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Obesity Drug Dosing Calculator
Enter weight and height to get BMI and ideal, lean and adjusted body weight. For adults with a BMI of 30 or more, each dose below uses the weight chosen for that drug, shown under the dose. Most choices follow the Association of Anaesthetists and SOBA guidance (2015); where a product licence differs, as for sugammadex, the calculator follows the licence. Each source is listed under Formulas & sources.
No matching items.
Patient
Body size and the weights used for drug dosing.
Physiology & fluids
Normal values, ventilation and fluid requirements.
Induction
Hypnotics, sedatives and opioids. Titrate to effect.
Neuromuscular block
Relaxants and reversal.
Local anaesthetics
Maximum recommended doses and volumes, not a guarantee of safety. Obesity: lean body weight.
Worked example
What this calculator shows for a typical patient. Change the values above for yours.
| Ideal body weight | 57 kg |
|---|---|
| Lean body weight | 59 kg |
| Adjusted body weight | 86.2 kg |
| Propofol induction (LBW) | 88.5–147 mg |
| Propofol infusion (AjBW) | 345–1034 mg/h |
| Suxamethonium (TBW) | 130–195 mg |
| Rocuronium (LBW) | 35.4 mg |
| Sugammadex (TBW) | 260–520 mg |
| Lidocaine maximum (LBW) | 177 mg |
Frequently asked questions
How do you calculate lean body weight?
With the Janmahasatian formula: men 9270 × weight / (6680 + 216 × BMI), women 9270 × weight / (8780 + 244 × BMI), with weight in kg. Unlike the older James formula, it does not fall at very high BMI.
How do you calculate ideal body weight?
With the Devine formula: 50 kg for men or 45.5 kg for women, plus 0.91 kg for every cm of height above 152.4 cm. It is the same as the predicted body weight used for tidal volumes. The calculator does not apply it below 152.4 cm, and it is not used for children.
What is adjusted body weight?
Adjusted body weight = ideal body weight + 0.4 × (total body weight − ideal body weight), the 40% correction factor. It is used for drugs that partly distribute into fat, such as propofol infusions, alfentanil and neostigmine.
Why is suxamethonium dosed on total body weight?
Plasma cholinesterase activity and extracellular fluid volume both increase in obesity, so a dose on total body weight gives more reliable intubating conditions (Lemmens and Brodsky, 2006). The Association of Anaesthetists and SOBA guidance (2015) also doses it on total body weight.
Formulas & sources
How each value is calculated. Tap next to an item to jump here.
Patient
BMI
Back to dose ↑BMI = weight (kg) / height (m)². WHO adult classes: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30–34.9 obese class I, 35–39.9 class II, 40 or more class III.
In children, plot BMI on BMI-for-age centile charts instead of using adult cut-offs.
Related terms: Body mass index, habitus
Ideal body weight
Back to dose ↑Devine: 50 kg (male) or 45.5 kg (female) + 0.91 × (height (cm) − 152.4). It is identical to the ARDSnet predicted body weight used for tidal volumes.
Adults only, and not defined below 152.4 cm (5 ft).
Related terms: IBW, predicted body weight, PBW
Lean body weight
Back to dose ↑Janmahasatian (2005), recommended by the Association of Anaesthetists:
- Male: 9270 × weight / (6680 + 216 × BMI)
- Female: 9270 × weight / (8780 + 244 × BMI)
Unlike the James formula, it does not fall at very high BMI.
Related terms: LBW, fat-free mass
Adjusted body weight
Back to dose ↑AjBW = IBW + 0.4 × (TBW − IBW). It is used for drugs that partly distribute into fat, such as propofol infusions, alfentanil, neostigmine and antibiotics.
Related terms: AjBW, ABW, dosing weight
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
Induction
Propofol
Back to dose ↑Adults 1.5–2.5 mg/kg. Children (1 month–16 years) 2.5–4 mg/kg (BNF/BNFc). Give less in the elderly, in ASA 3–4 and after premedication.
Obesity: induction on lean body weight.
Related terms: Diprivan
Propofol infusion
Back to dose ↑Adults 4–12 mg/kg/h. Children over 3 years 9–15 mg/kg/h (BNFc). Obesity: adjusted body weight.
Prefer target-controlled infusion with processed EEG monitoring.
Related terms: TIVA, maintenance
Fentanyl
Back to dose ↑1–3 µg/kg at induction. Higher doses are used to obtund the pressor response and in cardiac anaesthesia. Obesity: lean body weight.
Related terms: Sublimaze
Alfentanil
Back to dose ↑10–20 µg/kg at induction, for example to blunt the response to laryngoscopy in RSI. Obesity: adjusted body weight (Association of Anaesthetists).
Related terms: Rapifen
Neuromuscular block
Suxamethonium
Back to dose ↑Infants 2 mg/kg, children 1–2 mg/kg, adults 1–1.5 mg/kg. IM in children (no IV access): 4 mg/kg, maximum 150 mg.
Obesity: total body weight, because plasma cholinesterase activity is increased.
Avoid with hyperkalaemia, burns or crush injury over 48 h old, denervation or myopathy, and a history of malignant hyperthermia.
Related terms: succinylcholine, scoline, sux
Rocuronium
Back to dose ↑Intubation 0.6 mg/kg; rapid sequence 1–1.2 mg/kg. Reversible with sugammadex. Obesity: lean body weight (Association of Anaesthetists 2015).
Related terms: Esmeron
Sugammadex
Back to dose ↑2 mg/kg when T2 has reappeared, 4 mg/kg at 1–2 post-tetanic counts, and 16 mg/kg for immediate reversal after rocuronium 1.2 mg/kg (adults; not investigated in children). Base the dose on neuromuscular monitoring.
Obesity: actual body weight, as in the product licence (SmPC). This replaces the adjusted body weight of the 2015 Association of Anaesthetists guideline.
Not recommended in severe renal impairment (SmPC).
Related terms: Bridion, reversal
Neostigmine
Back to dose ↑50 µg/kg (maximum 2.5 mg in children and 5 mg in adults) with glycopyrronium 10 µg/kg (maximum 500 µg). It works best at minimal block: give it once all four twitches are present, ideally with a train-of-four ratio of at least 0.4 on a quantitative monitor, and confirm a ratio of at least 0.9 before extubation (ASA 2023; ESAIC 2023 allows from a ratio above 0.2).
Related terms: reversal, anticholinesterase
Local anaesthetics
Lidocaine
Back to dose ↑3 mg/kg, maximum 200 mg. A 1% solution is 10 mg/mL.
Related terms: lignocaine, Xylocaine
Bupivacaine
Back to dose ↑2 mg/kg, maximum 150 mg. It is the most cardiotoxic.
Related terms: Marcain
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.