a diluted drug is the province of an infusion rate calculation. Medication doses on the same order set are almost all weight-based, which is the weight-based dosage calculation, and chemotherapy and some other agents are dosed by body surface area instead.
An alternative to weight tiers is the body surface area method, which allows roughly 1,500 mL/m²/day. It tracks metabolic rate more closely in very large or very small patients and is preferred in some oncology and burns protocols, but it needs height as well as weight and is harder to do in the head. Veterinary medicine uses a third approach, an allometric power law of the form 132 × BW0.75 mL/day for dogs, which is worked through in the veterinary fluid therapy calculation; the underlying idea — that requirement scales with metabolic rate rather than with mass — is exactly Holliday and Segar's.
For patients on insulin infusions, fluid and glucose management run together, and the bolus arithmetic in an insulin correction dose calculation is usually being done on the same chart.
Key terms
- Maintenance fluid
- The volume needed over 24 hours to replace normal urinary, stool and insensible losses in a patient who is not eating or drinking.
- Insensible losses
- Water lost through skin evaporation and respiration, roughly a third of the maintenance requirement in a child, and higher with fever, tachypnoea or radiant warmers.
- Deficit
- Fluid already lost before treatment starts, estimated as percentage dehydration × weight × 10 mL per kg per percent.
- Isotonic fluid
- A solution with sodium concentration close to plasma, such as 0.9% sodium chloride or lactated Ringer's. Now the recommended maintenance fluid in children outside the newborn period.
- SIADH
- Syndrome of inappropriate antidiuretic hormone secretion, in which retained free water dilutes serum sodium. Post-operative state, meningitis, head injury and some lung disease all provoke it.
- Two-thirds maintenance
- A restricted order equal to 66.7% of the calculated maintenance volume, used where fluid retention is expected.