ABI Calculator: Ankle-Brachial Index
The ankle-brachial index compares systolic blood pressure at the ankle with systolic blood pressure at the arm. Clinicians use the ratio to assess blood flow to the legs and to place a result in the range associated with peripheral artery disease (PAD).
What the number describes
In a healthy leg, ankle systolic pressure is usually equal to or slightly higher than arm systolic pressure, so the ratio sits near or just above 1.00. When plaque narrows a leg artery, pressure falls downstream of the narrowing, and the ankle pressure drops relative to the arm pressure. The ratio therefore falls as flow limitation grows, which is why the same simple division carries both a screening signal and a severity signal.
ABI is a vascular measurement, not a heart-risk score on its own. A low value also correlates with higher overall cardiovascular event rates in population studies, because leg artery disease often travels with disease elsewhere. The result is therefore read together with symptoms, examination findings, and other risk information, never as a stand-alone verdict.
Enter Your Pressures
Enter systolic pressures in mmHg, measured with the same method for every site. The calculator uses the higher ankle value per leg and the higher brachial value across both arms.
Your ABI Result
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How Is the Ankle-Brachial Index Calculated?
The American Heart Association method uses the highest pressure at each site, not an average. For each leg, the ankle value is the higher of the dorsalis pedis and posterior tibial systolic pressures for that leg. The denominator for both legs is the higher of the two brachial systolic pressures.
Right ABI = max(right dorsalis pedis, right posterior tibial) ÷ max(right brachial, left brachial)
Left ABI = max(left dorsalis pedis, left posterior tibial) ÷ max(right brachial, left brachial)
Using the higher value at each site protects the calculation from a single low reading caused by cuff placement or a weak signal at one artery. The two legs are reported separately, because narrowing often affects one leg more than the other. For a person-level summary, clinical reports commonly cite the lower of the two leg values.
Variable table
| Component | Clinical measurement | Units |
|---|---|---|
| Brachial systolic pressure (each arm) | Systolic pressure at the upper arm, measured with a cuff and Doppler probe after rest in a lying position | mmHg |
| Dorsalis pedis systolic pressure (each ankle) | Systolic pressure at the artery on the top of the foot | mmHg |
| Posterior tibial systolic pressure (each ankle) | Systolic pressure at the artery behind the inner ankle bone | mmHg |
| ABI (each leg) | Higher ankle pressure for that leg divided by the higher brachial pressure | Ratio (no unit) |
Pressures are systolic only. Diastolic pressures do not enter the ABI formula. A related blood-pressure estimate, mean arterial pressure, uses both systolic and diastolic values and answers a different question about average arterial pressure.
How Is an ABI Result Interpreted?
The ranges below follow the American Heart Association scientific statement and the widely used stratification built on it. The threshold of 0.90 or below marks the range associated with peripheral artery disease, while the bands inside that range describe severity in the same scale clinicians use for follow-up.
| ABI range | Interpretation used on this page | What it reflects |
|---|---|---|
| Above 1.40 | Noncompressible arteries | Artery wall stiffness or calcification prevents normal cuff compression, so the reading reflects stiffness rather than flow |
| 1.00 – 1.40 | Normal | No significant flow limitation at rest by this measure |
| 0.91 – 0.99 | Borderline | Early or mild flow reduction; results near the threshold are commonly repeated or checked with further testing |
| 0.70 – 0.90 | Mild PAD range | Flow limitation consistent with mild peripheral artery disease |
| 0.40 – 0.69 | Moderate PAD range | More marked flow limitation, often with exertional leg symptoms |
| Below 0.40 | Severe PAD range | Severe flow limitation, with the highest concern for rest pain and tissue loss |
A change of more than 0.15 on repeat testing is commonly treated as a meaningful change rather than measurement noise, according to the same AHA statement. That detail matters for follow-up: a fall from 0.95 to 0.78 carries a different meaning from a wobble between 0.95 and 0.92.
ABI also carries prognostic weight beyond the legs. Population studies link a low ABI with higher rates of heart attack, stroke, and cardiovascular death, because atherosclerosis is usually a systemic process. A low leg value therefore prompts attention to overall cardiovascular risk, including cholesterol, blood pressure, smoking, and diabetes, alongside leg-specific assessment. Readers who want the lipid side of that picture can use the site's LDL cholesterol calculator and the AHA PREVENT risk calculator.
When Is ABI Less Reliable?
No single ratio is reliable in every body, and ABI has well-documented blind spots. Naming them is part of reading the result honestly.
- Calcified, noncompressible vessels. In diabetes, chronic kidney disease, and advanced age, medial calcification stiffens the artery wall. The cuff cannot compress the vessel, ankle pressures read falsely high, and a diseased leg can return a normal or even high ABI. In that setting, clinicians commonly use a toe-brachial index, because toe vessels calcify less, or turn to imaging.
- Exercise-only disease. A resting ABI can be normal when narrowing only limits flow during walking. When symptoms point to exertional disease and the resting value is normal or borderline, an exercise ABI or other vascular testing is commonly used next.
- Technique and cuff size. Cuffs that are too small over-read pressure, cuffs that are too large under-read it, and a rushed measurement without rest in a lying position adds noise. The AHA method specifies rest, a lying position, and a Doppler probe for a reason.
- One-sided arm disease. Using the higher brachial pressure protects against a low arm reading, but a narrowed subclavian artery on one side can still distort the denominator. A large between-arm difference is itself a finding worth noting in a clinical report.
- Acute symptoms. Sudden leg pain, pallor, loss of pulse, or new weakness is an emergency presentation, not a home-calculator scenario. The disclaimer at the top of this page applies with full force there.
When ABI is unreliable or borderline, the alternative is not a different formula on the same numbers. It is a different measurement: toe pressures, exercise testing, duplex ultrasound, or angiographic imaging, selected in a clinical setting.
Worked Example
A person has the following systolic pressures, all in mmHg: right brachial 120, left brachial 118, right dorsalis pedis 132, right posterior tibial 128, left dorsalis pedis 110, left posterior tibial 112.
- Higher brachial pressure: max(120, 118) = 120 mmHg.
- Right ankle pressure: max(132, 128) = 132 mmHg. Right ABI = 132 ÷ 120 = 1.10, in the normal range.
- Left ankle pressure: max(110, 112) = 112 mmHg. Left ABI = 112 ÷ 120 = 0.93, in the borderline range.
- Lower leg value: 0.93, so the person-level summary is borderline, driven by the left leg.
The example shows why both legs matter. A single averaged value would have hidden the left-leg finding entirely. The calculator on this page reproduces exactly this arithmetic, and the “Fill example” button loads these same numbers.
Frequently Asked Questions
How is the ankle-brachial index calculated?
For each leg, divide the higher ankle systolic pressure (dorsalis pedis or posterior tibial) by the higher brachial systolic pressure from either arm. The result is reported separately for the right leg and the left leg.
What is a normal ABI value?
In the ranges used on this page, 1.00 to 1.40 is the normal range. Values of 0.91 to 0.99 are borderline, 0.90 or below fall in the range associated with peripheral artery disease, and values above 1.40 are read as noncompressible arteries.
What does an ABI below 0.90 mean?
An ABI of 0.90 or below is the commonly used threshold for the range associated with peripheral artery disease. Within that range, 0.70 to 0.90 is described as mild, 0.40 to 0.69 as moderate, and below 0.40 as severe, in the stratification used on this page.
Why is an ABI above 1.40 a problem if it is high?
A value above 1.40 usually means the artery wall does not compress normally under the cuff, often because of calcification in diabetes or chronic kidney disease. The reading then reflects stiffness rather than blood flow, and clinicians commonly turn to a toe-brachial index or imaging instead.
Can one leg have a normal ABI and the other an abnormal ABI?
Yes. ABI is calculated and reported separately for each leg, because arterial narrowing often affects one leg more than the other. For an overall person-level summary, clinical reports commonly cite the lower of the two leg values.
Does a normal ABI rule out leg artery disease?
No. A resting ABI can be normal when narrowing only limits flow during exercise, when the measurement technique differs, or when calcified vessels mask a low value. Symptoms, examination findings, and further testing carry weight alongside the resting number.
How is an ABI test performed in a clinic?
After rest in a lying position, systolic pressures are measured with a cuff and a handheld Doppler probe at both arms and at the dorsalis pedis and posterior tibial arteries of each ankle. The higher ankle pressure for each leg is divided by the higher brachial pressure.
Sources and Editorial Review
Reviewed by the Heart Score Calculator Editorial Team, November 2026. The interpretation ranges and measurement method on this page follow the sources below.
- 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. DOI: 10.1161/CIR.0b013e318276fbcb
- 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.
- Fowkes FGR, Murray GD, Butcher I, et al. Ankle brachial index combined with Framingham Risk Score to predict cardiovascular events and mortality: a meta-analysis. JAMA. 2008;300(2):197-208.
- Grundy SM, Stone NJ, Bailey AL, et al. 2018 AHA/ACC Guideline on the Management of Blood Cholesterol. Circulation. 2019;139(25):e1082-e1143. DOI: 10.1161/CIR.0000000000000625
Related reading on this site: the calcium score test explainer covers a different direct measurement of arterial plaque, and the cardiovascular calculators hub lists every heart and circulation tool on the site.