Architectural scale calculator
Convert a measurement on a scaled drawing into the real-world dimension it represents, or work backwards from real dimensions to what you will draw on the sheet. Works at any ratio from 1:1 (full size) to 1:2500 (rural mapping).
How to use this calculator
Pick the scale of your drawing (or chart) from the presets below or type it directly into the scale field. Choose whether you want to go from drawing to real dimensions or the other way around, then enter the measurement and press Convert. Output is in millimetres, centimetres, metres, inches and feet so you can move between the metric and imperial conventions without a second tool.
The calculator assumes a printed-at-1:1 drawing on calibrated paper. If your printer silently shrank the sheet by 4 percent (a common default on consumer drivers), the conversion will be off by the same 4 percent. The how-to-print guide covers the calibration check that fixes this in under a minute.
Common architectural and engineering scales
| Scale | 1 mm on paper = | Typical use | Try it |
|---|---|---|---|
| 1:1 | 1 mm | Full size, no reduction. | 10 mm → |
| 1:2 | 2 mm | Detail of furniture and fittings. | 10 mm → |
| 1:5 | 5 mm | Joinery and construction details. | 10 mm → |
| 1:10 | 10 mm | Stair, balustrade, window details. | 10 mm → |
| 1:20 | 20 mm | Detail plans and elevations. | 10 mm → |
| 1:50 | 50 mm | Working plans for residential and small commercial. | 10 mm → |
| 1:100 | 100 mm | General arrangement plans for housing and offices. | 10 mm → |
| 1:200 | 200 mm | Whole-building plans for larger projects. | 10 mm → |
| 1:500 | 500 mm | Site plans and small urban blocks. | 10 mm → |
| 1:1000 | 1.00 m | Master plans and large site work. | 10 mm → |
| 1:1250 | 1.25 m | UK ordnance-survey urban base mapping. | 10 mm → |
| 1:2500 | 2.50 m | Ordnance-survey rural base mapping. | 10 mm → |
About the maths
A scale ratio of 1:n means one unit on paper represents n units in reality. So a 45 mm line on a 1:100 plan is 4 500 mm (4.5 m) in the world. Going the other way, a 4 500 mm wall on a 1:100 plan is 45 mm on paper. The conversion is just multiplication or division by the scale denominator. There is no trick, no rounding, no metric-imperial conversion factor inside the ratio. The reason a scale ruler exists is purely ergonomic: it saves you doing the arithmetic for every measurement.
Where to go from here
Once you have your scale fixed, pick the right printable paper for it. The architectural scale guide covers when each ratio is used in practice; the scale grid catalogue has pre-scaled paper for every common ratio so you can sketch directly on it without doing the conversion by hand.