An architect's scale
is a specialized ruler. It is used in making or measuring from reduced scale drawings
, such as blueprints
and floor plans
. It is marked with a range of calibrated scales (ratios
For accuracy and longevity the material used should be dimensionally stable and durable. Scales were traditionally made of wood, but today they are usually made of rigid plastic or aluminum. Architect's scales may be flat, with 4 scales, or have a symmetric 3-lobed cross-section, with 6.
Architect's scale rulers used in Britain and other metric areas are marked with ratios without reference to a base unit. Therefore a drawing will indicate both its scale and the unit of measurement being used.
In Britain, the standard units used on architectural drawings are the (SI) units millimetres (mm) and metres (m), where as in France centimetres (cm) and metres are most often used.
In Britain, for flat rulers, the paired scales often found on architect's scales are:
For triangular rulers, the paired scales are:
United States and Imperial units
In the United States
, and prior to metrification
, architect's scales are/were marked as a ratio of x inches
(typically written as x"=1'-0"). For example one inch measured from a drawing with a scale of "one-inch-to-the-foot" is equivalent to one foot in the real world (a scale of 1:12) whereas one inch measured from a drawing with a scale of "two-inches-to-the-foot" is equivalent to six inches in the real world (a scale of 1:6). It is not to be confused with a true unitless ratio (inches to inches)-- a 1:50 architectural scale would be a 1:60 unitless scale.
Typical scales used in the United States are:
- Full scale, with inches divided into sixteenths of an inch
The following scales are generally grouped in pairs using the same dual-numbered index line:
- three-inches-to-the-foot (3"=1'-0") (ratio equivalent 1:4)/one-and-one-half-inch-to-the-foot (1-1/2"=1'-0") (1:8)
- one-inch-to-the-foot (1"=1'-0") (1:12)/one-half-inch-to-the-foot (1/2"=1'-0") (1:24)
- three-quarters-inch-to-the-foot (3/4"=1'-0") (1:16)/three-eighths-inch-to-the-foot (3/8"=1'-0") (1:32)
- one-quarter-inch-to-the-foot (1/4"=1'-0") (1:48)/one-eighth-inch-to-the-foot (1/8"=1'-0") (1:96)
- three-sixteenths-inch-to-the-foot (3/16"=1'-0") (1:64)/three-thirty-seconds-inch-to-the-foot (3/32"=1'0") (1:128)