Calculate ankle-brachial index (ABI) from ankle and brachial systolic pressures in mmHg, cmHg, or kPa and view the unitless ratio.
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Medical disclaimer: This calculator is for educational purposes only and does not provide medical advice or a diagnosis. ABI measurement technique and interpretation should be performed or confirmed by a qualified healthcare professional (especially if you have diabetes, kidney disease, or suspected calcified/noncompressible arteries). If you have severe symptoms such as rest pain, a suddenly cold/pale/blue foot, sudden worsening leg pain, or non-healing wounds, seek urgent medical care or call your local emergency number.
Ankle-Brachial Index (ABI) Formula
The ankle-brachial index is a unitless ratio that compares the systolic blood pressure measured at the ankle with the systolic blood pressure measured at the arm.
- ABI = ankle-brachial index, a unitless ratio
- P_ankle = ankle systolic pressure, converted to mmHg
- P_brachial = brachial systolic pressure, converted to mmHg
If you enter pressure values in cmHg or kPa, the calculator first converts each value to mmHg, then applies the ABI formula.
- P_mmHg = pressure in millimeters of mercury
- P_cmHg = pressure in centimeters of mercury
- P_kPa = pressure in kilopascals
The calculatorâs main function is to divide ankle systolic pressure by brachial systolic pressure after both values are in the same unit. It also flags entries that are outside typical adult pressure ranges and notes when the ABI is unusually high.
ABI Result Ranges
The table below gives common ABI interpretation ranges. ABI results should be interpreted with symptoms, exam findings, and clinician judgment.
| ABI value | Common interpretation | What it may suggest |
|---|---|---|
| 1.00 to 1.40 | Normal range | Typical blood flow ratio between ankle and arm |
| 0.91 to 0.99 | Borderline | May need follow-up if symptoms or risk factors are present |
| 0.41 to 0.90 | Abnormal | May suggest peripheral artery disease |
| 0.40 or lower | Severely abnormal | May suggest severe arterial disease |
| Above 1.40 | Noncompressible range | Can occur with stiff or calcified arteries; additional testing may be needed |
Pressure Unit Conversions Used
| Entered unit | Conversion to mmHg | Example |
|---|---|---|
| mmHg | Multiply by 1 | 120 mmHg = 120 mmHg |
| cmHg | Multiply by 10 | 12 cmHg = 120 mmHg |
| kPa | Multiply by 7.50062 | 16 kPa = 120.010 mmHg |
Example ABI Calculations
Example 1: You measure an ankle systolic pressure of 110 mmHg and a brachial systolic pressure of 125 mmHg.
The ABI is 0.880, which falls in the abnormal range in the table above.
Example 2: You enter an ankle pressure of 15 cmHg and a brachial pressure of 18 kPa.
The ABI is about 1.111, which is within the normal range.
FAQ
Which ankle pressure should you use for ABI?
When more than one ankle artery pressure is measured, clinicians commonly use the higher ankle systolic pressure for that leg. If you were given specific instructions by a clinician or testing protocol, follow those instructions.
Which brachial pressure should you use?
ABI is often calculated using the higher brachial systolic pressure from the two arms, unless a clinician advises a different method. Using the higher arm pressure helps avoid overestimating the ABI when one arm pressure is lower because of arterial narrowing.
Can ABI diagnose peripheral artery disease by itself?
ABI is an important screening and assessment value, but it is not the whole diagnosis by itself. Symptoms, medical history, pulse exam, risk factors, and sometimes additional vascular tests are used to interpret the result. An ABI above 1.40 can also be hard to interpret because arteries may be noncompressible.
