North American Architectural (ARCH) Series

The ARCH Series: North American Architectural Drawing Sizes

While the ISO 216 standard dominates the global market, the ARCH series is the primary standard used by architects and engineers in the United States and Canada. Unlike the metric-based A series, the ARCH series is defined in inches and is specifically tailored to the requirements of large-scale construction and design blueprints.

Main Characteristic

The defining characteristic of the ARCH series is its alternating aspect ratios. Unlike the ISO "A" series, which maintains a single constant ratio, ARCH sizes alternate between a 4:3 ratio (ARCH A, C, and E) and a 3:2 ratio (ARCH B and D). This structure is designed so that proportions remain perfect when scaling by a factor of two. For example, a "full-size" ARCH D drawing (24 x 36) can be reduced to a "half-size" ARCH B sheet (12 x 18) without any cropping or distortion, as both share the same 3:2 geometry.

The Measurement System

The Starting Point: The series is built around the ARCH A size (9 x 12 inches). This serves as the basic unit, often used for small sketches or detail callouts.

The Scaling: Each size in the series doubles the surface area of the preceding one. This is achieved by doubling the shorter dimension of the sheet. For example, if you place two ARCH A sheets side-by-side along their 12-inch sides, you get the dimensions of one ARCH B sheet (12 x 18 inches). This pattern continues throughout the series.

Common Usage: The industry workhorse is ARCH D (24 x 36). This is the "gold standard" for architectural blueprints, providing enough surface area for detailed floor plans and elevations while remaining manageable on a job site.