Percent Stenosis Calculator (NASCET Carotid & Area %)

Calculate percent stenosis from residual and reference lumen diameters (NASCET or ECST) or areas, or find the residual diameter for a known stenosis.

This calculator applies arithmetic to user-supplied measurements; it does not select a reference, interpret an image, or grade disease. Diameter reduction and area reduction are different quantities. NASCET and ECST use different reference diameters, so values cannot be relabeled or converted by changing the dropdown. If you have symptoms of stroke or heart attack, seek emergency care.

Percent Stenosis Formula

The following formula is used to calculate the percent stenosis for a given normal and stenotic diameter.

PS = ((Dn – Ds) / Dn) × 100

Measurement note: This calculator computes percent diameter reduction (PS = (1 − D_s/D_n) × 100). Ensure D_n matches the reference diameter used in your imaging/reporting standard or lab protocol (for example, different carotid stenosis conventions), and note that some reports may use different definitions (including area-based stenosis). Results may differ from radiology/vascular lab reports.

Variables:

PS is the percent stenosis
D_n is the normal diameter
D_s is the stenotic diameter

To calculate the percent stenosis, subtract the stenotic diameter from the normal diameter. Divide the result by the normal diameter and multiply by 100 to get the percentage.

What is Percent Stenosis?

Percent stenosis is a measure used in medical diagnostics to quantify the narrowing of a blood vessel or other tubular structure. It is expressed as a percentage and indicates how much the diameter of the vessel has been reduced compared to a chosen reference (“normal”) diameter. This measurement can be one part of a broader clinical evaluation, and clinicians may interpret it alongside symptoms, imaging standards, and other findings to guide further evaluation.

How to Calculate Percent Stenosis?

The following steps outline how to calculate the Percent Stenosis.

Measure the reference (normal) diameter of the vessel (D_n) using the same standard your lab/report uses.
Measure the narrowest (stenotic) diameter (D_s) using the same imaging modality and units.
Compute PS = ((D_n − D_s) / D_n) × 100.
Sanity check: D_n must be greater than 0 and D_s should not exceed D_n; otherwise re-check measurements/units.
Optionally, compare your result with the calculator above.

Example Problem :

Use the following variables as an example problem to test your knowledge.

Normal Diameter (D_n) = 5 mm

Stenotic Diameter (D_s) = 2 mm

Percent Stenosis (PS) = ((5 − 2) / 5) × 100 = 60%