How do you calculate ideal body weight for ventilator settings?

Men: 50 kg + 2.3 kg for each inch over 5 feet. Women: 45.5 kg + 2.3 kg for each inch over 5 feet. Height and sex only — actual weight is not used.

What tidal volume should be set in ARDS?

6 mL/kg of predicted body weight is the ARDSNet target, reduced toward 4 mL/kg if plateau pressure remains above 30 cmH₂O.

Why not use actual body weight?

Lung size tracks height rather than body mass. Using actual weight in an obese patient delivers a much larger breath to a lung that has not grown, which is how lung-protective targets are exceeded without anyone noticing.

Does ideal body weight change if the patient gains weight?

No. It is derived from height and sex, so it is fixed for an adult regardless of what the scale says.

Clinical Tool

IBW Calculator

Ideal Body Weight · Tidal Volume targets

Biological sex

Height

Result

66.1 kg

VT at 6 ml/kg

397 mL

VT at 8 ml/kg

529 mL

Formula (Devine)

Male: 50 + 2.3 × (height_in − 60)

Female: 45.5 + 2.3 × (height_in − 60)

▸NBRC uses IBW to calculate tidal volume targets — always use IBW, not actual body weight.

▸ARDSNet protocol: 6 ml/kg IBW. Standard ventilation: 6–8 ml/kg IBW.

▸For obese patients, never base TV on actual weight — lung size correlates to height, not weight.

About the ideal body weight calculator

Tidal volume on a ventilator is set from predicted body weight, which is calculated from height and sex — never from what the patient actually weighs. Lungs do not grow with body mass, and a 6 mL/kg breath sized on actual weight can be a very large breath.

Why predicted, not actual

Lung volume tracks height, not weight. Two patients of the same height have similar lung volumes whether one weighs 60 kg or 130 kg, so sizing a breath on actual weight delivers a proportionally larger volume to the same-sized lung.

This is the single most common way lung-protective ventilation is got wrong in practice, and it is invisible on the ventilator screen: the set tidal volume looks reasonable until it is divided by the correct weight.

Choosing the target

The ARDS Network protocol uses 6 mL/kg of predicted body weight, reduced toward 4 mL/kg if the plateau pressure stays above 30 cmH₂O. Around 6 to 8 mL/kg is a common starting range for lungs without ARDS, with the same pressure limits applied.

The volume target is not the only constraint. Plateau pressure and driving pressure describe what the breath is doing to the lung, and a volume within target can still be too much for a stiff or small lung.

The formula

The Devine formulas give 50 kg plus 2.3 kg per inch over 5 feet for men, and 45.5 kg plus 2.3 kg per inch over 5 feet for women. Below 5 feet the formulas are extrapolated and should be treated with more caution.

Common questions

How do you calculate ideal body weight for ventilator settings?: Men: 50 kg + 2.3 kg for each inch over 5 feet. Women: 45.5 kg + 2.3 kg for each inch over 5 feet. Height and sex only — actual weight is not used.
What tidal volume should be set in ARDS?: 6 mL/kg of predicted body weight is the ARDSNet target, reduced toward 4 mL/kg if plateau pressure remains above 30 cmH₂O.
Why not use actual body weight?: Lung size tracks height rather than body mass. Using actual weight in an obese patient delivers a much larger breath to a lung that has not grown, which is how lung-protective targets are exceeded without anyone noticing.
Does ideal body weight change if the patient gains weight?: No. It is derived from height and sex, so it is fixed for an adult regardless of what the scale says.

Educational reference for respiratory therapy students and clinicians preparing for the NBRC examinations. It supports clinical reasoning and does not replace institutional protocol, a device's instructions for use, or the judgement of the team caring for the patient.