Actual vs Ideal vs Adjusted Body Weight for Dosing

At a glance

Weight	How it is obtained	What it represents	Typical role
Actual (total) body weight	Scales	Everything: lean tissue, fat, fluid	Default for most mg/kg orders in adults of normal size and in children
Ideal body weight (Devine)	Height and sex	Expected weight for height, ignoring fat	Some dosing in obesity, tidal volume, Cockcroft-Gault in some protocols
Adjusted body weight	Ideal + 0.4 × (actual − ideal)	A compromise that adds part of the excess	Some drugs and renal estimates in obesity
Lean body weight	Weight and height (e.g. Janmahasatian)	Estimated fat-free mass	Pharmacokinetic research, some anaesthetic agents

Actual (total) body weight

This is simply what the person weighs. It is the reference for most weight-based orders and for paediatric dosing, and it is the number written on the drug chart. It is also the weight to use for BMI, which in turn tells you how far the person is from an average build.

The catch is that fat and lean tissue handle drugs differently. Fat has little blood flow and holds little water, so water-soluble drugs barely enter it; clearance by liver and kidney rises with lean mass far more than with fat mass. Scaling a dose to total weight in a person with a large fat mass can therefore overshoot for many drugs.

Ideal body weight (Devine)

The Devine formula was published in 1974 for gentamicin dosing and has spread far beyond it, though it was never derived from population data on body composition:

Men: 50 kg + 2.3 kg per inch over 5 ft · Women: 45.5 kg + 2.3 kg per inch over 5 ft

The ideal body weight calculator converts height from centimetres for you. Ideal weight depends only on height and sex, so two people of the same height share the same ideal weight whatever they weigh. Below 152.4 cm (5 ft) the formula subtracts weight and becomes unreliable.

The predicted body weight used to set tidal volumes in mechanical ventilation, from the ARDS Network trial, is the same Devine equation expressed in centimetres.

Adjusted body weight

Adjusted weight adds back part of the excess above ideal weight:

Adjusted = ideal + 0.4 × (actual − ideal)

The 0.4 reflects the idea that roughly 40% of excess weight behaves like lean tissue for drug distribution. Some protocols use 0.3 or 0.5 for specific drugs. The adjusted body weight calculator uses 0.4, and it warns when actual weight is not above ideal weight, because adjustment then has no purpose.

Adjusted weight is also one of the options in the Cockcroft-Gault equation; in a large comparison with measured clearance it was the least biased choice in overweight and obese adults.

Lean body weight

Lean body weight estimates fat-free mass. The Janmahasatian equation (2005) uses weight and BMI and is one of the most cited:

Men: 9270 × weight ÷ (6680 + 216 × BMI) · Women: 9270 × weight ÷ (8780 + 244 × BMI)

Unlike ideal weight, it increases slightly as total weight increases, and it was built from measured body composition. It is widely used in pharmacokinetic research and in anaesthesia, but it is less familiar on the ward. This site does not currently have a lean body weight calculator.

Same patient, different answers

A woman 165 cm tall weighing 110 kg (BMI 40.4):

Weight	Value	10 mg/kg would be
Actual	110 kg	1100 mg
Ideal (Devine)	56.9 kg	569 mg
Adjusted (0.4)	78.1 kg	781 mg
Lean (Janmahasatian)	54.7 kg	547 mg

The 10 mg/kg figure is purely illustrative arithmetic, not a dose for any drug. Her actual weight is 193% of her ideal weight, so the choice of weight alone changes the result twofold. For a woman of the same height weighing 57 kg, actual, ideal and adjusted weight would be practically identical; only lean weight, which leaves out all fat, would still be lower. The mg/kg dose calculator multiplies by whichever weight you enter, so the decision about which weight comes first.

Which body weight should you use for dosing?

There is no single correct dosing weight. The right choice depends on how a drug distributes and how it is cleared, which is why product information and protocols specify it drug by drug. General patterns described in the pharmacokinetic literature:

Water-soluble drugs that stay mostly in lean tissue and extracellular fluid tend to be scaled to ideal or adjusted weight in obesity; aminoglycoside protocols commonly use adjusted weight.
Highly fat-soluble drugs can have a volume of distribution that grows with total weight, so loading doses are sometimes based on actual weight, while maintenance doses follow clearance, which tracks lean mass more closely.
Drugs with fixed doses (many tablets, for example) are not weight-based at all, though some labels set separate doses above a weight threshold.
Renal function: the weight entered into Cockcroft-Gault changes the estimate in direct proportion; see the guide on eGFR vs creatinine clearance.
Ventilation: tidal volume is set on predicted (ideal) weight, not on actual weight.

We recommend following local protocols and the product information for the specific drug.

Children

Paediatric doses are usually expressed per kilogram of actual weight, with a ceiling at the usual adult dose, which is why the mg/kg calculator has a Maximum single dose (optional) field. Devine and the adult adjusted weight formula do not apply to children. In children with obesity, some references use an ideal weight derived from growth charts (for example the weight at the 50th BMI centile for age and height), but practice varies widely.

Common pitfalls

Using an estimated or remembered weight. Weighing the patient whenever possible is advisable: estimated weights can be well off the mark.
Applying ideal weight to an underweight person. If actual weight is below ideal, ideal weight overstates the dose; most protocols then use actual weight.
Mixing weights within one calculation, for example a dose on actual weight and a renal adjustment on ideal weight without realising it.
Using Devine below 152 cm or in children.
Forgetting fluid. Oedema and ascites raise actual weight without adding drug-clearing tissue.
Not stating the weight used. Writing which weight underpins a dose makes later checks much easier.

Measuring height and weight

Head straight, looking ahead, with the headboard resting on the top of the head.
Barefoot, heels together and against the wall or the measuring rod.
Scales on a hard, flat floor, in light clothing; weighing at the same time of day makes changes easier to compare.

Frequently asked questions

What is the difference between ideal and adjusted body weight?

Ideal body weight is calculated from height and sex alone and ignores what the person actually weighs. Adjusted body weight starts from ideal weight and adds 40% of the excess, so it sits between ideal and actual weight. For a woman 165 cm tall weighing 110 kg, that means 56.9 kg ideal and 78.1 kg adjusted.

Is ideal body weight the same as lean body weight?

No. Ideal body weight comes from a height-based formula such as Devine; lean body weight estimates fat-free mass from weight and height, for example with the Janmahasatian equation. The two can be similar in obesity, but in a person of normal build lean weight is well below ideal weight because it leaves out all fat.

When is adjusted body weight used?

Mainly when actual weight is well above ideal weight, commonly once it exceeds ideal weight by 20–30% or BMI is 30 or more. The exact trigger and the factor (usually 0.4, sometimes 0.3 or 0.5) depend on the protocol or drug. When actual weight is not above ideal weight, adjusting has no purpose.

Why is the Devine formula unreliable in short people?

Because Devine adds 2.3 kg per inch above 5 feet (152.4 cm), so below that height it subtracts weight. Results quickly become implausible, and the site’s ideal body weight calculator shows a warning under 152 cm. The formula was also never designed for children, so it does not apply to them either.

Which weight is used for children?

Most paediatric doses are written per kilogram of actual weight, with a cap at the usual adult dose. For children with obesity some protocols use an ideal weight for age and height instead; adult formulas such as Devine do not apply.

Is predicted body weight for ventilation the same as Devine ideal weight?

Practically, yes. The predicted body weight used to set tidal volumes, from the ARDS Network trial, is the Devine formula expressed in centimetres: 50 kg for men or 45.5 kg for women, plus about 0.91 kg per cm above 152.4 cm. In that protocol, tidal volume is set on this weight, not on actual weight.

Actual, ideal, adjusted or lean body weight: which weight is which