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Maintenance Fluids Calculator

Calculates maintenance fluid requirements from body weight with the Holliday-Segar method: the hourly 4-2-1 rule and the daily 100-50-20 mL/kg volume, plus the NICE 25–30 mL/kg/day adult range.

When to use

You need a baseline maintenance fluid rate for a child or adult who cannot take fluids orally.

Do not use when: There are ongoing losses, dehydration, heart or kidney failure, or the patient is a neonate; those need individual assessment and the estimate is informational only.

Formula

hourly = 4 × min(W, 10) + 2 × min(max(W − 10, 0), 10) + 1 × max(W − 20, 0); daily = 100 × min(W, 10) + 50 × min(max(W − 10, 0), 10) + 20 × max(W − 20, 0); W = weight_kg

Holliday and Segar (1957) linked water needs to caloric expenditure (1 mL per kcal); the 4-2-1 rule is the bedside approximation of the daily formula, and adults usually receive less (NICE: 25–30 mL/kg/day) because the rule over-predicts at high weights. Results are informational only and not a substitute for clinical judgement or medical advice.

Inputs

Parameter	Type	Unit	Required	Description
`weight_kg`	number	kg	yes	Body weight in kilograms. Range: ≥ 1, ≤ 300

Outputs

Output	Type	Unit	Description
`hourly_rate_ml_per_h`	number	mL/h	4 mL/kg/h for the first 10 kg, 2 for the next 10 kg, 1 for each kg above 20.
`daily_volume_ml`	number	mL/day	100 mL/kg for the first 10 kg, 50 for the next 10 kg, 20 for each kg above 20.
`daily_from_hourly_ml`	number	mL/day	The 4-2-1 rule extrapolated to a day (slightly below the 100-50-20 volume).
`nice_adult_min_ml_per_day`	number	mL/day	25 mL/kg/day, the lower bound NICE CG174 recommends for adults; only reported for weights of 40 kg or more.
`nice_adult_max_ml_per_day`	number	mL/day	30 mL/kg/day, the upper bound for adults; only reported for weights of 40 kg or more.

Example

25 kg child: {"weight_kg":25} → {"hourly_rate_ml_per_h":65,"daily_volume_ml":1600,"daily_from_hourly_ml":1560}

70 kg adult: {"weight_kg":70} → {"hourly_rate_ml_per_h":110,"daily_volume_ml":2500,"nice_adult_min_ml_per_day":1750,"nice_adult_max_ml_per_day":2100}

GET https://tttkmbb.com/api/v1/calculate/maintenance-fluids?weight_kg=25

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/maintenance-fluids (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/maintenance-fluids · Markdown: https://tttkmbb.com/medical/maintenance-fluids.md · JSON definition: https://tttkmbb.com/medical/maintenance-fluids.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="maintenance-fluids"
OpenAPI operationId: calculate_maintenance_fluids
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

FAQ

Why do the hourly and daily results disagree?

100 mL/kg/day is 4.17 mL/kg/h; the 4-2-1 rule rounds this down, so hourly × 24 is about 2–3% below the daily formula.

Which fluid should be used?

The method gives volume only. Current paediatric guidance favours isotonic fluids with glucose; the choice of fluid and electrolytes is a clinical decision.

Related calculators

Parkland Formula Calculator — Burn resuscitation volume on top of maintenance.
IV Drip Rate Calculator — Convert the hourly rate into a drip rate.
Water Intake Calculator — Oral fluid guidance for healthy adults.