Floor Plan Scale Calculator

Work out the scale of a floor plan from one known dimension, read real room sizes off a plan, and convert a room's floor area between plan and reality. Useful when a plan arrives with no scale in the title block, or when it has been printed or scanned at an unknown size.

On this page: Find the scale · Room size · Floor area · Plan scales · How to use · FAQ

Find the Scale of a Plan

Pick any feature whose real size you know — a door, a wall, a stated dimension — measure it on the plan, and the scale follows.

Measured on the plan

Real size of that feature

A standard internal door leaf is about 0.9 m (2'-6" to 3'-0") and a brick course is 75 mm — both make useful references.

Room Size from the Plan

Once the scale is known, measure a room and read its real dimensions.

Plan scale

Measured — length width

Floor Area at Scale

Area does not scale the same way as length. At 1:50 every length is divided by 50, but every area is divided by 50 × 50 = 2500. Enter a real room and see both.

Real room — length width

Plan scale

Common Floor Plan Scales

Scale	Imperial equivalent	1 m in real life is	Typical use
1:20	—	50 mm	Kitchens, bathrooms, details
1:25	—	40 mm	Small room layouts
1:50	≈ 1/4" = 1'-0"	20 mm	Houses and apartments
1:100	≈ 1/8" = 1'-0"	10 mm	Whole-building plans
1:200	≈ 1/16" = 1'-0"	5 mm	Large or multi-storey buildings
1:500	—	2 mm	Site plans
1:1250	—	0.8 mm	Location plans

The imperial column is approximate. 1/4" = 1'-0" is exactly 1:48, not 1:50, and 1/8" = 1'-0" is 1:96, not 1:100. They look alike on a sheet but are about 4 % apart, so never treat them as interchangeable when dimensions matter.

How to Use the Floor Plan Scale Calculator

If the plan has no stated scale, start with the first calculator and a feature whose real size you know.
Round the result to the nearest standard scale. A plan that computes to 1:51.3 was almost certainly drawn at 1:50 and printed slightly off.
Use the second calculator to read room dimensions, and the third to see how a room's area behaves at that scale.

Written dimensions on a drawing always take priority over anything you measure. Measuring is for the dimensions that were never written down.

Formula

scale denominator = real length ÷ plan length

Both lengths must be in the same unit first. A door 18 mm wide on the plan that is really 0.9 m wide: 900 ÷ 18 = 50, so the plan is at 1:50.

real area = plan area × scale factor²

Floor Plan Scale Questions

How do I find the scale of a floor plan with no scale marked?

Measure something whose real size you know — a door, a window, a brick course, or any dimension written on the drawing — and divide the real length by the measured length, using the same unit for both. Round to the nearest standard scale.

What scale are most floor plans drawn at?

In metric countries, 1:50 for houses and apartments and 1:100 for whole buildings. In the United States, 1/4" = 1'-0" and 1/8" = 1'-0" fill the same roles.

What does 1:100 mean on a floor plan?

One unit on the plan equals one hundred of the same units in the building. One centimetre on the plan is one metre in reality, and one millimetre is 100 mm.

Why does my measurement disagree with the written dimension?

Almost always because the sheet was printed or scanned at the wrong size. "Fit to page" is the usual culprit. Check against a scale bar if the drawing has one, and trust the written dimension.

How do I calculate floor area from a plan?

Convert each measured length to its real value first, then multiply. Do not measure the area on the plan and multiply by the scale factor once — area scales by the factor squared, so at 1:50 you would need to multiply by 2500.