Name the indication before you name the bag
Every fluid prescription is one of four: resuscitation (500 mL of balanced crystalloid over 15 minutes, 250 mL if frail or cardiac, then reassess), routine maintenance, replacement of losses still happening, or redistribution. This calculator works the arithmetic; the patient in front of you decides the prescription.
Adult maintenance fluids for 24 hours
For a patient who is nil by mouth with no abnormal losses. Losses are replaced separately, on top.
Paediatric maintenance: the 4-2-1 rule
4 mL/kg/h for the first 10 kg, 2 mL/kg/h for the next 10 kg, 1 mL/kg/h for every kilogram above 20.
Burns: the Parkland formula
4 mL × weight (kg) × % burned area of Hartmann’s in 24 hours, half in the first 8 hours from the time of injury. Count partial- and full-thickness burns only, not erythema.
Drip rate
mL/h = volume ÷ hours. Drops per minute = mL/h × drops per mL ÷ 60.
Sodium: the arithmetic of correction
Adrogué–Madias: change in sodium from 1 litre = (infusate Na − serum Na) ÷ (total body water + 1). Free water deficit = TBW × (Na ÷ 140 − 1).
Potassium: what the number hides
Roughly 100 mmol of total body potassium is missing for every 0.3 mmol/L the plasma level falls below 4.0.
The numbers behind it
| Quantity | Rule used |
|---|---|
| Maintenance water | 25–30 mL/kg/day; 20–25 mL/kg/day if elderly, frail, cardiac or renal |
| Sodium, potassium, chloride | About 1 mmol/kg/day each |
| Glucose | 50–100 g/day, to limit starvation ketosis |
| Children | 4-2-1 mL/kg/h |
| Burns (Parkland) | 4 mL × kg × %TBSA in 24 h, half in the first 8 h from the injury; urine 0.5–1 mL/kg/h |
| Drops per minute | mL/h × drops per mL ÷ 60 (standard set 20, blood set 15, burette 60) |
| Total body water | 0.6 × kg (men); 0.5 × kg (women, elderly) |
| Sodium change per litre | (infusate Na − serum Na) ÷ (TBW + 1); no more than 10 mmol/L in 24 h (8 if high risk) |
| Free water deficit | TBW × (Na ÷ 140 − 1) |
| Potassium | Peripheral maximum 40 mmol/L and 10 mmol/h; about 100 mmol deficit per 0.3 mmol/L below 4.0 |
Bag compositions used
| Fluid | Na+ mmol/L | Glucose g/L | Main role |
|---|---|---|---|
| 0.9% sodium chloride | 154 | 0 | Resuscitation; gastric losses; head injury |
| Hartmann’s / Ringer’s lactate | 131 | 0 | Resuscitation; replacement; burns |
| Plasma-Lyte 148 | 140 | 0 | As Hartmann’s |
| 0.45% NaCl / 5% glucose | 77 | 50 | Adult maintenance |
| 0.18% NaCl / 4% glucose | 30 | 40 | Maintenance where less sodium is wanted |
| 5% glucose | 0 | 50 | Free water |
| 3% sodium chloride | 513 | 0 | Symptomatic hyponatraemia |
Compositions vary between manufacturers. Confirm them against the products stocked in your own hospital.
A calculator is not a prescription
These are teaching calculations for medical students and junior doctors. Doses, rates and products differ between hospitals, and every figure must be checked against your own formulary, local protocols and laboratory reference intervals. Assess the patient — pulse, blood pressure, perfusion, JVP, chest, urine output and weight — before and after every bag, and ask a senior when in doubt.