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Fluid and Blood Calculator
Enter the weight and starting Hb to get routine maintenance fluids, the urine output target, estimated blood volume and allowable blood loss, with weight-based volumes of blood products. Adults get the NICE 25–30 mL/kg/day range; children the 4-2-1 rule. Allowable blood loss is a planning estimate, not a transfusion trigger: decide from measured Hb, ongoing bleeding and the clinical picture.
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Physiology & fluids
Normal values, ventilation and fluid requirements.
Blood
Blood volume, allowable loss and blood products.
Worked example
What this calculator shows for a typical patient. Change the values above for yours.
| Maintenance fluid (25–30 mL/kg/day) | 72.9–87.5 mL/h |
|---|---|
| Maintenance per 24 hours | 1750–2100 mL |
| 4-2-1 rule, for comparison | 110 mL/h |
| Estimated blood volume | 4900 mL |
| Allowable blood loss (simplified estimate) | 2450 mL |
| Fresh frozen plasma | 1050–1400 mL |
Frequently asked questions
How do you calculate allowable blood loss?
Allowable blood loss = estimated blood volume × (starting Hb − threshold Hb) / starting Hb. For a 70 kg man (blood volume 4900 mL) with an Hb of 14 g/dL and a threshold of 7 g/dL, it is about 2450 mL. It is a simplified planning estimate: it is not a transfusion trigger or a reason to delay transfusion, and the threshold should suit the patient.
How do you estimate blood volume?
In mL/kg: neonates 85, infants 80, children 75, adult men 70 and adult women 65. For adults with a BMI of 30 or more, the calculator uses 70 / √(BMI / 22) mL/kg (Lemmens), because fat holds less blood per kg than lean tissue. These are estimates, not measurements.
How much FFP should be given?
Give FFP only for an indication, such as clinically significant bleeding with abnormal coagulation tests or as a transfusion protocol directs (NICE NG24). The usual dose is 15 to 20 mL/kg (British Society for Haematology): 1050 to 1400 mL for a 70 kg adult, about 4 to 5 units, though unit volumes vary. Reassess clinically and with coagulation tests after each dose.
What urine output should be the target?
At least 0.5 mL/kg/h in adults, 1 mL/kg/h in children and 1 to 2 mL/kg/h in infants. These are monitoring references: low urine output alone does not call for a fluid bolus; consider perfusion, renal function, diuretics and the surgical context.
Formulas & sources
How each value is calculated. Tap next to an item to jump here.
Physiology & fluids
Maintenance fluid
Back to dose ↑Adults: NICE CG174 routine maintenance, 25–30 mL/kg/day, shown per hour. The 4-2-1 rate is shown only for comparison. In obese adults use ideal body weight and the lower end: patients rarely need more than 3 L/day, and seek expert advice if the BMI is over 40 (NICE CG174 1.4.2).
Children: Holliday–Segar 4-2-1 hourly rate (4 mL/kg/h for the first 10 kg, 2 mL/kg/h for the next 10 kg, then 1 mL/kg/h), and the daily volume of 100, 50 and 20 mL/kg/day for the same bands (NICE NG29). Use isotonic fluids and reassess fluid balance and electrolytes.
These are starting points for routine maintenance, not resuscitation or replacement of losses.
Related terms: 4-2-1 rule, Holliday Segar, fluids
Urine output target
Back to dose ↑Adults at least 0.5 mL/kg/h, children 1 mL/kg/h, infants 1–2 mL/kg/h.
Related terms: UO, oliguria
Blood
Estimated blood volume
Back to dose ↑mL/kg: neonate 85, infant 80, child 75, adult male 70, adult female 65.
For adults with BMI ≥ 30, Lemmens (2006) uses 70 / √(BMI / 22) mL/kg, because fat is poorly perfused.
Related terms: EBV, circulating volume
Allowable blood loss (estimate)
Back to dose ↑ABL = EBV × (Hb start − Hb threshold) / Hb start. Enter Hb in g/dL or g/L (both fields use the chosen unit). The threshold defaults to 7 g/dL (70 g/L).
This is a planning estimate only, not a transfusion trigger. Use measured Hb, ongoing bleeding and the clinical picture to decide on transfusion.
Related terms: ABL, MABL
Red cells
Back to dose ↑Children: 10–15 mL/kg. About 4 mL/kg raises Hb by about 1 g/dL (10 g/L) (BSH 2016).
Adults: transfuse one unit at a time and reassess.
Related terms: RBC, packed red cells, transfusion
Fresh frozen plasma
Back to dose ↑15–20 mL/kg (BSH), for clinically significant bleeding with abnormal coagulation tests or as a transfusion protocol directs (NICE NG24). Unit volumes vary; reassess after each dose.
Related terms: FFP, plasma
Platelets
Back to dose ↑Under 15 kg: 10–20 mL/kg. From 15 kg: one adult dose (BSH).
Related terms: PLT
Cryoprecipitate
Back to dose ↑Children 5–10 mL/kg. Adults: two pools (about 10 donor units).
Related terms: cryo, fibrinogen
Tranexamic acid
Back to dose ↑15 mg/kg IV (maximum 1 g) over 10 minutes. In adult trauma, follow it with 1 g over 8 hours (CRASH-2).
Reduce in renal impairment.
Related terms: TXA, Cyklokapron
Main sources
- BNF and BNF for Children (NICE), current editions.
- Association of Paediatric Anaesthetists (APA) and Advanced Paediatric Life Support (APLS).
- Resuscitation Council UK, 2025 Guidelines (paediatric and adult life support) and 2021 anaphylaxis guidance.
- Association of Anaesthetists: peri-operative management of the obese surgical patient (2015), Quick Reference Handbook (local anaesthetic toxicity 3-10, malignant hyperthermia 3-8); European MH Group (2024).
- British Society for Haematology (BSH) transfusion guidelines, NICE CG174 (intravenous fluids) and product licences (SmPC, emc).
For education only. These figures support, but do not replace, clinical judgement, local protocols and the current BNF/BNFc and product literature. Check every dose before giving it and titrate to effect. See also the disclaimer.