Floor plan scale calculator
Ruler on the drawing, tape in the room — convert either direction at any standard scale, or work out what scale an unmarked plan was printed at.
Import the drawing as an underlay, calibrate it against one known dimension, and every measurement after that is exact — no ruler, no scale arithmetic.
The arithmetic
real = paper × N and paper = real ÷ N at scale 1:N — same units
both sides. The default example: 87 mm measured on a 1:50 plan is 87 × 50 = 4350 mm, or
4.35 m built. Going the other way, a 4.35 m
wall on a 1:100 sheet is 4350 ÷ 100 = 43.5 mm of ink.
Finding an unknown scale is the same equation solved for N:
N = real ÷ paper. A known 4.2 m wall that measures 84 mm gives
4200 ÷ 84 = 50 — the drawing is 1:50. Gridsnap's
trace workflow does this calibration for you, once,
against the image itself.
Common questions
Related: editing an existing floor plan and converting a plan image to DXF — both start by calibrating a scale; how to read a floor plan for everything else on the sheet.
What does a scale like 1:50 actually mean?
One unit on paper is fifty of the same unit in reality: 1 mm on the drawing is 50 mm built, 10 mm is half a metre. That is the whole system — multiply a paper measurement by the scale number for reality, divide a real dimension by it for paper.
Which scale is used for what?
1:20 and 1:25 for construction details and joinery; 1:50 for room-level floor plans, the workhorse of residential drawings; 1:100 for a whole floor or small building on one sheet; 1:200 and 1:500 for site plans; 1:1000 and up for location plans. Gridsnap's PDF export offers standard scales from 1:20 to 1:2000 and prints a scale bar on the sheet.
How do I find the scale of a drawing that doesn't state one?
Use the "find the scale" mode: pick one dimension you know in reality — a written dimension on the drawing, a standard door opening (900 mm), a bath (1700 mm) — measure it on paper with a ruler, and the calculator gives the ratio and snaps it to the nearest standard scale. If the exact ratio lands far from any standard, the print was probably resized ("fit to page" is the usual culprit) and no ruler measurement on it can be trusted.
My PDF was printed "fit to page" — can I still use it?
Not with a ruler: fit-to-page shrinks the drawing by an arbitrary factor, so the stated scale is wrong and even the found scale will be a non-standard number. You can still recover the geometry by tracing: import the image into the Gridsnap editor, calibrate against one known dimension, and draw over it — the calibration replaces the broken paper scale entirely. See converting a floor plan image to DXF.
Why measure in millimetres on paper but metres in reality?
Because that is what each ruler reads: a desk ruler is graduated in millimetres, and building dimensions are quoted in metres or millimetres depending on size. The calculator shows real results both ways. Construction drawings themselves are dimensioned in millimetres throughout — a 4.35 m wall is written 4350.
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Skip the ruler entirely
Trace the drawing once in the editor and export it back out at any standard scale — 1:20 to 1:2000, true vector, with a scale bar on the sheet.