Does open area affect sheet strength?

Substantially. Removing metal reduces both stiffness and load capacity, and it does so unevenly depending on the pattern direction. For structural applications treat perforated sheet as an engineered component rather than scaling solid-sheet figures.

Why do published tables disagree slightly?

Rounding of the constants, mostly. Some tables print 78.5 where the exact value is 78.54, and some use 0.215 for the capsule correction where the exact figure is 0.2146. The differences are well inside manufacturing tolerance.

What open area is achievable in practice?

Up to about 60% is routine. Between 60% and 75% is possible but the sheet will bow during perforation and needs levelling. Above 75%, expanded metal is usually cheaper and generates no scrap.

Open area formulas for perforated sheet

Every pattern reduces to one idea: hole area divided by the area of the cell that contains it. Here is that model, the formulas it produces, and how to check your own numbers.

6 min read · updated 30 October 2026

Open area is the percentage of a perforated sheet that is hole rather than metal. It governs airflow, light transmission, acoustic behaviour, weight and — more than most people expect — whether the sheet can be made flat.

Published tables give a separate formula for each hole shape and arrangement, which makes the subject look harder than it is. All of them are the same calculation.

The whole subject

Open area % = (area of one hole ÷ area of the repeating cell containing one hole) × 100

Everything below is this expression with the geometry filled in.

Hole areas

Shape	Dimension	Area
Round	diameter d	π·d² / 4
Square	side c	c²
Hexagon	across flats d	(√3/2)·d² ≈ 0.866·d²
Slot / capsule	width w, overall length L	w·L − 0.2146·w²
Rectangle	width w, length L	w·L

A capsule is a rectangle with two semicircular ends; the correction term removes the corners the radii cut away.

Cell areas

The cell is the smallest repeating tile that contains exactly one hole. This is where arrangement enters the calculation.

Arrangement	Pitch notation	Cell area
60° staggered (triangular)	pitch T	(√3/2)·T² ≈ 0.866·T²
Straight / rectangular	pitches U₁ × U₂	U₁ · U₂
Staggered, general	pitches Z₁, Z₂	0.5 · Z₁ · Z₂

The resulting formulas

Combine the two tables and the familiar published constants appear on their own.

Pattern	Formula	Where the constant comes from
Round, 60° staggered	d² × 90.69 / T²	(π/4) ÷ (√3/2) × 100 = 90.69
Round, straight pitch	d² × 78.54 / (U₁·U₂)	(π/4) × 100 = 78.54
Square, straight pitch	c² × 100 / (U₁·U₂)	c² ÷ U₁U₂
Hexagon, 60° staggered	d² × 100 / T²	0.866 ÷ 0.866 cancels
Capsule, staggered	(w·L − 0.215·w²) × 100 / (0.5·Z₁·Z₂)	capsule area ÷ staggered cell

A worked example

Ø5 mm round, 8 mm pitch, 60° staggered, 2 mm SS 304, 1250 × 2500 mm sheet with 20 mm margins

Hole area: π × 5² / 4 = 19.63 mm²
Cell area: 0.866 × 8² = 55.43 mm²
Open area: 19.63 / 55.43 = 35.4%
Ligament: 8 − 5 = 3.0 mm
Perforated zone: 1210 × 2460 mm
Hole count: (1210 × 2460) / 55.43 ≈ 53,704
Gross weight: 1.25 × 2.5 × 0.002 × 7930 = 49.56 kg
Metal removed: zone area × thickness × density × 35.4% = 16.72 kg
Net weight: 49.56 − 16.72 = 32.84 kg

Ligament: the number that decides feasibility

The ligament is the web of metal left between two adjacent holes. For a staggered pattern it is simply pitch minus hole size. It is the figure that most often makes a pattern unmakeable, because a web thinner than roughly 0.7 times the sheet thickness will break the tooling.

Minimum pitch for a given thickness

pitch ≥ hole size + 0.7 × sheet thickness

Combined with the rule that hole size must be at least the sheet thickness, this fixes the densest pattern a given plate can take.

You do not have to do this by hand — we built the calculator that runs these numbers.

Calculate it instead

Common questions

Does open area affect sheet strength?: Substantially. Removing metal reduces both stiffness and load capacity, and it does so unevenly depending on the pattern direction. For structural applications treat perforated sheet as an engineered component rather than scaling solid-sheet figures.
Why do published tables disagree slightly?: Rounding of the constants, mostly. Some tables print 78.5 where the exact value is 78.54, and some use 0.215 for the capsule correction where the exact figure is 0.2146. The differences are well inside manufacturing tolerance.
What open area is achievable in practice?: Up to about 60% is routine. Between 60% and 75% is possible but the sheet will bow during perforation and needs levelling. Above 75%, expanded metal is usually cheaper and generates no scrap.