What this calculator does
Some medicines distribute mainly through lean tissue and some through everything. For a patient well above ideal weight, dosing on actual weight overshoots and dosing on ideal weight undershoots, so a blended figure is used instead.
The convention adds 40 per cent of the excess back. At 175 cm the Robinson ideal weight is 68.9 kg, so an actual weight of 100 kg gives an adjusted weight of 81.3 kg rather than either extreme.
The formula
Ideal body weight is calculated from height using the Robinson formula. The difference between actual and ideal weight is multiplied by 0.4 and added back to the ideal figure.
| Term | Meaning |
|---|---|
| Ideal body weight | A height-based reference weight, here from the Robinson formula. |
| Adjustment factor | The 0.4 multiplier applied to the excess over ideal weight. |
| Adjusted body weight | The blended figure used for dosing certain drugs. |
The inputs explained
| Field | What to enter |
|---|---|
| Actual weight (kg) | Actual body weight in kilograms. |
| Height (cm) | Height in centimetres, used to derive ideal body weight. |
| Sex | Sex, which selects the Robinson coefficients. |
When to use it
Dosing a renally cleared drug
Several dosing protocols specify adjusted rather than actual body weight.
Calculating creatinine clearance
Which weight to use in Cockcroft-Gault depends on body composition and local protocol.
Comparing the three weights
Seeing actual, ideal and adjusted together shows how far apart they can be.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How far does the adjustment move the figure?
The same height at three actual weights.
| Actual weight | Adjusted body weight | Ideal body weight (Robinson) |
|---|---|---|
| 70 kg | 69.3 kg | 68.9 kg |
| 100 kg | 81.3 kg | 68.9 kg |
| 130 kg | 93.3 kg | 68.9 kg |
Questions
Where does the 0.4 factor come from?
From observations that adipose tissue contributes roughly 30 to 40 per cent as much as lean tissue to the distribution volume of several drugs. It is an empirical convention rather than a derived constant, and some protocols use 0.3 instead.
Which drugs use adjusted weight?
It varies by agent and by local protocol. Aminoglycosides are the classic example, and several other renally cleared drugs use it. The dosing reference for the specific medicine is what determines this, not a general rule.
Which ideal body weight formula should be used?
Devine, Robinson, Miller and Hamwi all exist and give slightly different answers. This uses Robinson. Where a dosing protocol specifies a formula, that one should be used rather than whichever is to hand.
What if actual weight is below ideal?
Actual weight is normally used, since the adjustment exists to moderate dosing in patients above ideal weight. The calculation still runs but has little practical effect.
For estimating height where it cannot be measured, see the knee height calculator. For the kidney function estimate it often feeds, see the creatinine clearance calculator.