Architectural Scale Calculator

Convert measurements on a drawing to real size, and real size back to the drawing, using architect's scales written in imperial notation such as 1/4" = 1'-0", engineer's scales such as 1" = 20', or metric ratios such as 1:50. Results are given in feet and inches, decimal feet and metric units at the same time.

On this page: Drawing to real · Real to drawing · Scale chart · How to use · Formula · FAQ

Drawing Measurement to Real Size

Real Size to Drawing Measurement

Architect's and Engineer's Scale Chart

Every imperial drawing scale has an equivalent ratio. These are the values the calculator uses.

Architect's scales (imperial)

Engineer's scales (imperial)

Metric drawing scales

Metric practice writes the ratio directly: 1:50 and 1:100 are the usual plan scales, 1:20 and 1:10 are used for details, and 1:200 to 1:1000 for site plans. The nearest metric equivalent of 1/4" = 1'-0" is 1:50, and 1/8" = 1'-0" is close to 1:100 — close, but not identical, so never mix the two on one drawing.

How to Use the Architectural Scale Calculator

1. Pick the drawing scale from the list. Imperial, engineer's and metric scales are grouped together.
2. Measure the distance on the drawing with a ruler and enter it, choosing the matching unit.
3. Read the real-world size in feet and inches, decimal feet, and metric at the same time.
4. To go the other way, use the second calculator: enter the real size and read the length to draw.

If the drawing has been printed at anything other than 100 %, the ratio no longer holds. Check the printed length of the drawing's own scale bar before measuring, or measure a dimension you already know.

Architectural Scale Formula

An imperial scale converts to a ratio by expressing both sides in the same unit. For 1/4" = 1'-0", one foot is 12 inches, so:

scale factor = 12 ÷ 0.25 = 48, giving 1:48

For an engineer's scale such as 1" = 20', convert the feet to inches:

scale factor = (20 × 12) ÷ 1 = 240, giving 1:240

The measurements then follow the usual relationship:

real size = drawing measurement × scale factor

drawing measurement = real size ÷ scale factor

Example: 3 1/2 inches measured on a 1/4" = 1'-0" plan. 3.5 × 48 = 168 inches, which is 14'-0".

Architectural Scale Questions

What is 1/4 inch scale?

1/4" = 1'-0" means a quarter of an inch on the drawing represents one foot in reality. Its ratio is 1:48, because one foot is 48 quarter-inches. It is the standard scale for residential floor plans in the United States.

What is the ratio for 1/8 inch scale?

1/8" = 1'-0" is 1:96. It is used when a plan is too large to fit a sheet at 1/4" scale, typically for commercial buildings and overall floor plans.

What is the difference between an architect's scale and an engineer's scale?

An architect's scale divides the inch into fractions such as 1/4 and 1/8 and is used for buildings. An engineer's scale divides the inch into decimal parts, so one inch equals a round number of feet such as 20 or 50, and is used for site, civil and survey drawings.

What is 1/4 inch scale in metric?

1/4" = 1'-0" is exactly 1:48. The closest standard metric scale is 1:50, which is about 4 % smaller. They are near enough for a visual comparison but must not be treated as interchangeable when dimensions matter.

Why does my measurement not match the drawing's dimension?

The most common cause is printing. A plan printed with "fit to page" or at a reduced percentage is no longer at its stated scale. Written dimensions always take priority over a measured distance.

How do I convert a scale ratio into feet and inches?

Multiply the measured length in inches by the scale factor, then divide by 12 to get feet. The remainder is the inches. The calculator above does this and rounds to the nearest 1/16 inch.

Working with models or maps instead? Use the scale converter and scale factor calculator, the Map Scale Calculator, or the on-screen Scale Ruler.