# 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.

- Calculator id: `maintenance-fluids` · Category: Medical & Clinical (`medical`) · Tool name: `calculate_maintenance_fluids`
- Canonical page: https://tttkmbb.com/medical/maintenance-fluids · This document: https://tttkmbb.com/medical/maintenance-fluids.md · JSON definition: https://tttkmbb.com/medical/maintenance-fluids.json

## Purpose

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.

**Use when:** 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.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `weight_kg` | number | kg | required | Body weight in kilograms. (min 1, max 300) |

## Output

| Field | 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. |

## 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.

## Data Sources

- Holliday MA, Segar WE (1957) The maintenance need for water in parenteral fluid therapy, Pediatrics 19:823-832 — https://pubmed.ncbi.nlm.nih.gov/13431307/ (peer_reviewed, retrieved 2026-09-24)
- NICE CG174 (2013, updated 2017) Intravenous fluid therapy in adults in hospital — https://www.nice.org.uk/guidance/cg174 (standard, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/maintenance-fluids?weight_kg=…`
- `POST https://tttkmbb.com/api/v1/calculate/maintenance-fluids` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/maintenance-fluids · OpenAPI operationId `calculate_maintenance_fluids` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "maintenance-fluids", "inputs": {…}}`

## Example

- 25 kg child: inputs `{"weight_kg":25}` → `{"hourly_rate_ml_per_h":65,"daily_volume_ml":1600,"daily_from_hourly_ml":1560}`
- 70 kg adult: inputs `{"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
```

## Limitations

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. 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. All values are computed from the formula above; no measurement or live data is involved.

## 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

- [Parkland Formula Calculator](https://tttkmbb.com/medical/parkland-formula.md) — Burn resuscitation volume on top of maintenance.
- [IV Drip Rate Calculator](https://tttkmbb.com/medical/iv-drip-rate.md) — Convert the hourly rate into a drip rate.
- [Water Intake Calculator](https://tttkmbb.com/health/water-intake.md) — Oral fluid guidance for healthy adults.