Ankle-Brachial Index Calculator

The Ankle-Brachial Index (ABI) is a simple, non-invasive test used to screen for peripheral artery disease (PAD), a condition where arteries in the legs become narrowed or blocked by plaque. It is one of the most reliable early indicators of cardiovascular disease, and a low ABI predicts heart attack and stroke risk, not just leg trouble.

How ABI is Calculated

The formula is straightforward:

ABI = Ankle Systolic Pressure ÷ Arm (Brachial) Systolic Pressure

You measure the systolic blood pressure (the top number) at the ankle and at the upper arm (brachial artery), then divide. The result is a ratio, a pure number with no units.

Which pressures, exactly

This is where a home attempt usually goes wrong, and it matters because using the wrong reading pushes the answer in the reassuring direction.

A proper ABI takes four ankle readings and two arm readings. Each ankle has two arteries worth measuring, the dorsalis pedis on the top of the foot and the posterior tibial behind the inner ankle bone. For each leg you use the higher of that leg’s two ankle pressures, and you divide by the higher of the two arm pressures, left or right.

Two consequences worth knowing:

• You get one ABI per leg, not one per person. Disease is frequently one-sided, so an average would hide it. Report the lower leg as the patient’s ABI for risk purposes, but treat each leg on its own.
• If you divide by the lower arm reading, you get a higher ABI and a falsely clean result. A difference of more than 10 to 15 mmHg between arms is itself a finding, usually subclavian stenosis, and needs looking at.

The measurement also needs a Doppler probe rather than a stethoscope, because pulses at the ankle are often too faint to hear when there is disease, which is precisely the situation you are testing for. Lie flat for five minutes first. Sitting up or standing changes the numbers.

Interpreting Your ABI Score

• Above 1.40: Arteries may be stiff or calcified (common in diabetics). Requires further evaluation.
• 1.00 – 1.40: Normal. Arteries are open and flowing well.
• 0.91 – 0.99: Borderline. Mild narrowing may be present. Monitor closely.
• 0.71 – 0.90: Mild PAD. Some restriction in blood flow to the legs.
• 0.41 – 0.70: Moderate PAD. Significant narrowing. Medical review recommended.
• 0.40 or below: Severe PAD. High risk for leg tissue damage and cardiovascular events.

Who Should Check Their ABI?

ABI screening is recommended for anyone over 50 with risk factors such as smoking, diabetes, high blood pressure, high cholesterol, or a family history of heart disease. People with leg pain while walking (claudication) should also be tested.

Practical Example

If your ankle systolic pressure is 120 mmHg and your brachial pressure is 130 mmHg: ABI = 120 ÷ 130 = 0.92

This falls in the borderline range, suggesting a follow-up with your doctor is wise.

A normal resting ABI does not rule out disease

Someone with real narrowing can still have a resting ABI of 1.0, because at rest the leg muscles need little blood and a partly blocked artery keeps up. Walk them on a treadmill and the demand rises, the pressure downstream of the narrowing drops, and the ABI falls. That is the exercise ABI, and a post-exercise drop of 20% or more is diagnostic even when the resting number looked fine.

So if you have classic claudication, cramping in the calf that starts after a predictable distance and stops within a few minutes of standing still, a normal resting ABI is not the end of the conversation. Ask about an exercise test.

When the number is above 1.40

A high ABI is not a good result, it is an uninterpretable one. Calcified vessels, common in long-standing diabetes and chronic kidney disease, resist the cuff and give an artificially high ankle pressure. The artery underneath may be badly diseased. This needs a different test: a toe-brachial index, since the small toe arteries rarely calcify, or a Doppler waveform.

Important Note

ABI is a screening tool, not a diagnosis. Always discuss your result with a healthcare provider. This calculator is for educational purposes only.