Free tool
Drawing scale calculator
Three quick calculations for construction drawings: the true scale of a resized print, the scale implied by a known dimension, and converting between paper and real lengths. Runs in your browser; nothing is sent anywhere.
Calculator
A drawing made at one paper size and printed or saved at another. What scale is it really?
Measure a printed dimension on the sheet, with a ruler on paper or a 1:1 measurement in a PDF app.
Convert a length on the paper to a real length at a given scale, or back again.
How the calculator works
- Resized print: the true scale is the stated scale multiplied by the ratio of the long edges of the two paper sizes. A1 is 841 mm long and A3 is 420 mm, so a 1:100 A1 drawing on A3 is 1:100 × 841 ÷ 420, about 1:200.
- Scale from a dimension: the real length divided by the measured length, both in millimetres.
- Area error: if you measure at the wrong scale, lengths are off by the ratio of the two scales and areas by that ratio squared. That is why a resized sheet can make areas four times too big or small.
In Project Blueprint you don't need to work this out by hand. Measure › Calibrate sets the scale from a known dimension, Measure › Verify Scale checks it against a printed figure, and each sheet keeps its own scale. Read the full guide to drawing scale.
Frequently asked questions
What scale is an A1 drawing printed on A3?
About half the stated scale. A 1:100 drawing at A1 printed on A3 is about 1:200, because A3 is half the length and width of A1. Lengths measured at 1:100 come out half size, and areas a quarter.
How do I find the scale from a dimension?
Measure the dimension on the page, then divide the real length by the measured length in the same units. If a 6,000 mm dimension measures 60 mm on the print, the scale is 6,000 ÷ 60, which is 1:100.
Why doesn't my result match a standard scale exactly?
Small differences come from measuring precision and from A-series paper sizes not being exact multiples of each other. If the result is within about one per cent of a standard scale, use the standard scale. If it is further off, the sheet may be stretched or resized to an odd size.