What a scale of 1 : n means
One unit on the plan stands for n of the same unit in real life. At 1 : 100, one centimetre on the plan is 100 centimetres, or one metre, in real life.
The units must match before converting, which is why the calculator asks which unit you measured in and reports the answer in metres.
Worked example
A wall measures 5 cm on a 1 : 100 plan. One centimetre stands for one metre, so the real wall is 5 m long.
Going the other way, a 5 m wall on a 1 : 100 plan is drawn 5 / 100 = 0.05 m, which is 5 cm on the paper.
Choosing a scale
A larger number after the 1 shows a smaller drawing. 1 : 1000 fits a whole site on a page, while 1 : 20 shows a detail.
Architectural drawings often use 1 : 50 or 1 : 100, model railways use 1 : 76 or 1 : 87, and road maps use scales in the tens of thousands.
Inputs for the scale calculator
Which way are you converting?
Choose whether you measured on the map or model, or on the real thing.
Scale (1 : n)
The number after the 1. For a 1 : 100 scale, type 100.
Measurement
The length you measured on the drawing or on the real object.
Unit you measured in
The unit on your ruler or plan. The answer is also given in metres so both directions can be compared.
CALCULATOR QUESTIONS
Scale calculator questions
How do I use a 1 : 100 scale?
Multiply the measured length by 100 in the same unit. 5 cm on the plan is 500 cm, or 5 m, in real life.
How do I convert a real length to a drawing length?
Divide by the scale number. A 5 m wall at 1 : 100 is drawn 0.05 m, or 5 cm, long.
Does the unit I choose change the answer?
No. The measurement is converted to metres first, so 5 cm and 0.05 m give the same real length.
What is the difference between scale and scale factor?
The scale is the ratio written as 1 : n. The scale factor is the number n you multiply by.