Perforation Measurement Guide
October 5, 2025
This guide explains how to interpret perforation measurements from the AI Stamp ID application, how to round them to traditional gauge values, and why small variations can occur due to paper aging or scanning tolerances.
Why Your Perforation Numbers Have Decimals
When AI Stamp ID analyzes a stamp, it measures the spacing between perforation holes in millimeters based on the size of the quarter reference. It then calculates the average distances of the perforations and divides that into 2mms for the final perforation density. Because this is a more detailed measurement within a digital image, you may see non-integer values such as 10.41 or 11.03 rather than whole numbers like 10 or 11.
Traditional perforation gauges, however, report results as whole numbers — 'Perforation 10' or 'Perforation 11' that represent the approximate number of holes per 2 cm. Therefore, a digital measurement like 10.41 simply reflects higher precision but maps back to the same familiar gauge scale.
How to Round Your Results
To convert your AI-measured perforation into the conventional gauge number, use the following table as a guide:
| AI Measurement Range (mm) | Rounded Gauge Value |
|---|---|
| 9.75 – 10.24 mm | Perf 10 |
| 10.25 – 10.74 mm | Perf 10½ |
| 10.75 – 11.24 mm | Perf 11 |
| 11.25 – 11.74 mm | Perf 11½ |
| 11.75 – 12.24 mm | Perf 12 |
In practice, round to the nearest 0.5 step (½-increment). For example:
- 10.41 → Perf 10½
- 11.03 → Perf 11
You can use either whole or half-steps, depending on how your reference catalog lists the issue.
Accounting for Paper Shrinkage
Over decades, paper fibers can shrink slightly due to humidity, temperature, or the original drying process of stamp paper. This shrinkage can make the printed design or perforation spacing smaller by about 0.5 mm across the measured area.
Because of this, two genuine examples of the same stamp might show slightly different readings—say 10.95 vs 11.10 mm. Both are correctly interpreted as Perforation 11.
If your measurement is close to a boundary (for example 10.48 mm), consider normal material variation before deciding between Perf 10½ and Perf 11.
Practical Guidance
- Always measure all perforation sides if possible; average them for best accuracy.
- Round to the nearest 0.5 increment to match catalog listings.
- Small deviations (≤ 0.1 mm) usually result from paper shrinkage or scanning tolerances and are normal.
Sources and Acknowledgments
The information in this guide was developed specifically for AI Stamp ID using a synthesis of philatelic measurement standards and general knowledge of image-based measurement practices. No copyrighted or proprietary content was copied or reproduced. The following sources and references informed the development of this document:
- General philatelic measurement conventions as defined by major cataloging systems, including Scott Catalogue, Stanley Gibbons, and Michel references.
- Industry-standard understanding of perforation measurement (holes per 2 centimeters) used in physical gauges and collector tools.
- Common rounding and tolerance practices in digital measurement and computer vision analysis, applied to perforation spacing in millimeters.
- Conservation literature noting that paper fibers can shrink over time due to humidity and temperature changes, leading to small dimensional variations (e.g., National Postal Museum, Smithsonian guidance).
- Original interpretation and adaptation by AI Stamp ID for translating physical gauge standards into AI-generated measurement outputs.
This guide is an original work prepared for the AI Stamp ID platform and is intended to help users interpret automated perforation measurements in line with traditional philatelic practice.