What this calculator does
In a healthy circulation, blood pressure at the ankle is at least as high as at the arm. When the leg arteries narrow, ankle pressure falls, and the ratio between the two detects that before symptoms become obvious.
The bands run in both directions. Below 0.90 suggests peripheral artery disease, with severity increasing as the ratio falls, while above 1.40 suggests arteries too calcified to compress, which is its own problem.
The formula
Each ankle systolic pressure is divided by the higher of the two arm pressures. The lower of the two resulting values is the figure normally reported.
| Term | Meaning |
|---|---|
| ABI | Ankle systolic pressure divided by the higher arm systolic pressure. |
| Peripheral artery disease | Narrowing of the leg arteries, which the index screens for. |
| Noncompressible | An index above 1.40, indicating arteries stiff enough that the cuff cannot compress them. |
The inputs explained
| Field | What to enter |
|---|---|
| Right arm systolic pressure (mmHg) | Right arm systolic pressure in mmHg. |
| Left arm systolic pressure (mmHg) | Left arm systolic pressure in mmHg. The higher of the two arms is used as the denominator. |
| Right ankle systolic pressure (mmHg) | Right ankle systolic pressure in mmHg. |
| Left ankle systolic pressure (mmHg) | Left ankle systolic pressure in mmHg. |
When to use it
Screening for peripheral artery disease
The index detects narrowing before claudication becomes limiting.
Assessing leg pain on walking
A low index supports an arterial cause for exertional leg pain.
Before compression therapy
Compression bandaging is unsafe with significant arterial disease, so the index is checked first.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What does each ankle pressure indicate?
Four ankle pressures against the same arm pressure.
| Right ankle pressure | Right ABI | Higher arm systolic pressure used |
|---|---|---|
| 50 mmHg | 0.42 (Mild to moderate PAD) | 120 mmHg |
| 90 mmHg | 0.75 (Mild to moderate PAD) | 120 mmHg |
| 120 mmHg | 1.00 (Normal) | 120 mmHg |
| 170 mmHg | 1.42 (Noncompressible: arterial calcification suspected) | 120 mmHg |
Questions
Why use the higher arm pressure?
Because a difference between arms can itself indicate arterial narrowing in the arm, which would give a falsely reassuring ratio. Taking the higher value guards against that.
Why is a high index also a problem?
Because it means the arteries are too stiff to compress with a cuff, usually from calcification, which is common in diabetes and chronic kidney disease. The measurement becomes unreliable rather than reassuring, and toe pressures are used instead.
Can a normal resting index rule out disease?
Not entirely. Some people with significant disease have a normal resting index that falls after exercise, which is why exercise ABI testing exists when symptoms suggest disease despite a normal resting figure.
How is the measurement taken?
With a blood pressure cuff and a Doppler probe rather than a stethoscope, since ankle pulses can be too weak to hear. Technique affects the result, and it is normally performed by a trained clinician.
For average circulatory pressure, see the mean arterial pressure calculator. For circulatory stress from vital signs, see the shock index calculator.