RSBI Calculator (rapid shallow breathing index, f/VT for weaning)

Understanding the RSBI calculator — concept and principle

The rapid shallow breathing index (RSBI) divides breathing rate by tidal volume in litres. It is a bedside test used during weaning from mechanical ventilation to judge whether a patient is likely to breathe on their own after the breathing tube is removed.

Rate ÷ tidal volume: with the default inputs the result is RSBI 60 (below 105).

How it is calculated

RSBI (breaths/min per litre) = breathing rate ÷ tidal volume (L)

Minute ventilation (L/min) = breathing rate × tidal volume (L)

Rate and tidal volume are measured during about a minute of unsupported breathing.
The result is compared with the threshold of 105.

The principle

When the breathing muscles cannot cope with the load, people breathe fast and shallow, because small breaths take less effort each time. That raises the rate and lowers the tidal volume together, so their ratio rises sharply. Yang and Tobin (1991) found that an index under 105 predicted successful weaning better than other bedside measures.

Worked example

Default inputs: rate 24 breaths/min, tidal volume 400 mL.

Tidal volume = 0.4 L.
RSBI = 24 ÷ 0.4 = 60 → Below 105 — weaning success more likely.
Minute ventilation = 24 × 0.4 = 9.6 L/min.

Each point is the calculator re-run with only “Spontaneous tidal volume (mL)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Spontaneous tidal volume (mL)	RSBI (breaths/min per litre)
100	240
200	120
300	80
400	60
500	48
600	40
800	30

How to read the result

Where does your result fall?

An RSBI below 105 supports proceeding with a spontaneous breathing trial or extubation, but readiness also depends on the underlying illness, oxygen levels, cough strength, secretions, mental state and circulation. An index of 105 or more suggests the patient may not yet manage without support. Values measured on pressure support or CPAP are usually lower than truly unassisted values.

Common mistakes

Measuring on high pressure support and comparing with the 105 threshold.
Measuring for only a few breaths rather than about a minute.
Using RSBI alone to decide on extubation.

Keep in mind: This is a calculation aid for clinicians, not a decision rule. Weaning and extubation are decided by the critical care team.

Frequently asked questions

What is a good RSBI?

Below 105 is the classic threshold; lower values are generally more favourable.

Why does fast shallow breathing matter?

It signals that breathing muscles are struggling, which often precedes weaning failure.

Is RSBI used in children?

Paediatric thresholds differ and are usually adjusted for body weight.

Sources: Yang KL, Tobin MJ, "A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation", N Engl J Med 1991;324:1445–1450

Checked 9 Nov 2026. Standard values: index f/V_T and the 105 breaths/min/L threshold introduced by Yang KL and Tobin MJ, “A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation”, N Engl J Med 1991;324:1445–1450.

Your numbers