How to use the architectural scale calculator
A drawing scale tells you how many real-world units one unit on paper stands for. To convert, the calculator turns every scale into a single number — the scale factor — and multiplies or divides by it. Choose a direction (real to drawing, or drawing to real), pick the scale from the list, type the length and choose units. The result updates as you type, and the panel on the right also shows what one centimetre and one inch on paper represent at that scale.
Metric scales are already ratios: 1:100 has a factor of 100. Imperial architectural scales are written as "so many inches on paper equals one foot". Because one foot is 12 inches, the factor is 12 divided by the paper inches: 1/4" = 1'-0" gives 12 ÷ 0.25 = 48, so it is 1:48. Engineering scales compare inches to feet directly: 1" = 20' is 20 × 12 = 1:240.
real length = drawing length × scale factor
scale factor = real length ÷ measured drawing length (same units)
Worked example
A wall is 5 m long and the plan is drawn at 1:100. 5 m is 5000 mm, and 5000 ÷ 100 = 50 mm on paper. Going the other way, if you measure 37 mm on a 1:50 detail, the real length is 37 × 50 = 1850 mm, or 1.85 m.
For a US plan at 1/4" = 1'-0", a 10 ft room is 120 inches; 120 ÷ 48 = 2.5 inches on the sheet. To find an unknown scale, say a 12 m façade measures 120 mm on a print: 12 000 ÷ 120 = 100, so the drawing is 1:100. If you get something like 1:97, the print was probably slightly reduced — check that it was printed at 100%.
Common architectural scales and what they are used for
| Scale | Ratio | 1 cm on paper | Typical use |
|---|---|---|---|
| 1:5000 – 1:1000 | 5000 – 1000 | 50 – 10 m | Location and master plans |
| 1:500 / 1:200 | 500 / 200 | 5 m / 2 m | Site plans, block plans |
| 1:100 | 100 | 1 m | Floor plans, sections, elevations |
| 1:50 | 50 | 50 cm | Detailed plans, apartments, interiors |
| 1:20 / 1:10 / 1:5 | 20 / 10 / 5 | 20 / 10 / 5 cm | Construction details, joinery |
| 1/8" = 1'-0" | 96 | 96 cm | US floor plans of larger buildings |
| 1/4" = 1'-0" | 48 | 48 cm | US residential floor plans |
| 1/2" = 1'-0" | 24 | 24 cm | US enlarged plans, stairs |
| 1 1/2" = 1'-0" | 8 | 8 cm | US wall sections, details |
| 1" = 20' | 240 | 2.4 m | US civil and site plans |
Choosing a scale that fits the sheet
Before you set up a layout, check the building against the paper. Enter the overall length and width of the building and the calculator shows which ISO sheets (A4 to A0) the drawing fits on at the selected scale, in landscape with a small margin. Leave extra room for the title block, dimensions and annotation — in practice the drawing should use about 70–80% of the sheet. If nothing fits, step down to the next scale (for example from 1:100 to 1:200) or split the plan over two sheets with match lines.
If you work in Revit, the scale is a property of each view, and sheets print correctly only when the PDF is printed at actual size. A PDF that was printed "fit to page" no longer has a true scale — measure a known dimension and use the "find an unknown scale" box to see what it actually is.
Frequently asked questions
What does 1:100 scale mean?
At 1:100, one unit on the drawing equals 100 of the same units in reality. 1 cm on paper is 1 m on site, and 10 mm on paper is 1 m.
What is 1/4 inch = 1 foot as a ratio?
1/4" = 1'-0" is 1:48, because one foot is 12 inches and 12 divided by 0.25 is 48. 1/8" = 1'-0" is 1:96 and 1/2" = 1'-0" is 1:24.
How do I find the scale of a drawing?
Measure a dimension on the drawing that you know in reality, for example a door or a dimensioned wall. Divide the real length by the measured length, in the same units. The result is the scale factor; the calculator rounds it to the nearest standard scale.
Why does my printed PDF not match the scale?
Usually because the PDF was printed with Fit to page. Print at 100% / Actual size and on the paper size the sheet was made for. Then measure a known dimension to confirm.
Which scale should I use for floor plans?
In metric practice floor plans are usually 1:100 or 1:50, site plans 1:200 to 1:500 and details 1:20 to 1:5. In US practice plans are commonly 1/4" = 1'-0" or 1/8" = 1'-0".
Is this scale calculator free?
Yes. It runs entirely in your browser, needs no account and sends nothing to a server.