How do I find a missing dimension on a floor plan?

Take the other dimensions in the same string away from the overall dimension. The dimensions in a string always add up to the overall.

How do I add feet and inches?

Turn everything into inches first (12'-6" is 150 in), do the sum, then turn the answer back: divide by 12 for feet, the rest is inches.

What are floor plan dimensions measured to?

It depends on the office: face of stud, center of wall or face of finish. The general notes say which. Always check before you set out.

Construction plans: lesson 2 of 6

Reading dimensions on a floor plan

Find the missing dimension in the string. Get 10 right to finish. Pick feet and inches or metric below.

What is the missing dimension?

How to read it

Plans carry strings of dimensions: an overall dimension on the outside and smaller ones inside it. The smaller ones add up to the overall, which is how you check a drawing and how you find a dimension that is not written.

9000 - 3600 - 2400 = 3000 mm

Build to the written dimensions, never by measuring the drawing. See why written dimensions win.

Common mistakes

Mixing inches and feet in a sum. Work in inches, then convert back.
Using a dimension from a different string. Only the dimensions on the same line add up to its overall.

Questions

How do I find a missing dimension on a floor plan?: Take the other dimensions in the same string away from the overall dimension. The dimensions in a string always add up to the overall.
How do I add feet and inches?: Turn everything into inches first (12'-6" is 150 in), do the sum, then turn the answer back: divide by 12 for feet, the rest is inches.
What are floor plan dimensions measured to?: It depends on the office: face of stud, center of wall or face of finish. The general notes say which. Always check before you set out.

Explained in our own words and checked against several textbooks and trade school resources. Office standards vary, so always check the legend on your drawing set.