What is open area in a strainer screen?

Open area is the fraction of the straining material that is physically open to flow, expressed as a percentage of the gross screen area. It is a property of the screen material and pattern alone, and says nothing on its own about how much flow area the finished element presents to the pipe.

What is open area ratio (OAR)?

Open area ratio compares the effective open flow area of the complete element with the internal flow area of the pipe. At 100 percent OAR the clean element has only the same open flow area as the pipe, so any debris makes the element the controlling restriction. Practical designs carry a loading margin selected for the actual solids duty.

Why does a staggered hole pattern give more open area?

At the same hole diameter and pitch, a 60 degree staggered layout packs the holes into a smaller repeating cell than a straight layout. The constants reflect this: 90.69 for 60 degree staggered against 78.54 for straight centres, a ratio of about 1.155.

Open Area Calculator for Perforated Plate, Mesh & Wedge Wire

Q: Does mesh count alone define the opening?

No. For a plain square weave the clear opening is 1/N minus the wire diameter, so two screens of the same mesh count with different wire diameters have different openings and different open areas. Mesh count must always be stated with wire diameter.

Purpose

What This Tool Does

Open area is the fraction of the straining material physically open to flow. It is a property of the screen material and pattern alone. What governs the hydraulics of a finished strainer is the effective open area of the whole element measured against the bore of the pipe — the open area ratio. This calculator takes you from pattern geometry to that ratio in one step, and shows every figure in between.

When to use it

Comparing two screen specifications on a like-for-like basis.
Checking whether a proposed element has enough area for the duty.
Writing an enquiry that states open area rather than only a mesh count.
Sanity-checking a supplier's quoted open area against the pattern.

When not to use it

As a pressure-drop calculation — area is only one input to that.
As proof of element strength. Collapse resistance is a separate check.
As a final design figure without certified manufacturer data.

Calculator

Open Area and Area Ratio

Results update as you type. Nothing is hidden behind a form.

Inputs

Enter the screen pattern, then the element and pipe dimensions.

Screen pattern

Hole size (mm)

Pitch, centre to centre (mm)

Pitch must be larger than the hole — holes would overlap.

Element form

Basket diameter (mm)

D in the diagram — across the open end of the basket body.

Basket length (mm)

L in the diagram — the straight perforated body. Do not include the top flange, the handle or the closed bottom.

Inactive area (%)

Seams, end rings, handles and reinforcement that carry no flow.

Blockage allowance (%)

The loaded condition you intend to clean at.

Pipe internal diameter (mm)

Bore, not nominal size. 6″ Sch 40 is 154.1 mm.

Results

Open area of the screen material

—

Clear opening	—
Gross screen area	—
Net active screen area	—
Effective open area, clean	—
Effective open area, loaded	—
Pipe bore area	—
Gross screen : pipe area	—
Effective open area : pipe area (OAR)	—
OAR at selected blockage	—

Pattern drawn to the entered dimensions

Methodology

Formulae, Sources and Assumptions

Every constant this calculator uses is published and traceable. Nothing is fitted, guessed or proprietary.

Open area of the material

OA% = 90.69 × (d / p)² Round holes, 60° staggered centres. d hole diameter, p centre-to-centre pitch. Source: McNICHOLS, Perforated Metal Open Area, round and square hole percentage of open area tables.

OA% = 78.54 × (d / p)² Round holes, straight centres. Same source. The ratio 90.69 / 78.54 = 1.155 is the tighter repeating cell of a staggered layout, nothing more.

OA% = 100 × (s / p)² Square holes, straight or staggered centres. s side of square. Same source.

OA% = (1 − N × d_w)² × 100 Plain square woven mesh. N mesh count per inch, d_w wire diameter in inches. Clear opening a = 1/N − d_w. Standard weaving formula.

OA% = s / (s + w) × 100 Wedge wire, continuous slot. s slot opening, w wire width at the screening surface. Slots run one way, so the relationship is linear, not squared.

From material to element

A_gross = π D L Cylindrical basket, curved surface only.

A_net = A_gross × (1 − inactive%) Seams, end rings, handles and reinforcement carry no flow.

A_eff = A_net × OA% Effective open flow area of the clean element.

OAR = A_eff / A_pipe  
        A_pipe = πd²/4

Open area ratio, on the pipe bore, not the nominal size. Hydrotherms Engineering Handbook, chapter 8.

Assumptions

The pattern is uniform across the whole active area.
Blockage is applied in direct proportion to open area. Real blinding is not uniform and is often worse, so treat the loaded figure as indicative.
A support plate behind a fine mesh is not deducted. Where mesh is carried on perforated plate, the controlling open area is the more restrictive of the two. State which convention you are using in an enquiry.
Nothing here addresses element strength, collapse pressure or pressure drop.

Limitations

Preliminary engineering guidance, not a certified design calculation.
Manufacturing tolerance on wire diameter and hole pitch moves real open area by a few percent.
Mesh count alone never defines an opening. Always state wire diameter and the applicable sieve standard.

Reference

Published Screen Data

From the Hydrotherms Engineering Handbook, chapter 7 — to look up a standard screen, or to check a calculated figure.

Perforated plate — round hole, 60° staggered

The right-hand column re-derives open area from 90.69 × (d/p)². Amber rows are where the published figure and the formula disagree by more than three points; those pitches are worth re-checking.

Hole dia (in)	Pitch (in)	Published OA %	Formula OA %
0.020	0.043	20	19.6
0.027	0.066	17	15.2
0.033	0.077	20	16.7
0.045	0.086	28	24.8
0.057	0.121	25	20.1
0.062	0.094	41	39.7
0.094	0.156	33	32.8
0.100	0.156	37	37.1
0.125	0.188	40	40.3
0.156	0.188	63	63.0
0.188	0.250	51	51.0
0.250	0.375	40	40.3
0.312	0.438	47	46.3
0.375	0.500	51	51.0
0.438	0.594	49	49.2
0.500	0.688	48	48.0
0.625	0.812	54	53.7
0.750	1.000	51	51.0

Woven wire mesh — plain square weave

Mesh	Wire dia (in)	Clear opening (in)	Opening (µm)	Open area %
2	0.0630	0.4370	11100	76.4
2	0.0920	0.4070	10360	66.6
3	0.0630	0.2700	6860	65.6
4	0.0470	0.2080	5160	65.9
4	0.0630	0.1870	4750	56.0
5	0.0410	0.1590	4040	63.2
6	0.0350	0.1320	3350	62.7
7	0.0350	0.1080	2740	57.2
8	0.0280	0.0970	2460	60.2
10	0.0250	0.0750	1905	56.3
11	0.0180	0.0730	1850	64.5
12	0.0230	0.0600	1520	51.8
14	0.0200	0.0510	1300	51.0
16	0.0180	0.0440	1130	50.7
18	0.0170	0.0380	980	48.3
20	0.0160	0.0340	872	46.2
30	0.0130	0.0200	513	37.1
40	0.0100	0.0150	384	36.0
50	0.0090	0.0110	282	30.3
60	0.0070	0.0090	231	33.9
80	0.0050	0.0075	180	36.0
100	0.0045	0.0055	141	30.0
120	0.0037	0.0046	118	30.1
150	0.0026	0.0041	105	37.4
170	0.0024	0.0035	79	35.1
200	0.0021	0.0029	74	33.6
250	0.0016	0.0024	62	36.0
300	0.0015	0.0018	46	29.7
325	0.0014	0.0017	44	30.0
400	0.0010	0.0015	39	36.0

The micron column carries the nominal standard aperture, which is not always the inch figure converted exactly — sieve designations are standardised values. That is precisely why mesh equivalence between standards is approximate and must never be quoted as absolute retention.

Wedge wire — continuous slot

Slot opening (in)	Approx. opening (µm)	Open area %
0.003	75	7.7
0.005	127	14.3
0.010	254	25.0
0.015	381	25.0
0.020	500	25.0
0.031	775	34.0
0.034	864	20.0
0.062	1550	51.0
0.063	1600	50.0
0.094	2350	44.0
0.125	3175	66.0
0.156	3962	71.0

Technical FAQ

Questions Engineers Ask

What is open area?

The fraction of the straining material physically open to flow, as a percentage of gross screen area. It is a property of the material and pattern only.

What is open area ratio?

The effective open flow area of the complete element divided by the pipe bore area. At an OAR of 1.0 the clean element has only the same flow area as the pipe, so the first debris makes it the controlling restriction. Practical designs carry a margin chosen for the solids duty, the allowable pressure drop and the cleaning interval.

Does mesh count define the opening?

No. Clear opening is 1/N minus the wire diameter, so two screens of the same mesh count with different wire diameters have different openings and different open areas. Never specify a screen by mesh count alone.

Why does staggering help?

At the same hole size and pitch a 60° staggered layout fits the holes into a smaller repeating cell than a straight layout, so more of the sheet is open — 90.69 against 78.54, about 15 percent more.

Next Step

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This calculator gives you the geometry. Turning that into a strainer needs pressure drop at your flow, element strength at your differential, and certified dimensions. Send us the duty and our engineers will come back with a checked selection.

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Calculation version OAC-1.0 Last technical review 28 August 2026 Reviewed by Hydrotherms engineering

Engineering disclaimer. This tool provides preliminary engineering guidance. It does not replace the applicable design code, project specification, piping class, statutory requirement or certified manufacturer data. Results must be verified before use in a purchase specification or a final design.