Ankle-Brachial Index (ABI) Calculator
Compute the ankle-brachial index for each leg and classify peripheral artery disease severity using the AHA / ACC interpretation bands.
Lower-limb ABI — Normal
1.02
- Right-leg ABI
- 1.04 — Normal
- Left-leg ABI
- 1.02 — Normal
- Higher arm systolic (denominator)
- 130 mmHg
ABI is the systolic ankle pressure divided by the higher of the two arm (brachial) systolic pressures, computed separately for each leg. The lower of the two ABIs is used to classify peripheral artery disease (PAD): >1.40 non-compressible (calcified vessels); 1.00–1.40 normal; 0.90–0.99 borderline; 0.71–0.89 mild PAD; 0.41–0.70 moderate PAD; ≤0.40 severe PAD. Source: AHA Scientific Statement, Aboyans et al., Circulation 2012;126:2890–2909.
How to use this calculator
Measure the systolic blood pressure in both arms (brachial) and both ankles. For each ankle, take the higher of the posterior tibial and dorsalis pedis readings — that is the value to enter. The calculator returns the ABI for each leg, the lower-leg ABI used clinically, and the AHA / ACC interpretation band for each.
How the calculation works
For each leg, ABI = ankle systolic pressure ÷ higher of the two arm systolic pressures. The higher arm pressure is always used as the denominator, even when the legs are calculated separately, so the index reflects ankle perfusion against the best available central perfusion. The lower of the two leg ABIs is the value used for diagnosis, since unilateral peripheral artery disease is common. Interpretation bands come from the AHA Scientific Statement (Aboyans et al., Circulation 2012;126:2890–2909) and are re-affirmed in the 2016 AHA / ACC Lower-Extremity PAD Guideline: above 1.40 the vessels are non-compressible (often from medial calcinosis in diabetes or chronic kidney disease), 1.00–1.40 is normal, 0.90–0.99 borderline, 0.71–0.89 mild PAD, 0.41–0.70 moderate PAD, and 0.40 or below severe PAD.
Worked example
A 68-year-old patient with claudication has right arm 142 mmHg, left arm 138 mmHg, right ankle 96 mmHg, left ankle 124 mmHg. Higher arm = 142 mmHg. Right ABI = 96 / 142 = 0.68 (moderate PAD). Left ABI = 124 / 142 = 0.87 (mild PAD). The lower-leg ABI is 0.68, placing the patient in the moderate-PAD band. This is consistent with the right-leg claudication on history.
Frequently asked questions
What is the ankle-brachial index?
The ankle-brachial index (ABI) is a non-invasive ratio that compares the systolic blood pressure at the ankle to the systolic pressure at the arm. A lower ratio means lower perfusion pressure reaches the leg, which is the haemodynamic signature of peripheral artery disease (PAD). It is the screening test of choice for PAD in primary care and vascular clinics worldwide.
What is a normal ABI?
The AHA / ACC defines a normal ABI as 1.00 to 1.40. Values from 0.90 to 0.99 are borderline and warrant repeat measurement and clinical correlation. Values at or below 0.90 are diagnostic of PAD. Above 1.40 the result is reported as non-compressible — the calculation is not reliable because the calf arteries cannot be fully occluded by the cuff, usually because of medial arterial calcinosis seen in long-standing diabetes or chronic kidney disease.
How is the ABI actually measured?
The patient lies supine for 5–10 minutes. Systolic pressure is measured in both arms with a hand-held Doppler probe over the brachial artery. At each ankle the Doppler is placed over both the posterior tibial artery and the dorsalis pedis artery; the higher of these two values is taken as the ankle pressure for that leg. The ABI for each leg is then computed against the higher of the two arm pressures. Manual oscillometric cuffs without Doppler are less accurate and should not be used for diagnostic ABI.
Which leg ABI do I use for the diagnosis?
Clinical guidelines diagnose PAD using the lower of the two leg ABIs because peripheral artery disease is often unilateral or asymmetric. Reporting both legs separately is still required so the clinician can distinguish bilateral disease from one-sided disease and follow each leg over time.
Can the ABI be falsely high or non-compressible?
Yes. Medial arterial calcinosis stiffens the calf vessels so that even very high cuff pressures cannot fully occlude them, producing falsely elevated ankle readings and an ABI above 1.40. This is common in diabetes, end-stage renal disease, and very elderly patients. In that situation the calculator returns a "non-compressible" label; the appropriate next step is a toe-brachial index (TBI) or arterial duplex ultrasound, both of which are unaffected by medial calcinosis.
Does a normal resting ABI rule out peripheral artery disease?
No. Patients with exertional claudication can have a normal resting ABI that drops with exercise. If the history is suspicious, the AHA / ACC recommends an exercise (post-exercise) ABI: the patient walks on a treadmill until symptoms or for 5 minutes, then the ABI is measured immediately. A post-exercise ABI drop of more than 20% from baseline is diagnostic of PAD even when the resting ABI is normal.
Is the ABI a substitute for a vascular specialist review?
No. This calculator implements the AHA interpretation bands accurately but cannot substitute for a clinician. A low or non-compressible ABI should always be interpreted alongside the patient's symptoms, cardiovascular risk profile, and imaging where indicated. The calculator is suitable for clinicians performing the measurement, for students learning the test, and for patients trying to understand a result they were given — not for self-diagnosis.