Rapid shallow breathing index (RSBI): how to calculate f/VT
The rapid shallow breathing index (RSBI) is respiratory rate divided by tidal volume in liters. For a patient breathing 24 times a minute with a tidal volume of 400 mL (0.4 L), the RSBI is 24 ÷ 0.4 = 60.0 breaths/min/L.
Not a medical device. Not a diagnosis or dosing recommendation.
What is the rapid shallow breathing index?
The RSBI puts a number on rapid, shallow breathing. Yang and Tobin introduced it in 1991 as the ratio of respiratory frequency to tidal volume, written f/VT. Fast breathing raises the ratio, and small breaths raise it too, so a pattern of many small breaths gives a high number.
It's also called the Yang–Tobin index, Tobin's RSBI, or simply f/VT. The unit is breaths per minute per liter.
This page explains the arithmetic. It doesn't give a cutoff, and it doesn't say what a value means for a patient.
The RSBI formula
rsbi = respiratory_rate / tidal_volume_lIn words: divide the respiratory rate (breaths per minute) by the tidal volume in liters. Magent's calculator reports the result to 1 decimal place.
What each part means
| Term | Plain-English meaning | Unit | Accepted range |
|---|---|---|---|
respiratory_rate | Breaths per minute (f) | breaths/min, whole number | 4–80 |
tidal_volume_ml | Size of each breath (VT), in milliliters | mL | 50–2000 |
tidal_volume_l | Size of each breath (VT), in liters | L | 0.05–2.0 |
rsbi | Respiratory rate ÷ tidal volume in liters | breaths/min/L | 1 decimal place |
You give tidal volume one way or the other, in milliliters or liters, not both.
How the formula works
- Take the respiratory rate in breaths per minute.
- Put the tidal volume in liters. If you have milliliters, divide by 1,000.
- Divide the respiratory rate by the tidal volume in liters.
- Round to 1 decimal place.
The index moves in opposite directions with its two inputs. Doubling the respiratory rate doubles it. Halving the tidal volume also doubles it. So 20 breaths of 0.4 L and 40 breaths of 0.8 L give the same RSBI, 50.0, even though the second pattern moves twice as much air.
Units
The formula needs tidal volume in liters. That's the step most worth checking, because tidal volume is usually written in milliliters. If you divide by milliliters by mistake, 24 ÷ 400 gives 0.06, a thousand times smaller than the real index of 60.0. The calculator accepts milliliters and does the conversion for you (tidal_volume_ml).
Worked examples
These use Magent's own expected values for the calculator.
Example 1: rate 24, tidal volume 400 mL
- Tidal volume: 400 ÷ 1,000 = 0.4 L
- RSBI: 24 ÷ 0.4 = 60.0 breaths/min/L
Example 2: rate 20, tidal volume 0.4 L
- Tidal volume is already in liters: 0.4 L
- RSBI: 20 ÷ 0.4 = 50.0 breaths/min/L
Example 3: rate 40, tidal volume 200 mL
- Tidal volume: 200 ÷ 1,000 = 0.2 L
- RSBI: 40 ÷ 0.2 = 200.0 breaths/min/L
Example 4: rate 12, tidal volume 0.6 L
- RSBI: 12 ÷ 0.6 = 20.0 breaths/min/L
Examples 3 and 4 are the two extremes of the pattern: many small breaths give a large index, and a few large breaths give a small one.
RSBI reference table
These values are computed from the formula and rounded to 1 decimal, as Magent's calculator reports them. They show how the index moves; they aren't targets, reference ranges or categories.
| Breaths/min \ tidal volume | 0.25 L (250 mL) | 0.3 L (300 mL) | 0.4 L (400 mL) | 0.5 L (500 mL) | 0.6 L (600 mL) |
|---|---|---|---|---|---|
| 10 | 40.0 | 33.3 | 25.0 | 20.0 | 16.7 |
| 15 | 60.0 | 50.0 | 37.5 | 30.0 | 25.0 |
| 20 | 80.0 | 66.7 | 50.0 | 40.0 | 33.3 |
| 25 | 100.0 | 83.3 | 62.5 | 50.0 | 41.7 |
| 30 | 120.0 | 100.0 | 75.0 | 60.0 | 50.0 |
| 35 | 140.0 | 116.7 | 87.5 | 70.0 | 58.3 |
Find the respiratory-rate row and the tidal-volume column. Moving down a column raises the index, and moving right along a row lowers it.
History of the RSBI
The index comes from K. L. Yang and M. J. Tobin, A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation, published in the New England Journal of Medicine in 1991 (volume 324, issue 21, pages 1445–1450).
The abstract explains the motivation. The traditional predictors at the time, minute ventilation and maximal inspiratory pressure, were frequently inaccurate. The authors developed two new indexes: f/VT, which quantified rapid shallow breathing, and a second index called CROP. The study had a derivation group of 36 patients and a prospective test group of a further 64.
A 2026 systematic analysis of later studies describes how the original measurement was taken. The patient stayed intubated but was disconnected from the ventilator and breathed room air spontaneously for one minute, and the measurements were made with spirometry.[1]
When it's used
Published work describes the RSBI as a measurement used to assess readiness for liberation from mechanical ventilation.[1] Clinical references on ventilator weaning describe extubation-readiness assessment tools, and note that for most patients weaning leads to extubation after the first spontaneous breathing trial (SBT).[2] The RSBI is one of the measurements discussed in that process.[1]
Where it's used
It's an intensive care measurement. Modern ventilators commonly calculate the RSBI and show it directly on screen.[1] Originally the inputs were measured with spirometry while the patient breathed without ventilator support.[1]
Who uses it
Published work notes that clinicians, particularly respiratory therapists and chest physicians, often obtain the RSBI under the patient's current ventilator mode and pressure settings.[1]
Limits of the calculation
- It's a ratio of two numbers. It isn't an order to wean or extubate, and it doesn't classify the result.
- As published, the index was measured during spontaneous breathing. Magent's calculator divides the two numbers you supply; it doesn't know how they were measured.
- Measurement conditions change the number. Most studies in the 2026 analysis reported lower values under ventilator support, particularly pressure support or CPAP, than during unsupported breathing; trigger sensitivity and base flow also mattered.[1]
- Magent's calculator doesn't perform a spontaneous breathing trial.
- Inputs outside the accepted ranges are rejected, not estimated. The respiratory rate must be a whole number.
Clinical decisions belong to a qualified clinician who knows the patient.
How it differs from related calculators
- PaO2/FiO2 ratio divides arterial PO2 by the inspired oxygen fraction. It describes oxygenation, not breathing pattern.
- Oxygenation index combines inspired oxygen fraction, mean airway pressure and arterial PO2.
- Minute ventilation multiplies rate by tidal volume instead of dividing. Yang and Tobin compared their index with it.
Glossary
- Respiratory rate (f): breaths per minute.
- Tidal volume (VT): the volume of one breath, here in liters.
- f/VT: respiratory rate divided by tidal volume; another name for the RSBI.
- Spontaneous breathing trial (SBT): a period of breathing with little or no ventilator support before extubation is considered.[2]
- Pressure support / CPAP: ventilator settings that assist spontaneous breaths; the RSBI can read differently under them.[1]
Common mistakes
- Using milliliters in the division. Convert to liters first, or enter
tidal_volume_mland let the calculator convert. - Multiplying instead of dividing. Rate × tidal volume is minute ventilation, a different measure.
- Ignoring how it was measured. The same patient can give different values with and without ventilator support.[1]
- Entering a decimal respiratory rate. The calculator accepts whole breaths per minute.
Source
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(21):1445-1450. DOI: 10.1056/NEJM199105233242101
Further reading
- Lay YY, Chuang YT, Pai SC, Lin CS, Feng JY, Chen TA. Ventilator-derived Versus Standard Rapid Shallow Breathing Index for Weaning Assessment: A Systematic Review. In Vivo. 2026;40(4):1872-1883.
- Fadila M, Rajasurya V, Regunath H. Ventilator Weaning. StatPearls. 2022.
Frequently asked questions
How do you calculate the rapid shallow breathing index?
Divide the respiratory rate (breaths per minute) by the tidal volume in liters. For 24 breaths per minute and 400 mL, that's 24 ÷ 0.4 = 60.0 breaths/min/L.
What is the RSBI formula?
RSBI = f ÷ VT, where f is the respiratory rate and VT is the tidal volume in liters.
What does f/VT stand for?
Frequency over tidal volume: respiratory rate (f) divided by tidal volume (VT). It's the same thing as the RSBI.
What is a normal RSBI?
This page doesn't give a normal range or a cutoff. It calculates the ratio; what a value means depends on how it was measured and on the clinical context.
Who developed the RSBI?
K. L. Yang and M. J. Tobin, in a 1991 paper in the New England Journal of Medicine.
Does tidal volume go in mL or liters?
Liters. If you have milliliters, divide by 1,000 first: 400 mL is 0.4 L.