ABI Calculator - Free Online Tool

Compute the Ankle-Brachial Index for PAD screening — bilateral results with six-tier clinical interpretation

The Ankle-Brachial Index (ABI) is the gold-standard non-invasive test for detecting Peripheral Artery Disease (PAD), a circulatory condition in which narrowed arteries reduce blood flow to the limbs. Our free ABI Calculator lets you compute the index for both legs simultaneously, interpret your results against six clinical tiers, and understand what the numbers mean for your vascular health — all without needing expensive equipment or a specialist's appointment. PAD affects roughly 12% of adults worldwide and is strongly associated with serious cardiovascular events including heart attack and stroke, even in people who have no classic leg symptoms. Studies show that up to 75% of individuals with PAD experience no claudication (leg pain during walking), making asymptomatic screening critical. The ABI is the simplest and most cost-effective way to screen for this hidden disease: a ratio close to 1.0 indicates healthy circulation, while values below 0.90 point toward arterial disease of varying severity. Our calculator supports two input modes. The simplified mode matches how most clinical guidelines describe ABI testing: you enter the highest systolic blood pressure recorded from either arm, plus the systolic pressure at each ankle. The advanced mode mirrors the full clinical protocol used in vascular labs — you enter separate readings for the right arm, left arm, right dorsalis pedis (DP), right posterior tibial (PT), left dorsalis pedis, and left posterior tibial arteries. The calculator then automatically selects the highest value from each site group, exactly as clinical standards require. This matters because using both ankle sites per leg reduces the chance of missing a focal stenosis that affects only one vessel. Results are presented for both legs simultaneously. Each leg's ABI value is displayed to two decimal places and mapped to one of six clinical tiers: Non-compressible (>1.40), Normal (1.00–1.40), Borderline (0.91–0.99), Mild PAD (0.71–0.90), Moderate PAD (0.41–0.70), or Severe PAD (≤0.40). A visual severity gauge shows exactly where each result falls on the spectrum so you can see the comparison at a glance. When both legs are measured, the calculator automatically computes the inter-leg difference and flags it when it exceeds 0.15, the threshold at which clinicians consider asymmetry to be clinically significant. For values above 1.40, the calculator displays a Toe-Brachial Index (TBI) advisory. In this scenario, the arteries are too calcified to compress properly during measurement — a condition common in diabetes, chronic kidney disease, and advanced age — so the ABI overestimates the true pressure and cannot be used reliably. The TBI, which uses a specialized small cuff on the toe, is the recommended alternative in these cases. The tool also includes optional patient information fields (name, date, clinician) that are used in the printable report, making it useful for clinicians, wound-care nurses, and vascular rehabilitation therapists who need a quick bedside record. All calculations run entirely in your browser — no data is sent to any server. ABI screening is recommended by major cardiology and vascular societies for all adults over age 70, and for adults over 50 who have a history of smoking or diabetes. It is also indicated for any patient with exertional leg pain, non-healing foot wounds, or absent peripheral pulses. ABI has approximately 90% sensitivity and 98% specificity for detecting significant PAD compared to angiography. In wound care settings, an ABI above 0.80 is generally required before applying full compression bandaging, making this calculator directly relevant to nurses and wound-care practitioners. Use this tool as a starting point for a conversation with your healthcare provider. A single ABI measurement is one piece of a comprehensive cardiovascular risk assessment, and results should always be interpreted alongside your full clinical picture including symptoms, risk factors, and physical examination findings.

Understanding the Ankle-Brachial Index

The ABI is a simple ratio of ankle blood pressure to arm blood pressure. A value close to 1.0 means the blood pressure at the ankle is nearly the same as at the arm, indicating healthy circulation. Lower values indicate arterial narrowing in the legs.

What Is the ABI and How Is It Measured?

The Ankle-Brachial Index is calculated by dividing the highest systolic blood pressure recorded at the ankle by the highest systolic blood pressure recorded at the arm (brachial artery). Clinically, the measurement is performed using a standard blood pressure cuff and a hand-held Doppler ultrasound probe to detect flow signals in the arteries. The Doppler probe is placed over the dorsalis pedis artery (top of the foot) and the posterior tibial artery (inner ankle) to capture both major ankle vessels, and the highest reading from either site is used. The same technique is applied to both brachial arteries. The highest arm pressure is used as the denominator to avoid underestimating the ABI in cases of subclavian artery stenosis. This non-invasive technique takes about 15 minutes per patient and requires no radiation or contrast agents.

How Is the ABI Calculated?

The formula is straightforward: ABI = Highest Ankle Systolic Pressure / Highest Brachial Systolic Pressure. In the simplified approach, the clinician records the highest pressure from either arm as the brachial reference, and the highest pressure from each ankle (DP or PT) as the ankle value for that leg. For the right leg: Right ABI = Highest Right Ankle Pressure / Highest Brachial Pressure. For the left leg: Left ABI = Highest Left Ankle Pressure / Highest Brachial Pressure. In the full clinical protocol, separate readings are taken at four ankle sites (right DP, right PT, left DP, left PT) and two brachial sites (right arm, left arm). The calculator automatically takes the maximum within each group, reducing measurement error. Results are expressed to two decimal places for precision.

Why the ABI Matters Beyond Leg Symptoms

An ABI below 0.90 is not just a sign of reduced leg circulation — it is an independent predictor of major adverse cardiovascular events (MACE) including heart attack and stroke, even in people who feel completely well. This is because atherosclerosis (arterial plaque buildup) is a systemic disease; hardening of the leg arteries reflects the same process happening in coronary and carotid arteries. For this reason, international cardiology guidelines from the ACC, AHA, and ESC classify a low ABI as a cardiovascular risk enhancer that can help clinicians decide whether to intensify preventive treatments such as statin therapy and antiplatelet agents. ABI testing is endorsed for population screening in high-risk groups and as an objective marker in cardiovascular risk stratification.

Limitations and When ABI Is Unreliable

The ABI has two main limitations. First, it can be falsely elevated in patients with heavily calcified arteries, most commonly seen in long-standing diabetes, chronic kidney disease, and older age. When arteries cannot be compressed by the cuff, the ABI reads above 1.40, which does not reflect true perfusion. In these cases, the Toe-Brachial Index (TBI), measured with a small digital cuff at the great toe, is preferred because toe arteries calcify less frequently. Normal TBI is above 0.70 (some guidelines use 0.60). Second, the ABI has roughly 90% sensitivity, meaning about 10% of people with significant PAD may have a near-normal ABI at rest. For borderline values of 0.91–0.99, an exercise ABI — measured immediately after standardized treadmill walking — can unmask PAD that is not apparent at rest.

ABI Formulas & Equations

Ankle-Brachial Index (ABI)

ABI = Ankle Systolic Pressure (mmHg) / Brachial Systolic Pressure (mmHg)

The primary ABI formula divides the highest systolic blood pressure measured at the ankle (dorsalis pedis or posterior tibial) by the highest systolic blood pressure measured at the arm (brachial artery). A separate ABI is calculated for each leg.

Toe-Brachial Index (TBI)

TBI = Toe Systolic Pressure (mmHg) / Brachial Systolic Pressure (mmHg)

Used when ABI exceeds 1.40 (non-compressible arteries). The TBI uses a small cuff on the great toe, which calcifies less frequently than ankle arteries. Normal TBI is above 0.70.

Inter-Leg ABI Difference

Difference = |Right Leg ABI − Left Leg ABI|

The absolute difference between the right and left leg ABI values. A difference greater than 0.15 is considered clinically significant asymmetry and may indicate unilateral arterial disease.

ABI from Advanced Protocol

ABI (per leg) = max(DP, PT) / max(Right Arm, Left Arm)

In the full clinical protocol, the highest of the dorsalis pedis (DP) and posterior tibial (PT) readings is used for the ankle, and the highest of both brachial readings is used as the denominator.

ABI Classification Reference Tables

ABI Classification and Clinical Interpretation

Six-tier classification of ABI values used by the American Heart Association (AHA) and European Society of Cardiology (ESC) for peripheral artery disease screening and cardiovascular risk assessment.

ABI Range	Classification	Clinical Meaning	Recommended Action
> 1.40	Non-Compressible	Calcified arteries; ABI is unreliable at this value	Request Toe-Brachial Index (TBI) measurement
1.00 – 1.40	Normal	Healthy peripheral circulation; no significant PAD	Routine cardiovascular monitoring every 5 years
0.91 – 0.99	Borderline	Possible early PAD; may be asymptomatic	Monitor every 6–12 months; consider exercise ABI
0.71 – 0.90	Mild PAD	Reduced blood flow; claudication may be present	Lifestyle modification; medical evaluation; supervised walking
0.41 – 0.70	Moderate PAD	Significant arterial stenosis; functional impairment likely	Vascular specialist referral; duplex ultrasound imaging
≤ 0.40	Severe PAD	Critical limb ischemia; limb-threatening condition	Urgent vascular surgery or interventional radiology evaluation

ABI Screening Recommendations by Risk Group

Summary of when ABI screening is recommended based on age, symptoms, and risk factors, per ACC/AHA vascular guidelines.

Risk Group	Screening Recommendation	Rationale
Adults ≥ 70 years	Screen all, regardless of symptoms	High baseline PAD prevalence in this age group
Adults 50–69 with smoking or diabetes	Screen all	Smoking and diabetes are the strongest PAD risk factors
Exertional leg pain (any age)	Screen if claudication suspected	Calf pain during walking that resolves with rest is classic PAD symptom
Non-healing foot/leg wounds	Screen before compression therapy	ABI > 0.80 generally required for safe compression bandaging
Known coronary or cerebrovascular disease	Consider screening	PAD frequently co-exists with coronary and carotid atherosclerosis

ABI Calculation Examples

Basic ABI Calculation — Single Patient

A 65-year-old patient has the highest brachial (arm) systolic pressure of 130 mmHg. The right ankle systolic pressure is 110 mmHg, and the left ankle systolic pressure is 126 mmHg.

Right ABI = 110 / 130 = 0.85

Left ABI = 126 / 130 = 0.97

Right leg classification: 0.85 falls in the 0.71–0.90 range → Mild PAD

Left leg classification: 0.97 falls in the 0.91–0.99 range → Borderline

Inter-leg difference = |0.85 − 0.97| = 0.12 → Within normal range (< 0.15)

The right leg shows mild peripheral artery disease (ABI 0.85), while the left leg is borderline (ABI 0.97). The inter-leg difference of 0.12 is within the normal range. Lifestyle modification and follow-up monitoring are recommended, with an exercise ABI test to further evaluate the right leg.

Bilateral Comparison with Significant Asymmetry

A 72-year-old diabetic smoker has brachial pressure of 140 mmHg. Right ankle pressure is 84 mmHg, left ankle pressure is 122 mmHg.

Right ABI = 84 / 140 = 0.60

Left ABI = 122 / 140 = 0.87

Right leg classification: 0.60 falls in the 0.41–0.70 range → Moderate PAD

Left leg classification: 0.87 falls in the 0.71–0.90 range → Mild PAD

Inter-leg difference = |0.60 − 0.87| = 0.27 → Clinically significant (> 0.15)

The right leg shows moderate PAD (ABI 0.60) and the left shows mild PAD (ABI 0.87). The inter-leg difference of 0.27 is clinically significant, indicating more severe disease on the right side. Vascular specialist referral and duplex ultrasound imaging of the right leg are recommended. Smoking cessation is critical.

Non-Compressible Result Requiring TBI

A 78-year-old patient with chronic kidney disease has brachial pressure of 135 mmHg. Right ankle pressure is 198 mmHg, left ankle pressure is 190 mmHg.

Right ABI = 198 / 135 = 1.47

Left ABI = 190 / 135 = 1.41

Both values exceed 1.40 → Non-compressible arteries

ABI is unreliable in this range due to arterial calcification

Recommend Toe-Brachial Index (TBI) for both legs

Both legs produce ABI values above 1.40 (right 1.47, left 1.41), indicating non-compressible calcified arteries. The ABI cannot be used to assess perfusion in this patient. A Toe-Brachial Index (TBI) measurement is needed — normal TBI is above 0.70.

How to Use This ABI Calculator

Choose Simple or Advanced Mode

Select Simple Mode if you have a single brachial (arm) pressure reading and one reading per ankle — this covers most clinical and screening scenarios. Select Advanced Mode if you have separate readings for the right and left arms, and separate dorsalis pedis (DP) and posterior tibial (PT) readings for each ankle. Advanced mode mirrors the full clinical protocol and automatically selects the highest value from each measurement group.

Enter Blood Pressure Readings in mmHg

Enter all systolic blood pressure values in millimetres of mercury (mmHg). In Simple Mode: enter the highest brachial (arm) pressure from either arm, the right ankle pressure, and the left ankle pressure. In Advanced Mode: enter right arm, left arm, right DP, right PT, left DP, and left PT pressures. All values must be positive numbers greater than zero. Leave a field blank if a reading was unobtainable.

Review ABI Values and Clinical Interpretation

The calculator computes a separate ABI for each leg to two decimal places and maps each result to one of six clinical categories: Non-Compressible, Normal, Borderline, Mild PAD, Moderate PAD, or Severe PAD. Read the interpretation text and recommended next steps for each leg. The severity gauge shows both legs' positions on the ABI scale simultaneously, and an inter-leg difference flag will appear if asymmetry exceeds 0.15.

Use the Report and Checklist for Next Steps

Complete the risk factor checklist to receive personalized context about whether ABI screening is especially important for you. Add optional patient information (name, date, clinician) and click Print Report to generate a printable clinical summary. Use the CSV export to save your readings for follow-up tracking. Share the printed or saved results with your healthcare provider at your next appointment.

Frequently Asked Questions

What is a normal ABI value?

A normal ABI falls between 1.00 and 1.40. This range indicates that the systolic blood pressure at the ankle is at least as high as the arm pressure, which is expected in healthy arteries because the lower limb vasculature amplifies pressure slightly. Values in this range are generally associated with no significant peripheral artery disease. Most major vascular societies, including the American Heart Association and the European Society of Cardiology, use 0.90 as the diagnostic threshold: an ABI below 0.90 is considered abnormal and indicates PAD of varying severity. Values above 1.40 are also abnormal but for a different reason — they suggest arterial calcification rather than stenosis.

What does an ABI below 0.90 mean?

An ABI below 0.90 indicates peripheral artery disease (PAD), meaning there is significant narrowing of the arteries supplying blood to the legs. The lower the value, the more severe the disease. Values from 0.71 to 0.90 suggest mild PAD, often causing calf pain during walking (claudication) that resolves with rest. Values from 0.41 to 0.70 suggest moderate PAD with greater functional impairment. Values at or below 0.40 suggest severe PAD or critical limb ischemia, which can threaten limb viability and requires urgent specialist evaluation. Importantly, ABI below 0.90 is also an independent cardiovascular risk marker associated with increased rates of heart attack and stroke, even in patients with no leg symptoms.

Why is ABI above 1.40 a problem?

An ABI above 1.40 is considered non-compressible and is not a sign of good circulation — rather, it indicates that the arteries are calcified and too stiff to compress with a blood pressure cuff. This makes the measured systolic pressure artificially high, so the ABI ratio is overestimated. This pattern is most common in people with diabetes mellitus, chronic kidney disease, and advanced age. When ABI exceeds 1.40, it cannot be used to rule out PAD. The recommended alternative is the Toe-Brachial Index (TBI), which uses a specialized small cuff placed on the great toe. Toe arteries calcify much less frequently, so TBI remains reliable in most calcification cases. A normal TBI is generally above 0.70.

How does the inter-leg ABI difference matter?

A difference in ABI between the right and left legs can indicate unilateral (one-sided) arterial disease. A difference of up to 0.10–0.15 can occur due to measurement variability and is considered normal. However, when the inter-leg difference exceeds 0.15, clinicians consider this clinically significant asymmetry. It suggests that one leg may have greater arterial obstruction than the other, which can guide imaging decisions — for example, duplex ultrasound of the more affected leg. Our calculator automatically computes the absolute difference between your right and left ABI values and flags it when it crosses this threshold, alerting you to discuss the finding with your provider.

Who should be screened with an ABI test?

Major cardiology and vascular guidelines recommend ABI screening for all adults aged 70 and over, regardless of symptoms. Screening is also recommended for adults aged 50–69 who have a history of smoking or diabetes. Additionally, ABI testing is indicated for any patient who has exertional leg pain (especially calf pain that stops with rest), non-healing foot or leg wounds, or absent or diminished peripheral pulses on physical examination. In wound-care settings, ABI is used before applying compression bandaging — an ABI above 0.80 is generally required for safe full compression therapy. Patients with known coronary artery disease or stroke may also benefit, as PAD and cardiovascular disease frequently co-exist.

What is the accuracy of the ABI test?

The ABI has approximately 90% sensitivity and 98% specificity for detecting significant PAD compared to digital subtraction angiography, which is the reference standard. This means ABI correctly identifies about 90 of every 100 people who truly have significant PAD, and correctly identifies 98 of every 100 people who do not have significant PAD. The high specificity makes it a reliable tool for ruling in disease. However, the 10% false-negative rate means some patients with PAD — particularly those with borderline values of 0.91–0.99 — may need exercise ABI testing or imaging to confirm or exclude the diagnosis. Measurement technique is important: values should be obtained with the patient supine and at rest for at least five minutes before the first reading.