Docuex Tools

Free Online Scale Calculator

Three calculators in one: convert drawing measurements to real-world dimensions, work the other way round, or enter both lengths and find the scale of an unknown drawing. Supports mm, cm, m and inches.

Real-world length
5.00m
50 mm × 100 = 5,000 mm
How scale works. A scale of 1:100 means every 1 unit on the drawing represents 100 units in the real world. So 50 mm on a 1:100 plan is 5,000 mm (5 m) in reality. Common architectural scales: 1:50, 1:100, 1:200 for floor plans; 1:1000, 1:1250, 1:2500 for site and OS plans.

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About the scale calculator

A scale calculator is the quickest way to translate measurements on a drawing into real-world dimensions, and the other way round. Architects working with 1:50 floor plans, engineers on 1:1250 site plans, surveyors handling unmarked drawings, the maths is the same and this tool does it in one click.

Docuex's free scale calculator gives you three modes in one: convert a drawing measurement to a real dimension, convert a real dimension to what it should measure on a drawing, or enter both values and let the calculator tell you the drawing's scale. It works in millimetres, centimetres, metres and inches, so it covers metric and imperial drawings equally well.

Three calculators in one tool

Common drawing scales at a glance

ScaleTypical use1 mm on drawing =
1:20Construction details, joinery20 mm (2 cm)
1:50Floor plans, sections, room layouts50 mm (5 cm)
1:100Building plans, large room layouts100 mm (10 cm)
1:200Multi-storey building plans, site context200 mm (20 cm)
1:500Site plans, small developments500 mm (50 cm)
1:1000Large site plans, master plans1 m
1:1250UK Ordnance Survey location plans1.25 m
1:2500UK OS block plans, urban context2.5 m

How to find the scale of an unknown drawing

If you've got an unmarked PDF or scanned drawing and need to know its scale, switch the calculator to Find the Scale. Measure any feature on the drawing whose real-world size you know, a door (usually 760, 838 or 900 mm in the UK), a standard parking bay (2.4 × 4.8 m), a room you've been in, anything reliable. Enter the measurement and the real length and the calculator returns the scale ratio. If the result is close to a standard scale (within 3%), it tells you which one, because in practice, a calculator output of "1 : 103" almost certainly means the original was drawn at 1:100.

Frequently asked questions

How do I use a scale calculator?

Pick a mode at the top of the tool. Measured → Real-world: enter the length you read off the drawing plus the scale, get the real dimension. Real-world → Measured: enter the real length plus the scale, get what it should measure on the drawing. Find the Scale: enter both lengths, get the scale.

What does a 1:100 scale mean?

A 1:100 scale means 1 unit on the drawing represents 100 units in reality. So 50 mm on the drawing is 5,000 mm (5 m) in the actual building. A 1:50 plan is half as zoomed-out, a 1:200 plan is twice as zoomed-out.

How do I find the scale of a drawing that isn't marked?

Switch to the Find the Scale mode. Measure any element on the drawing whose real-world size you know, a door, a parking space, a window. Enter the measured length and the known real length. The calculator gives you the exact ratio, and if it's near a standard scale (1:50, 1:100, 1:200, etc.) it tells you which one.

What are the most common architectural drawing scales?

In the UK and most of Europe: 1:50 and 1:100 for floor plans, 1:20 for construction details, 1:200 or 1:500 for site plans, 1:1250 and 1:2500 for Ordnance Survey location plans. The US uses imperial scales such as 1/4" = 1' (≈ 1:48) and 1/8" = 1' (≈ 1:96).

What's the difference between architectural and engineering scales?

Architectural scales (1:50, 1:100, 1:200) are tuned for building drawings, they make rooms and details readable. Engineering scales (1:500, 1:1000, 1:2500, 1:5000) are for site plans, civil engineering and surveys, where you need to fit a large area onto one sheet.

Can I use the calculator for imperial scales (inches)?

Yes. Pick inches as the unit. The calculator handles imperial input identically to metric, so 1/4" = 1' or 1/8" = 1' style scales work fine, just enter inches throughout.

Why does my result come out as 1:103 instead of 1:100?

Because your measurement or the assumed real-world length is slightly off the exact ratio. Drawings rarely measure perfectly because of printing tolerance, paper stretch or measurement error. The calculator suggests the closest standard scale when you're within 3%, that's usually the drawing's actual intended scale.

Is the calculator really free?

Yes. No sign-up, no usage limit, no ads. It runs entirely in your browser; we don't store any input. Why? Because most visitors who use our tools end up needing CAD conversion or large format scanning later, we'd rather help you now and earn the work then.

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