Actual weight
Measured / verified weight
Use only when the source asks for actual body weight.
Body measures for dosing
Which body measure do you need?
Actual weight, IBW, adjusted body weight and BSA answer different calculation questions. Start with the measure named by the clinical source, then calculate that exact value.
Source chooses the measure
Units stay visible
Clinical context matters
Decision aid
Four different body-size descriptors
kg
Ideal body weight
Height + named IBW equation
MedMaths uses Devine as the default IBW estimate.
Adjusted body weight
IBW + 0.4 × (actual − IBW)
Use the 0.4 method only when the relevant source specifies it.
Body surface area
Height + weight + named BSA formula
Use the equation named by the protocol or medicine reference.
Key rule: these values are not interchangeable just because they all describe body size.
Calculate now
What does your clinical source ask for?
Choose the calculator that returns the exact body measure named by the medicine reference, protocol or equation you are following.
Height + weight
→ BSA (m²)
Calculate body surface area in m² from height and weight using the formula named by the relevant protocol or reference.
Use when: Your source asks for BSA or an m² value.
Calculate BSAHeight + Devine equation
→ IBW (kg)
Calculate adult Devine ideal body weight when the clinical source specifically requires IBW as its reference weight.
Use when: Your source explicitly asks for Devine IBW.
Calculate IBWActual weight + IBW
→ AdjBW (kg)
Calculate the 0.4 adjusted-body-weight method from actual weight and Devine IBW when a clinical source explicitly requires it.
Use when: Your source explicitly asks for the 0.4 AdjBW method.
Calculate AdjBWThe calculation does not choose the body measure
MedMaths can calculate BSA, Devine IBW or the 0.4 adjusted-body-weight method. It does not decide which body measure a medicine, renal equation or protocol should use. Confirm the required measure in the applicable source first.
Understand
Understand body measures for medication maths
Use these Guides when the difficult part is choosing or understanding the measure rather than performing the arithmetic.
BSA formula choice
Which BSA Formula Should You Use?
Compare Mosteller, Du Bois, Haycock and Gehan & George, and follow the equation named by the protocol or medicine reference.
Read Guidemg/m² dosing
BSA Medication Dosing Explained
Follow an mg/m² order from a known BSA to the patient-specific medication dose, while keeping volume and protocol limits as separate steps.
Read GuideIBW vs AdjBW
Ideal vs Adjusted Body Weight for Dosing
Understand how IBW and AdjBW differ, what the 0.4 adjustment means and why the medicine-specific source must choose the weight method.
Read GuideLearn
Learn where body size enters the dose
MedMaths has two lessons that directly connect body-size values to medication calculations. IBW and AdjBW are covered through the Guides and calculators rather than a forced duplicate lesson.
1Weight-Based DosingUnderstand how a verified dosing weight fits into an mg/kg medication order.2BSA Dosing (mg/m²)Keep BSA in m² separate from an order in mg/m², then calculate the patient dose.
Open specialised dosing lessons Practise
Practise the dosing step
Practice focuses on using a body-size value inside a medication calculation rather than memorising a body-measure formula.
4 levels
Weight-Based Dosing
mg/kg × verified weight → dose
Open quiz levels4 levels
BSA Dosing
mg/m² × BSA → patient dose
Open quiz levelsCommon body-measure mistakes
Do not let similar-looking numbers become interchangeable
Choosing the measure yourself
The medicine reference, protocol or equation should tell you whether to use actual weight, IBW, AdjBW, BSA or another descriptor.
Treating IBW as a healthy-weight target
Clinical IBW equations are calculation tools. They are not a general recommendation for what a person should weigh.
Assuming AdjBW is universal
The 0.4 adjusted-weight method is not a universal dosing rule. Use it only when the applicable source specifies that method.
Mixing BSA with other weight methods
BSA is reported in m² and is not interchangeable with BMI, mg/kg dosing or a body weight reported in kilograms.
Used in clinical areas
See body measures alongside the other calculations and context commonly encountered in each specialty.
Related calculation topics
Continue when the body measure becomes an input to a medication dose or renal calculation.
Body measure calculator FAQs
Answers about choosing BSA, IBW, or adjusted body weight without duplicating the individual calculator formulas.
Body measure calculator FAQs
Answers about choosing BSA, IBW, or adjusted body weight without duplicating the individual calculator formulas.
Which body measure calculator should I use?
Use the calculator that matches the body measure explicitly named by the medicine reference, protocol, equation, prescriber, pharmacist, or local policy.
- Choose BSA when the source asks for body surface area in m².
- Choose IBW when the source specifically asks for adult Devine ideal body weight.
- Choose AdjBW when the source specifically asks for the 0.4 adjusted-body-weight method.
What is the difference between BSA, IBW, and adjusted body weight?
They are different body-size descriptors used for different calculation jobs; they should not be treated as interchangeable.
- BSA is an area estimate reported in m² and is calculated from height and weight.
- Devine IBW is an adult reference-weight equation based mainly on height and the selected sex equation.
- The AdjBW calculator on MedMaths starts with Devine IBW and adds 40% of the difference between actual weight and IBW when the method applies.
Does MedMaths decide which dosing weight or body measure a medicine should use?
No. MedMaths calculates the body measure you select; it does not decide which measure is clinically appropriate for a medicine or patient.
- The correct body measure is source- and medicine-specific.
- If the required measure is unclear, verify it with the applicable reference or clinical team before calculating or administering a medicine.
Where can I compare the different BSA formulas?
Use the MedMaths BSA Formula Guide for the Mosteller, Du Bois, Haycock, and Gehan & George comparison; use the BSA Calculator when you need the number itself.
- The calculator performs the arithmetic for the selected equation.
- The Guide explains why the formulas differ and how to follow a protocol-specified formula.
Formula origins in this topic
The equations used in these calculators were developed by published researchers. These researchers are not affiliated with MedMaths and have not reviewed or endorsed this site.
Devine Ideal Body Weight Formula
Equation attributed to: Ben J. Devine
References
Background sources for body surface area, the Mosteller BSA equation and the Devine ideal-body-weight equations.
References
Background sources for body surface area, the Mosteller BSA equation and the Devine ideal-body-weight equations.
Body Surface Area
NCBI Bookshelf / StatPearls
Clinical overview of body surface area and its use in physiological and medication calculations.
Simplified Calculation of Body-Surface Area
New England Journal of Medicine
Robert Mosteller's 1987 publication proposing the simplified square-root BSA equation.
The Origin of the ‘Ideal’ Body Weight Equations
Annals of Pharmacotherapy / SAGE
Historical review of the equations commonly described as Devine ideal body weight formulas.