ABI Calculator
Calculate the ankle-brachial index (ABI) for each leg to screen for peripheral artery disease. Enter the systolic pressure (mmHg) in both arms and both ankles — use the higher ankle reading per leg — to get each leg's ABI and its band.
Example: with Right arm systolic (mmHg) 130 · Left arm systolic (mmHg) 130 · Right ankle systolic (mmHg) 130 · Left ankle systolic (mmHg) 104 → Lower ABI (governs interpretation): Lower index 0.80 — mild PAD (0.71–0.90).
- Right leg ABI1.00 — normal (1.00–1.40)
- Left leg ABI0.80 — mild PAD (0.71–0.90)
Computed by the calculator below using its default values. Change any input to see your own numbers.
ABI (per leg) = higher ankle systolic ÷ higher of the two arm systolics. An ABI at or below 0.90 is the classic threshold for peripheral artery disease.
What the ankle-brachial index measures
Healthy leg arteries deliver blood to the ankle at a pressure at least as high as the arm. When plaque narrows the arteries of the pelvis or leg, ankle pressure falls below arm pressure, and the ratio of the two drops below 1. That ratio is the ankle-brachial index, a simple, non-invasive screen for peripheral artery disease.
Each leg is scored separately. The denominator is the higher of the two arm pressures, and the numerator is the higher of the two ankle readings (dorsalis pedis or posterior tibial) for that leg. The lower of the two legs' indices usually drives the clinical interpretation, since disease is often one-sided.
Reading the bands
An ABI of 1.00 to 1.40 is normal. From 0.91 to 0.99 is borderline. At or below 0.90 is diagnostic of peripheral artery disease, and the lower it goes the more severe the blockage. Above 1.40 the vessels are too stiff to compress — common in diabetes and advanced kidney disease — so the number is unreliable and a different test is needed.
How it’s calculated
For each leg, ABI = higher ankle systolic pressure ÷ higher of the right and left arm systolic pressures. Bands (ACC/AHA): >1.40 non-compressible; 1.00–1.40 normal; 0.91–0.99 borderline; 0.90 or below abnormal (0.71–0.90 mild, 0.41–0.70 moderate, ≤0.40 severe PAD).
A resting ABI can miss disease that only shows on exertion; an exercise ABI or toe-brachial index may be needed. Non-compressible arteries above 1.40 invalidate the ratio. Screening estimate only — confirm with a clinician.
Ankle-brachial index interpretation
| ABI | Interpretation |
|---|---|
| Over 1.40 | Non-compressible / calcified — unreliable |
| 1.00 – 1.40 | Normal |
| 0.91 – 0.99 | Borderline |
| 0.71 – 0.90 | Mild peripheral artery disease |
| 0.41 – 0.70 | Moderate PAD |
| 0.40 or less | Severe PAD |
Source: ACC/AHA ankle-brachial index interpretation categories.
Common mistakes
- Dividing by the same-side arm pressure instead of the higher of the two arms.
- Using the lower ankle reading; you take the higher of dorsalis pedis and posterior tibial per leg.
- Calling a value above 1.40 normal — it signals non-compressible vessels, not healthy flow.
- Screening only at rest when symptoms are exertional; an exercise ABI may be required.
Frequently asked questions
How do you calculate ABI?
For each leg, divide the higher ankle systolic pressure by the higher of the two arm systolic pressures. If both arms read 130 and the ankle reads 104, the ABI is 104 ÷ 130 = 0.80.
What ABI indicates peripheral artery disease?
An ABI of 0.90 or below is the classic threshold for PAD. Values of 0.91 to 0.99 are borderline, and 1.00 to 1.40 is normal. The lower the number, the more severe the arterial narrowing.
What does an ABI above 1.40 mean?
It usually means the leg arteries are calcified and too stiff to compress, common in diabetes and chronic kidney disease. The ratio is then unreliable, and a toe-brachial index or imaging is used instead.
Why is the lower leg's index used?
Peripheral artery disease is often one-sided, so the worse leg carries the clinical meaning. Reporting each leg plus the lower index keeps both the screen and the detail visible.
Can I rely on a home ABI?
Treat a home value as a rough screen only. Accurate ABI needs a proper Doppler and cuff technique. Any borderline or low result, or leg pain when walking, should be evaluated by a clinician.