How to use the scale converter
Architectural drawings are never shown at full size. A floor plan, section or site plan is shrunk by a fixed ratio so a whole building fits on one sheet, and that ratio is the drawing's scale. This converter does the arithmetic in both directions so you never have to reach for a scale ruler or a calculator mid-task.
- Pick the direction. Choose Drawing → real size if you measured something on a plan and want to know how big it is in reality. Choose Real size → drawing if you know the real dimension and need to know how long to draw it.
- Pick the scale. Metric ratios such as 1:50 and 1:100, architect's imperial scales such as 1/4" = 1'-0", and engineer's scales such as 1" = 20' are all listed. Use Custom for anything unusual, such as 1:75 or a scale you've worked out yourself.
- Enter the length and its unit. The results update as you type and are shown in millimetres, centimetres, metres, inches and feet-and-inches at once, so you can mix metric and imperial freely.
The formula behind architectural scale
Every scale can be written as a ratio 1:n, where n is the scale factor. Converting is a single multiplication or division:
Real length = Drawing length × n
Drawing length = Real length ÷ n
Imperial architect's scales look different but work the same way. To find n for a scale written as x" = 1'-0", divide 12 inches by x. For 1/4" = 1'-0", that is 12 ÷ 0.25 = 48, so the scale is 1:48. For engineer's scales written as 1" = y', multiply y by 12: 1" = 20' is 1:240.
Common scales at a glance
| Scale | Ratio | Typical use |
|---|---|---|
| 1:5 / 1:10 | 1:5, 1:10 | Joinery and construction details |
| 1:20 | 1:20 | Kitchen, bathroom and interior elevations |
| 1:50 | 1:50 | Detailed floor plans and sections |
| 1:100 | 1:100 | General arrangement plans, elevations |
| 1:200 – 1:500 | 1:200 – 1:500 | Site and block plans |
| 1/4" = 1'-0" | 1:48 | US residential floor plans |
| 1/8" = 1'-0" | 1:96 | US commercial plans |
| 1" = 20' | 1:240 | US site plans |
Worked examples
Metric plan to real size
You measure a bedroom wall on a 1:50 plan at 84 mm. Multiply by 50 and you get 4,200 mm, so the wall is 4.2 m long. On a 1:100 plan the same wall would measure 42 mm.
Imperial plan to real size
A living room on a 1/4" = 1'-0" plan measures 3 1/2 inches across. Each quarter inch is one foot, so 3.5 inches is 14 quarter inches, or 14 feet. Using the formula: 3.5 in × 48 = 168 in = 14'-0".
Real size to drawing
You need to draw a 900 mm wide door on a 1:20 interior elevation. Divide by 20: the door is drawn 45 mm wide. At 1:100 it would be just 9 mm, which is why door swings on general arrangement plans look so small.
Who this converter helps
- Architects checking dimensions on consultants' drawings, marking up printed sets on site, or setting up sheet layouts at a new scale.
- Architecture students building physical models, where a 1:200 massing model or a 1:50 sectional model needs every dimension converted before cutting.
- Interior designers who receive plans from architects or developers and need to confirm whether a sofa, island or bed will fit before specifying it. When the layout works, you can test finishes on a photo of the real room with AI Room Redesign.
- Builders and contractors reading real sizes off plans when a dimension string is missing, while remembering that written dimensions always take precedence over scaled ones.
- Homeowners trying to make sense of the drawings for an extension, renovation or new build, and checking room sizes against the furniture they already own.
Tips for accurate scaling
- Check the sheet size. A drawing marked "1:100 @ A1" printed on A3 is really at 1:200. Always confirm the paper size printed in the title block matches the sheet in your hand.
- Calibrate first. Measure a dimension that's already written on the drawing, convert it, and compare. If it doesn't match, the print has been scaled.
- Never scale for setting out. Construction should be built from written dimensions; scaling is for checking and estimating.
- Pair it with area. Once you know real lengths, the floor area ratio calculator tells you how much floor area your site allows, and the stair calculator checks whether a stair fits the floor-to-floor height you've drawn.
