# What is the ABI?

Works out the ankle-brachial index (ABI) for each leg from the arm and ankle systolic pressures, as the 2016 AHA/ACC guideline on peripheral artery disease defines it, and grades each result.

- Page: https://www.acalculator.org/health/abi-calculator
- JSON spec: https://www.acalculator.org/health/abi-calculator.json
- Version: 5c9ee647bc4b

## Default answer

Example with the default inputs (Right arm (brachial) 130, Left arm (brachial) 128, Right ankle, posterior tibial 120, Right ankle, dorsalis pedis 118, Left ankle, posterior tibial 100, Left ankle, dorsalis pedis 104): The ABI is 0.92 on the right (Borderline) and 0.8 on the left (Abnormal).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| rarm | Right arm (brachial) | Right brachial artery. Systolic pressure in mmHg, 20 to 300. |
| larm | Left arm (brachial) | Left brachial artery. Systolic pressure in mmHg, 20 to 300. |
| rpt | Right ankle, posterior tibial | Right posterior tibial artery. Systolic pressure in mmHg, 20 to 300. |
| rdp | Right ankle, dorsalis pedis | Right dorsalis pedis artery. Systolic pressure in mmHg, 20 to 300. |
| lpt | Left ankle, posterior tibial | Left posterior tibial artery. Systolic pressure in mmHg, 20 to 300. |
| ldp | Left ankle, dorsalis pedis | Left dorsalis pedis artery. Systolic pressure in mmHg, 20 to 300. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| lowest | ABI (lower leg) | The lower of the two legs’ ABI, to 2 decimals. |
| right | Right ABI | Higher right ankle pressure ÷ higher arm pressure, to 2 decimals. |
| rightGrade | Right leg | The guideline grade of the right ABI. |
| left | Left ABI | Higher left ankle pressure ÷ higher arm pressure, to 2 decimals. |
| leftGrade | Left leg | The guideline grade of the left ABI. |

## Method

ABI (each leg) = higher of the dorsalis pedis and posterior tibial systolic pressures ÷ higher of the right and left arm systolic pressures. Rounded to 2 decimals, then graded: 0.90 or less abnormal, 0.91 to 0.99 borderline, 1.00 to 1.40 normal, over 1.40 noncompressible.

## Assumptions

- Systolic pressures measured with a cuff and a Doppler probe while lying down.
- Each ABI is rounded once to 2 decimals (halves up) from the exact ratio of the typed pressures, then graded.

## Worked examples

1. rarm = 130, larm = 128, rpt = 120, rdp = 118, lpt = 100, ldp = 104 gives lowest = 0.8, right = 0.92, rightGrade = Borderline, left = 0.8, leftGrade = Abnormal. Source: 2016 AHA/ACC guideline rule: right 120 ÷ 130 = 0.923 → 0.92 borderline.
2. rarm = 120, larm = 124, rpt = 130, rdp = 126, lpt = 128, ldp = 132 gives lowest = 1.05, right = 1.05, rightGrade = Normal, left = 1.06, leftGrade = Normal.
3. rarm = 140, larm = 140, rpt = 126, rdp = 120, lpt = 200, ldp = 210 gives right = 0.9, rightGrade = Abnormal, left = 1.5, leftGrade = Noncompressible.
4. rarm = 100, larm = 98, rpt = 99, rdp = 90, lpt = 140, ldp = 135 gives right = 0.99, rightGrade = Borderline, left = 1.4, leftGrade = Normal.
5. rarm = 200, larm = 180, rpt = 181, rdp = 170, lpt = 199, ldp = 150 gives right = 0.91, rightGrade = Borderline, left = 1, leftGrade = Normal.

## FAQ

### How is the ABI calculated?

For each leg, take the higher of the two ankle pressures (dorsalis pedis and posterior tibial) and divide it by the higher of the two arm pressures. The 2016 AHA/ACC guideline on peripheral artery disease defines it this way.

### What is a normal ABI?

The guideline grades an ABI of 1.00 to 1.40 as normal, 0.91 to 0.99 as borderline, 0.90 or less as abnormal, and over 1.40 as noncompressible.

### What does an ABI of 0.90 or less mean?

The guideline calls it abnormal and uses it to support a diagnosis of peripheral artery disease (narrowed leg arteries), together with symptoms and an exam. The number alone is not a diagnosis.

### What does an ABI over 1.40 mean?

The ankle arteries are too stiff to squeeze shut with the cuff (noncompressible), which is common with diabetes or kidney disease. The ABI then cannot show narrowing, and the guideline suggests a toe-brachial index instead.

### Why use the higher arm pressure?

A narrowing in one arm artery can make that arm read low. The higher arm pressure is the better estimate of the pressure in the aorta.

### Why does the page round before it grades?

The guideline bands are written to 2 decimals (0.90, 0.91, 0.99, 1.00, 1.40). The page rounds each ABI once to 2 decimals, halves up, and grades that value, so 0.905 is 0.91, borderline.

## Sources

- Gerhard-Herman MD, Gornik HL, Barrett C, et al. 2016 AHA/ACC guideline on the management of patients with lower extremity peripheral artery disease. Circulation. 2017;135(12):e726-e779. https://doi.org/10.1161/CIR.0000000000000471 (open at https://pmc.ncbi.nlm.nih.gov/articles/PMC5477786/, retrieved 2026-10-05)
- Aboyans V, Criqui MH, Abraham P, et al. Measurement and interpretation of the ankle-brachial index: a scientific statement from the American Heart Association. Circulation. 2012;126(24):2890-2909. https://doi.org/10.1161/CIR.0b013e318276fbcb (retrieved 2026-10-05)