Ankle-Brachial Index (ABI) Calculator

Understanding the ankle-brachial index calculator — concept and principle

The ankle-brachial index (ABI) compares systolic blood pressure at the ankle with systolic pressure in the arm. In healthy legs, ankle pressure is equal to or slightly higher than arm pressure. When the leg arteries are narrowed by atherosclerosis, pressure falls beyond the blockage and the ratio drops. ABI is the standard first test for peripheral artery disease (PAD), a condition that also signals a higher risk of heart attack and stroke.

Healthy legs keep ankle pressure at or above arm pressure; 120 ÷ 130 = 0.92 is borderline.

How it is calculated

ABI = ankle systolic pressure ÷ higher brachial (arm) systolic pressure

Ankle pressure: for each leg, the higher of the dorsalis pedis (top of foot) and posterior tibial (inside of ankle) pressures, measured with a Doppler probe.
Arm pressure: the higher of the right and left brachial pressures.
Both pressures are in mmHg, so the ratio has no unit. Each leg gets its own ABI, and the lower one is used to describe the patient.

The principle

Pressure waves normally grow slightly stronger as they travel towards the feet, so a ratio of 1.00 or more is expected. A narrowed artery causes a pressure drop across the lesion, and the lower the ratio, the more severe the obstruction tends to be. Using the higher arm reading guards against an arm artery narrowing that would make the arm falsely low. Values above 1.40 do not mean better circulation; they usually mean the ankle arteries are stiff and calcified, as often happens in diabetes or chronic kidney disease, so the cuff cannot compress them.

Worked example

Inputs: ankle systolic 120 mmHg, higher arm systolic 130 mmHg.
ABI = 120 ÷ 130 = 0.923, shown as 0.92.
0.92 is between 0.91 and 0.99, so the result is borderline (0.91–0.99).

Each point is the calculator re-run with only “Higher arm systolic (mmHg)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Higher arm systolic (mmHg)	ABI
32.5	3.69
65	1.85
97.5	1.23
130	0.92
162	0.74
195	0.62
260	0.46

How to read the result

Where does your result fall?

1.00–1.40: normal.
0.91–0.99: borderline; an exercise ABI may be considered if symptoms suggest PAD.
0.90 or less: consistent with PAD; lower values generally mean more severe disease.
Above 1.40: non-compressible arteries; a toe-brachial index or other test is needed.

Common mistakes

Measuring without rest; the person should lie flat for about 5–10 minutes first.
Dividing by the lower arm pressure or by the pressure of the same-side arm only.
Using an automatic cuff instead of a Doppler, which can miss low ankle pressures.

Keep in mind: ABI is a screening and diagnostic aid, not a diagnosis by itself. A normal resting ABI does not rule out PAD when symptoms such as leg pain on walking are present; a doctor should interpret it with the clinical picture.

Frequently asked questions

What does a big difference between the two arms mean?

A difference of more than about 15 mmHg can point to narrowing of an arm artery and is worth mentioning to a doctor.

Why might a person with diabetes have a high ABI?

Calcified, stiff arteries resist the cuff, which falsely raises ankle pressure.

Can ABI be done at home?

Not reliably. It needs proper cuffs, a Doppler probe and training.

Sources: Aboyans V et al., Measurement and Interpretation of the Ankle-Brachial Index, AHA Scientific Statement, Circulation 2012 · Gornik HL et al., 2024 ACC/AHA Multisociety Guideline for the Management of Lower Extremity Peripheral Artery Disease, Circulation 2024

Checked 8 Nov 2026. AHA/ACC 2024 PAD guideline.

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