Ankle Brachial Index Calculator
Turn ankle and arm blood pressure readings into an ankle brachial index and see how it is classified.
What it does
The ankle brachial index is a screening test for peripheral artery disease, the narrowing of the arteries in the legs. It compares the pressure in the arteries of the ankle with the pressure in the artery of the arm, and the American Heart Association describes it as part of how the disease is diagnosed.
The calculation is a single division. The numerator is the higher of the two ankle readings for that leg, taken from the dorsalis pedis and the posterior tibial arteries, and the denominator is the higher of the two brachial readings. The 2024 ACC and AHA guideline for the disease spells that out, and the American Heart Association's scientific statement on perfusion assessment gives the same method with the measurement conditions.
The bands are what turn the number into meaning. The 2024 guideline reports an index of 0.90 or below as abnormal, 0.91 to 0.99 as borderline, 1.00 to 1.40 as normal, and anything above 1.40 as noncompressible, which happens when the arteries are so hard that the cuff cannot squeeze them and the reading is falsely high.
Measurement matters as much as the arithmetic. The patient lies flat after five to ten minutes of rest, the cuff is placed on the ankle and on the upper arm, and the pressure is taken with a Doppler probe over each of the foot arteries. Some patients with symptoms but a normal resting index are asked to walk on a treadmill and are measured again straight afterwards, which is a different test with its own reference values.
This page does arithmetic on readings you already have. A low index is a reason to talk to a clinician, not a diagnosis, and the same goes for a high one, which needs toe pressures and waveforms to interpret.
How to use
- Enter the right and left brachial systolic pressures.
- Enter the higher ankle pressure for each leg.
- Read the index for each leg and the band it falls in.
- Check which brachial pressure was used, since the higher one is the denominator.
- Compare the higher of the two leg readings, which is the figure usually quoted.
- Take an abnormal or noncompressible result to a clinician rather than treating it as a number on its own.
Formula
- ABI
- Ankle brachial index, the ankle pressure over the arm pressure.
- Ankle pressure
- The higher of the two pedal artery systolic pressures in that leg.
- Brachial pressure
- The higher of the two arm systolic pressures.
- mmHg
- Millimetres of mercury, the unit the pressures are measured in.
The index is reported to two decimal places in practice, and a value right on a boundary can land on either side depending on the exact readings, so the raw ratio matters. The American Heart Association notes that in severe disease the index is below 0.40, while the guideline reserves the word noncompressible for readings above 1.40, which need toe pressures and waveforms rather than a simple comparison.
Worked example
Example: arm 128 and 124, ankles 116 and 132
Defaults: right brachial 128, left brachial 124, right ankle 116 and left ankle 132.
- The denominator is the higher arm pressure: max(128, 124) = 128 mmHg.
- Right leg: 116 / 128 = 0.906, which is 0.91 to two decimals.
- That sits in the borderline band of 0.91 to 0.99.
- Left leg: 132 / 128 = 1.031, which is 1.03 to two decimals.
- That is inside the normal band of 1.00 to 1.40.
- The tool shows 0.91 (borderline), 1.03 (normal), 128 mmHg and 1.03 (normal).
Reference table
| Index | Category | What it means |
|---|---|---|
| 0.90 or below | Abnormal | Supports a diagnosis of peripheral artery disease |
| 0.91 to 0.99 | Borderline | Needs more testing in the right clinical context |
| 1.00 to 1.40 | Normal | No narrowing found on this test |
| Above 1.40 | Noncompressible | Vessels cannot be compressed, so toe pressures and waveforms are needed |
Limits
- Only handles the resting index, not the exercise version with its own reference values.
- Needs the ankle and brachial pressures to have been taken with the same method and equipment.
- Does not cover toe pressures or the toe brachial index, which are the next test when the index is not usable.
- Says nothing about symptoms, and a normal index in someone with calf pain when walking does not rule out disease.
Common mistakes
- Dividing by the lower arm pressure instead of the higher one.
- Using the dorsalis pedis reading only, when the higher of the two foot arteries is the correct numerator.
- Comparing the two legs with each other instead of with the arm pressure.
- Reading an index above 1.40 as a healthy result, when it means the test could not be performed properly.
- Treating a borderline index as nothing, when it is exactly the band that needs a second look.
Try it yourself
Quick checks. Answers sit tucked under each question.
With arm pressures of 120 and 120 and ankle pressures of 108 and 132, what are the two indexes?
The denominator is 120 mmHg, so the right leg is 108 / 120 = 0.90, which is abnormal, and the left leg is 132 / 120 = 1.10, which is normal.
An index of 0.85 in one leg falls in which band?
At or below 0.90 it is abnormal, which supports a diagnosis of peripheral artery disease in the right clinical setting.
Why is an index above 1.40 not a good result?
It means the arteries are so hard that the cuff could not compress them, so the reading is falsely high. Toe pressures and waveforms are the next test, not a reassuring number.
Questions
What does the ankle brachial index measure?
It compares the blood pressure in the arteries of the ankle with the pressure in the artery of the arm, which shows whether the leg arteries are narrowed.
What is a normal reading?
Between 1.00 and 1.40 according to the 2024 ACC and AHA guideline. Below 0.90 is abnormal and 0.91 to 0.99 is borderline.
How is the measurement taken?
With the patient lying flat after five to ten minutes of rest, using a blood pressure cuff on the ankle and on the upper arm, and a Doppler probe over the dorsalis pedis and posterior tibial arteries of each foot.
What is an exercise ABI?
The same measurements taken again straight after walking on a treadmill. It is used for patients whose symptoms appear with exercise even though the resting index is normal, and it has its own reference values.
Does age change the normal range?
No. The guideline does not give a separate range for older adults, and age is not a reason to call an abnormal result normal.
Is this a diagnosis?
No. It is a screening number. An abnormal result is a reason to see a clinician, who will add the history, the examination and further tests.
Sources
- American Heart Association: how is peripheral artery disease diagnosed?
- 2024 ACC and AHA guideline for lower extremity peripheral artery disease, in PubMed Central
- American Heart Association scientific statement: perfusion assessment in critical limb ischemia, in PubMed Central