What Is Open Area Percentage? Definition, Formula & Impact on Vibratory Screening

Open area percentage is the proportion of a woven wire screen's total surface that consists of open space (apertures) rather than wire, expressed as a percentage. It is a primary factor determining throughput capacity and screening efficiency on vibratory screeners and separators. A screen with 40% open area means that 40% of the screen surface is open space through which particles can pass, while 60% is occupied by wire.

Open area percentage is governed by two variables: the aperture (opening size) and the wire diameter. For any given mesh size, using a lighter (thinner) wire increases the open area, boosting throughput at the expense of screen durability. Using a heavier (thicker) wire decreases the open area but increases the screen's resistance to abrasion and fatigue. Understanding this trade-off is essential for selecting the right screen cloth for your vibratory separator.

Open Area Percentage Formula

For square-opening woven wire mesh, open area percentage is calculated as:

Open Area % = (A / (A + D))² x 100

Where:
A = aperture (opening size) in mm or inches
D = wire diameter in mm or inches

Open Area by Mesh Size — Reference Table

The following table shows typical open area percentages for common mesh sizes using ASTM E11 standard wire diameters. Actual values may vary when non-standard wire diameters are used.

Why Open Area Percentage Matters in Vibratory Screening

• Throughput capacity — Higher open area means more space for material to pass through per vibration cycle. Doubling the open area roughly doubles the theoretical throughput, all else being equal.
• Screening efficiency — Particles have more chances to contact an opening when open area is high, increasing the percentage of undersize material that actually passes through the screen.
• Blinding resistance — Screens with higher open area are somewhat more resistant to blinding because near-size particles have less wire surface to lodge against relative to opening area.
• Screen life — Higher open area (achieved with thinner wire) generally means shorter screen life under abrasive conditions. This is the central trade-off in screen specification.

How to Increase Open Area Without Changing Mesh Size

If your vibratory separator needs more throughput at the same cut point, you can increase open area by specifying a lighter wire diameter for the same mesh count. For example, a 60 mesh screen with standard wire has approximately 35.7% open area, but the same 60 mesh specification with a lighter wire may achieve 40% or more. ScreenerKing can manufacture replacement screens with custom wire diameter specifications for Sweco, Kason, Midwestern Industries, Cleveland Vibratory, and other OEM separators — contact our engineering team for recommendations specific to your material.

Related Glossary Terms

• Aperture — The individual opening size in the screen, one of two variables in the open area formula
• Wire Diameter — The thickness of the screen wire, the other variable in the open area formula
• Mesh Size — The number of openings per linear inch, which along with wire diameter determines open area
• Throughput / Capacity — Directly influenced by open area percentage
• Screening Efficiency — Improved by higher open area
• Blinding — Screen clogging, affected by the ratio of wire to opening

Open Area Percentage FAQs

How do you calculate open area percentage of a screen?

Open area percentage is calculated using the formula: Open Area % = (Aperture / (Aperture + Wire Diameter))² x 100 for square-opening woven wire mesh. For example, a screen with a 0.149 mm aperture (100 mesh) and a 0.104 mm wire diameter has an open area of approximately 34.7%. The larger the aperture relative to the wire diameter, the higher the open area percentage.

What is a good open area percentage for vibratory screening?

For most vibratory screening applications, an open area percentage between 30% and 50% provides a good balance of throughput and screen durability. Coarse mesh screens (4-20 mesh) typically have open areas above 50%, while fine mesh screens (200+ mesh) may drop below 30%. Using lighter wire diameters increases open area but reduces screen life, so the optimal percentage depends on balancing capacity against durability for your specific material.

Does open area percentage affect screening efficiency?

Yes. Higher open area percentage gives particles more opportunities to pass through the screen on each vibration cycle, which improves screening efficiency and throughput. Lower open area means particles must travel farther before encountering an opening, reducing the probability of passage and increasing the chance of oversize contamination in the fines fraction. This is why fine-mesh screening (which has inherently low open area) typically requires longer residence time or de-blinding aids.