Spirometry FEV1/FVC Ratio Calculator
Your FEV1/FVC ratio from a spirometry report, with the 0.70 threshold, the grey zone and the bronchodilator response.
What it does
A spirometry report is dense with numbers, but one of them carries most of the meaning. The FEV1/FVC ratio compares how much air you can push out in the first second with how much air you can push out at all. A low ratio means the airways narrow too early, which is the pattern behind asthma and COPD.
The threshold everyone quotes is 0.70. NICE guidance for COPD asks for post bronchodilator spirometry and treats a ratio below 0.70 as confirming airflow obstruction. GOLD sets the same line. Below it, the report is describing obstructed airways; at or above it, the ratio is in the normal range by that rule.
A fixed threshold is simple but blunt, so the guidelines add a grey zone. GOLD now recommends repeating the spirometry on a separate occasion when the post bronchodilator ratio lands between 0.60 and 0.80, because a value that close to the cut off can land on either side through ordinary day to day variation.
The second thing worth reading is the bronchodilator response. If FEV1 climbs after the inhaler, the obstruction is at least partly reversible. The GOLD rule is a rise of at least 200 mL and at least 12 percent of the pre test value, and both parts have to be met. The 2021 ERS and ATS standard now expresses the response as a change of more than 10 percent of the predicted FEV1 instead.
This tool works on the numbers on your report and nothing else. It cannot tell you why a ratio is low, it cannot check the quality of the test, and it does not replace the report card your technician gave you. What it does is turn two or three figures into the interpretation those guidelines describe, so you can read the numbers with the same rules your clinician used.
How to use
- Find the spirometry report, usually printed after the flow volume loop and the volume time curve.
- Enter FEV1 and FVC from the post bronchodilator column, the test you did after the inhaler.
- Read the ratio and the obstruction line, which uses the same 0.70 threshold as NICE and GOLD.
- Check the grey zone line, and if the ratio sits between 0.60 and 0.80, plan to repeat the test on another day.
- Add the pre bronchodilator FEV1 if the report has it, to see the response in millilitres and in percent.
- Use the needed FVC and FEV1 lines to see how far one of the two numbers would have to move to reach a 0.70 ratio.
Formula
- FEV1
- Forced expiratory volume in the first second, in litres
- FVC
- Forced vital capacity, the largest volume blown out after a full breath in, in litres
- 0.70
- The fixed ratio threshold used by NICE and GOLD for airflow obstruction
- post FEV1
- FEV1 measured after the bronchodilator inhaler
- pre FEV1
- FEV1 measured before the bronchodilator inhaler
The ratio has no units, so the same number comes out whether you use litres or millilitres, and this tool works in litres. The 0.70 threshold is fixed rather than age adjusted, which is why some clinicians prefer the lower limit of normal from reference equations instead. The bronchodilator rule needs both parts: an increase of at least 200 mL and at least 12 percent of the pre test FEV1. The 2021 ERS and ATS technical standard replaced that with a change of more than 10 percent of the predicted FEV1, so a positive result here does not always match the newest wording.
Worked example
Example: a report showing FEV1 2.80 L and FVC 4.20 L
A post bronchodilator FEV1 of 2.80 L and FVC of 4.20 L, with a pre bronchodilator FEV1 of 2.40 L.
- Ratio: 2.80 / 4.20 = 0.667, which is 66.7 %.
- That is below the 0.70 threshold, so the report shows airflow obstruction by the NICE and GOLD rule.
- It also falls between 0.60 and 0.80, so GOLD would suggest repeating the spirometry on another day.
- FVC needed for a 0.70 ratio: 2.80 / 0.70 = 4.00 L, which is 0.20 L below the measured FVC.
- FEV1 needed for a 0.70 ratio: 4.20 x 0.70 = 2.94 L, which is 0.14 L above the measured FEV1.
- Bronchodilator response: (2.80 - 2.40) x 1000 = 400 mL, and 400 / 2400 = 16.7 % of the pre test value.
- Both parts of the rule are met, so the response counts as a positive bronchodilator response.
Reference table
| Ratio | Reading | What it means |
|---|---|---|
| 0.80 and above | Normal ratio | No obstruction at the 0.70 threshold |
| 0.70 to 0.79 | Grey zone | Repeat the test on another day |
| 0.60 to 0.69 | Obstruction, grey zone | Below the threshold and close to the cut off |
| 0.40 to 0.59 | Obstruction | Clearly below the 0.70 threshold |
| Below 0.40 | Obstruction | Far below the threshold, refer to a clinician |
Limits
- This reads the numbers on a report. It does not check that the test was performed well enough to trust.
- The fixed 0.70 threshold over calls obstruction in older adults and under calls it in younger adults, so age matters.
- A ratio alone says nothing about severity, which comes from FEV1 as a percentage of the predicted value.
- It is not a diagnosis. If you have breathlessness, wheezing or a cough that will not settle, see a clinician.
Common mistakes
- Using the pre bronchodilator FEV1 instead of the post bronchodilator value when the guidelines ask for the post test ratio.
- Reading a ratio of 0.69 as normal because it is close to 0.70, when the guideline rule is a straight comparison.
- Treating the grey zone as a yes or no answer instead of a reason to repeat the test.
- Looking at FEV1 on its own and missing that the ratio is the number the diagnosis rests on.
- Forgetting that the ratio changes with age, so the same 0.68 means different things at 45 and at 75.
Try it yourself
Quick checks. Answers sit tucked under each question.
FEV1 is 3.50 L and FVC is 4.00 L after the bronchodilator. What is the ratio and what does it show?
87.5 %, and no obstruction at the 0.70 threshold. The ratio is 3.50 / 4.00 = 0.875, and it is above 0.80, so it sits outside the grey zone as well.
FEV1 is 1.20 L and FVC is 2.00 L after the bronchodilator. What is the ratio and what does it show?
60.0 %, with airflow obstruction and a grey zone result. The ratio is 1.20 / 2.00 = 0.60, which is below the 0.70 threshold and inside the 0.60 to 0.80 band.
The pre test FEV1 was 2.40 L and the post test FEV1 was 2.80 L. Does that count as a bronchodilator response?
Yes. The change is 400 mL and 16.7 percent of the pre test value, and the rule needs at least 200 mL and at least 12 percent, so both parts are met.
Questions
What FEV1/FVC ratio is normal?
The fixed threshold used by NICE and GOLD is 0.70. A ratio of 0.70 or above is read as no obstruction, and a ratio below 0.70 is read as airflow obstruction.
What is the grey zone?
It is the band around the cut off where the result is close enough to 0.70 that ordinary variation can move it. GOLD recommends repeating the spirometry on a separate occasion when the post bronchodilator ratio is between 0.60 and 0.80.
Why does the report have two sets of numbers?
One set is measured before the bronchodilator inhaler and the other after. Diagnosis uses the post bronchodilator values, and the difference between the two shows how reversible the narrowing is.
Does a positive bronchodilator response mean I have asthma?
No. A positive response means the airways widened after the inhaler, which happens in asthma but also in some people with COPD. The result has to be read with symptoms, history and the rest of the report.
Does a normal ratio rule out a lung condition?
No. A normal ratio with low FEV1 can still point to other problems, including poor effort, weak breathing muscles or a different condition. Only a clinician can interpret the whole test.
How many tests do I need?
Usually one good post bronchodilator test is enough for the ratio, and a second test on another day when the first lands in the grey zone. Spirometry is repeatable, so repeat it when you are well and rested.
Sources
- NICE guideline NG115, COPD in over 16s: diagnosis and management
- GOLD Science Committee recommendations on pre and post bronchodilator spirometry
- GINA 2022 update, bronchodilator response criteria