Type fractions (3/8, 9/16, 1 1/2) or decimals · Open area = K × (hole ÷ centre)² · K = 90.69 round staggered, 78.54 round straight, 115.47 square staggered, 100 square straight · Bar = centre − hole
How to read a perforated spec
Perforated metal is described by three things: the hole size, the distance between hole centres, and the pattern they sit in. A spec written as 3/8" hole on 9/16" staggered means 3/8 inch round holes whose centres are 9/16 inch apart, arranged in the 60° staggered pattern. Put those numbers in above — as fractions if that is how your drawing reads — and you get 40% open area, which matches how that sheet is described in our catalogue.
The number that usually matters is open area: the percentage of the sheet that is hole rather than metal. It governs airflow, drainage, light and sound transmission, and how much strength is left in the sheet. Two patterns with the same hole size can have very different open areas depending on how tightly the holes are spaced.
Which pattern gives the most open area
For the same hole size and spacing, the pattern alone changes the answer:
| Pattern | Coefficient | Relative open area |
|---|---|---|
| Square hole, staggered | 115.47 | Highest |
| Square hole, straight rows | 100 | High |
| Round hole, 60° staggered | 90.69 | Moderate — the industry default |
| Round hole, straight rows | 78.54 | Lowest |
Round holes on 60° staggered centres are the most common pattern because the staggering distributes stress more evenly than straight rows and keeps the remaining metal continuous in every direction.
The bar width is the limit
The bar — sometimes called the web or ligament — is the metal left between two holes, and it is simply the centre spacing minus the hole size. As you chase higher open area the bar gets thinner, and below a certain point the sheet becomes difficult to punch without distortion and loses meaningful strength.
As a general rule of thumb, the bar should not be thinner than the material gauge, and a very thin bar relative to the hole is worth a conversation before you order. Tell us the job and we will say whether the pattern is practical in the material you want.
Notes and limits
This calculator gives the theoretical open area of an ideal pattern. Real sheets have margins — unperforated borders at the edges — so the open area across a whole sheet is always slightly lower than the figure here. It also assumes the pattern runs uniformly, which is not the case for sheets perforated to a drawing.
Figures are for planning and comparison, not a certified specification. For anything where open area is a compliance or engineering requirement, confirm the finished part against the drawing.
Working in woven mesh rather than perforated sheet? Use the wire mesh opening and open area calculator instead — the maths is different. For a fuller walkthrough of the notation, see how to read a perforated metal spec, and expanded metal vs. perforated metal if you are still choosing between the two.
Know the pattern you need?
We stock round, square and slotted perforated sheet and cut to size in Hayward, CA.