How do I calculate a 1:50 scale?

A scale of 1:50 is one divided by fifty, a factor of 0.02, so every model length is 0.02 times the real one. An actual 10 m at 1:50 is 20 cm on the model, which the page works as 10 m x 0.02 = 0.2 m, then 0.2 m converted into the unit you chose for the model. Set Work out to Model size, type the ratio, type the length you measured, and pick the unit for each side.

How do I find the real size from a model measurement?

Set Work out to Actual size and give the page the scale and the model measurement. It divides instead of multiplying: a 20 cm model at 1:50 is 0.2 m divided by 0.02, which is 10 m in the real thing. The steps under the answer show the division, so you can check it rather than trust it.

How do I work out what scale a model is?

Set Work out to The scale and give both lengths. The page divides the model length by the actual length, with both sides converted to metres first, and writes the answer as a ratio: an actual 10 m beside a 20 cm model comes back as 1:50. A scale that is not a tidy one is left as it falls, so a 1.83 m person modelled at 12 cm reads 1:15.25 rather than being rounded to a nearby number.

Can the two sides use different units?

Yes, and that is the point of the two unit lists. Each side offers millimetres, centimetres, metres, kilometres, inches, feet, yards and miles, chosen separately. A 14.8 ft car at 1:24 is 7.4 in on the model, and a 3.7 km stretch of road on a 1:25,000 map is 14.8 cm. Conversions use the international definitions, where 1 in is 0.0254 m and 1 mi is 1,609.344 m exactly.

What happens with an odd scale like 1:87.1?

It is used exactly as written. HO is 1:87.1, and an actual 20 m at that scale comes back as 229.621125144 mm, not the answer you would get from rounding the ratio to 1:87 first. The same holds for a ratio that is not in 1:n form at all: 3:4 stays 3:4, a factor of 0.75.

Does it handle enlargements and percentages?

Both. A ratio with the larger number first enlarges, so 2:1 doubles an actual 30 mm into 60 mm on the model. A percentage is read as a scale as well, the way a photocopier or a print dialog states it: 150% is a factor of 1.5, which turns 10 cm into 15 cm, and 50% is the same as 1:2.

Why does a scale written as a single number get refused?

Because 50 on its own could mean 1:50 or 50:1, and those are opposite answers: one shrinks a length fifty times, the other grows it fifty times. Rather than guess, the page asks you to write the half you mean first. A single number below one is not ambiguous, so 0.02 is accepted and shown as 1:50.

Does the page keep my numbers?

The last entries are remembered in this browser only, so the page opens where you left it, and the Start over button at the top of the page forgets them. Nothing is sent anywhere, the calculation runs in your browser, and there is no account or upload at any point.

Scale Calculator

Type the scale and the length you have, and this page gives you the other length, with the conversion written out underneath. What it does differently is keep the two sides independent: the actual size can be in metres while the model is in centimetres, or a car in feet while the model is in inches, so the unit you measured in is never the unit you are forced to read. Any one of the three quantities can be the unknown, so the same page turns an actual 10 m into 20 cm at 1:50, turns a 20 cm model back into 10 m, and reads an actual 10 m beside a 20 cm model as 1:50. The ratio is used exactly as typed, including 1:87.1 and 3:4, and is never snapped to the nearest common modelling scale. Nothing is uploaded and there is no account: the numbers stay in your browser.

Model length20 cmscale 1:50 · actual 10 m · factor 0.02

Scale: 1:50
Actual: 10 m
Model: 20 cm
Factor: 0.02

The scale and the size

Leave the page one quantity to find. Each side keeps its own unit, so a 1:50 building can be measured in metres and built in centimetres without a conversion in your head.

Work out

Scale ratioActual lengthActual unitModel unit

The scale and the real thing are known: this is the size to build or draw.

Scale 1:50 is 1 / 50 = 0.02, so the model is 0.02 times the actual.
10 m x 0.02 = 0.2 m on the model.
0.2 m / 0.01 = 20 cm.

Worked cases

Each one runs through the same function the fields above do.

1:50, actual 10 msolve the model
20 cm
2:1, actual 30 mmenlarging
60 mm
1:50, model 20 cmsolve the actual
10 m

Unit conversions use the international definitions: 1 in is 0.0254 m and 1 mi is 1,609.344 m exactly. The US survey foot is a slightly different foot and is not offered here.
Numbers are shown to at most 12 significant figures. A browser works in binary floating point, so a longer tail would be noise rather than precision.
A ratio says nothing about material. Kerf, shrinkage, wall thickness and the tolerance of your printer or kit are not in this number, and a cut piece will not match it to the last decimal.

Accuracy. Exact arithmetic: model length is actual length times the scale ratio, with each side converted from the unit you picked for it. Any one of scale, actual and model can be the unknown. The ratio is used exactly as you wrote it and is never rounded to the nearest common modelling scale.

Common questions

How do I calculate a 1:50 scale?: A scale of 1:50 is one divided by fifty, a factor of 0.02, so every model length is 0.02 times the real one. An actual 10 m at 1:50 is 20 cm on the model, which the page works as 10 m x 0.02 = 0.2 m, then 0.2 m converted into the unit you chose for the model. Set Work out to Model size, type the ratio, type the length you measured, and pick the unit for each side.
How do I find the real size from a model measurement?: Set Work out to Actual size and give the page the scale and the model measurement. It divides instead of multiplying: a 20 cm model at 1:50 is 0.2 m divided by 0.02, which is 10 m in the real thing. The steps under the answer show the division, so you can check it rather than trust it.
How do I work out what scale a model is?: Set Work out to The scale and give both lengths. The page divides the model length by the actual length, with both sides converted to metres first, and writes the answer as a ratio: an actual 10 m beside a 20 cm model comes back as 1:50. A scale that is not a tidy one is left as it falls, so a 1.83 m person modelled at 12 cm reads 1:15.25 rather than being rounded to a nearby number.
Can the two sides use different units?: Yes, and that is the point of the two unit lists. Each side offers millimetres, centimetres, metres, kilometres, inches, feet, yards and miles, chosen separately. A 14.8 ft car at 1:24 is 7.4 in on the model, and a 3.7 km stretch of road on a 1:25,000 map is 14.8 cm. Conversions use the international definitions, where 1 in is 0.0254 m and 1 mi is 1,609.344 m exactly.
What happens with an odd scale like 1:87.1?: It is used exactly as written. HO is 1:87.1, and an actual 20 m at that scale comes back as 229.621125144 mm, not the answer you would get from rounding the ratio to 1:87 first. The same holds for a ratio that is not in 1:n form at all: 3:4 stays 3:4, a factor of 0.75.
Does it handle enlargements and percentages?: Both. A ratio with the larger number first enlarges, so 2:1 doubles an actual 30 mm into 60 mm on the model. A percentage is read as a scale as well, the way a photocopier or a print dialog states it: 150% is a factor of 1.5, which turns 10 cm into 15 cm, and 50% is the same as 1:2.
Why does a scale written as a single number get refused?: Because 50 on its own could mean 1:50 or 50:1, and those are opposite answers: one shrinks a length fifty times, the other grows it fifty times. Rather than guess, the page asks you to write the half you mean first. A single number below one is not ambiguous, so 0.02 is accepted and shown as 1:50.
Does the page keep my numbers?: The last entries are remembered in this browser only, so the page opens where you left it, and the Start over button at the top of the page forgets them. Nothing is sent anywhere, the calculation runs in your browser, and there is no account or upload at any point.

Exact arithmetic: model length is actual length times the scale ratio, with each side converted from the unit you picked for it. Any one of scale, actual and model can be the unknown. The ratio is used exactly as you wrote it and is never rounded to the nearest common modelling scale.