Architectural and Engineering Scale Factors
A scale written 1/4" = 1'-0" means a quarter inch on the sheet stands for a foot of building. Its factor is 12 divided by the drawing inches, so 12 ÷ 0.25 = 48, written 1:48. The metric scale sitting beside it in most title blocks is 1:50, and 1:50 is not 1:48. They are 4.17% apart.
Worked example
1/4" = 1'-0", wall measures 5 inches on the sheet. Factor is 48, so 5 × 48 = 240 inches = 20'-0", which is 240 × 25.4 = 6096 mm. The same 5 inches (127 mm) on a genuine 1:50 sheet gives 127 × 50 = 6350 mm. That is 254 mm of difference on one wall, out of a label that looked close enough.Conversion reference
| Architectural scale | Factor and nearest metric |
|---|---|
| 3" = 1'-0" | 1:4 · nearest 1:5, 25% off |
| 1 1/2" = 1'-0" | 1:8 · nearest 1:10, 25% off |
| 1" = 1'-0" | 1:12 · nearest 1:10, 16.7% off |
| 3/4" = 1'-0" | 1:16 · nearest 1:20, 25% off |
| 1/2" = 1'-0" | 1:24 · nearest 1:25, 4.17% off |
| 3/8" = 1'-0" | 1:32 · nearest 1:25, 21.9% off |
| 1/4" = 1'-0" | 1:48 · nearest 1:50, 4.17% off |
| 3/16" = 1'-0" | 1:64 · nearest 1:50, 21.9% off |
| 1/8" = 1'-0" | 1:96 · nearest 1:100, 4.17% off |
| 3/32" = 1'-0" | 1:128 · nearest 1:100, 21.9% off |
| 1/16" = 1'-0" | 1:192 · nearest 1:200, 4.17% off |
| 1/32" = 1'-0" | 1:384 · nearest 1:500, 30.2% off |
| 1/64" = 1'-0" | 1:768 · nearest 1:1000, 30.2% off |
| 1/128" = 1'-0" | 1:1536 · nearest 1:1250, 18.6% off |
Engineering scales
| Engineering scale | Factor and nearest metric |
|---|---|
| 1" = 10' | 1:120 · nearest 1:100, 16.7% off |
| 1" = 20' | 1:240 · nearest 1:250, 4.17% off |
| 1" = 30' | 1:360 · nearest 1:250, 30.6% off |
| 1" = 40' | 1:480 · nearest 1:500, 4.17% off |
| 1" = 50' | 1:600 · nearest 1:500, 16.7% off |
| 1" = 60' | 1:720 · nearest 1:500, 30.6% off |
| 1" = 100' | 1:1200 · nearest 1:1250, 4.17% off |
Why 4.17% keeps showing up
Twelve inches in a foot, ten of everything in metric. Imperial factors land on multiples of 12 and metric ones on multiples of 10, so every pair that ends up close together reduces to 24:25. 48 and 50. 96 and 100. 240 and 250. 1200 and 1250. Same ratio each time, 4.1667%. On a 30 m building that is 1.25 m of drift, which nobody spots on a plan because no individual dimension looks wrong.
The constants are exact
One inch is 25.4 mm. Not approximately, exactly, and it has been that way since the 1959 international yard and pound agreement pinned the yard at 0.9144 m. A foot is 304.8 mm and a square foot is 0.09290304 m². Worth saying out loud, because it means length conversion never introduces error on its own. If a converted set is wrong, somebody rounded on purpose.
One foot on paper
| Scale | One foot measures |
|---|---|
| 1/4" = 1'-0" (1:48) | 6.35 mm |
| 1:50 | 6.096 mm |
| 1/8" = 1'-0" (1:96) | 3.175 mm |
| 1:100 | 3.048 mm |
None of this holds if the sheet was printed at anything other than 100%. Fit to page, shrink oversized pages, an ARCH D sheet forced onto A3, all of them change the ratio and none of them tell you. That is what the graphic scale bar is for, so check the bar with a rule before trusting a measurement taken off paper.
Converting the dimensions themselves rather than the scale? See feet and fractional inches to mm, or open the calculator, which takes feet and fractional inches as typed. The tables on this page are generated from conversion-tables, our open dataset of conversion and scale factors.
Frequently asked
- What is the scale factor for 1/4 inch equals one foot?
- 48. A quarter inch on the drawing represents a foot of building, and 12 divided by 0.25 is 48, so the building is 48 times the drawing. Written 1:48.
- Is 1/4" = 1'-0" the same as 1:50?
- No. It is 1:48. 1:50 is the closest standard metric scale and it is 4.17% smaller, so the two should never carry the same label on a title block. On a 30 m building that gap is about 1.25 m.
- What is the metric equivalent of 1/8" = 1'-0"?
- Exactly 1:96. The metric scale used in its place is 1:100, which is the same 4.17% apart. The gap repeats across the close pairs (1:24 and 1:25, 1:48 and 1:50, 1:192 and 1:200) because imperial factors are multiples of 12 and metric ones multiples of 10, so every near pair reduces to 24:25.
- How do I convert an imperial drawing to a metric scale?
- Decide first whether you are keeping the geometry and restating the numbers in millimetres, or changing dimensions to metric-friendly values. Those are different buildings. Length conversion itself is exact, since one inch is exactly 25.4 mm, so any error in a converted set came from a rounding decision somebody made on purpose.
- Why is one inch exactly 25.4 mm?
- The 1959 international yard and pound agreement defined the yard as exactly 0.9144 m. Divide by 36 and the inch is exactly 0.0254 m. It is a definition rather than a measurement, which is why it carries no error.