Perforation gauge.

Answer

Perforation gauge = (holes - 1) x 20 mm / measured length in mm. Twelve holes spanning 19 mm gives 11.58, which is closest to 11.5.

Worked example

12 holes, centre of first to centre of last 19 mm. That span holds 12 - 1 = 11 spacings, so the gauge is 11 x 20 / 19 = 11.58 perfs per 2 cm. Nearest half gauge 11.5, nearest quarter 11.50. Hole spacing is 1.73 mm.

How it works

Perforation gauge is the number of holes in a 2 cm span. If n holes are counted from the centre of the first to the centre of the last, the span contains n - 1 spacings, so gauge = (n - 1) x 20 / length in mm. The spacing between holes is the length divided by n - 1.

Tips. Measure along a straight edge, over as many holes as practical to average out small errors, and measure both pairs of edges: many stamps have different gauges horizontally and vertically (a compound perforation). Catalogues state the gauges to a half or a quarter, and they differ on which edge to list first, so check your catalogue's convention. A printed perforation gauge is quicker, and this tool is for when you do not have one.

Sources and basis (checked 2026-10-05): Wikipedia, Postage stamp separation (gauge = holes in a 2 cm span, measured with a perforation gauge).

FAQ

What does perf 12 mean on a stamp?

There are 12 perforation holes in every 2 cm along the edge.

How do I measure perforations without a gauge?

Count holes along an edge, measure centre to centre from the first to the last hole, and use gauge = (holes - 1) x 20 / length in mm.

What is a compound perforation?

A stamp whose horizontal and vertical edges have different gauges, such as 12 x 11.5. Measure both pairs of edges.

Why is my result a fraction like 11.58?

Real stamps are never exactly on a standard gauge. Round to the nearest half or quarter to match the catalogue.

Does it matter how many holes I count?

More is better. A longer span averages out measurement error, so count at least 10 holes when the stamp allows.