BlueprintExercises
Scale reading: lesson 2 of 6

How to read an architect's scale

Read the blue line off the scale. Get 10 right to finish. The box below also lets you switch to a metric rule.

Done 0 of 10
3/86012345Scale 3/8" = 1'-0". Each small mark = 2 in.

The blue line is laid on the 3/8 edge of an architect's scale. How long is it at 3/8" = 1'-0"?

How to read it

The architect's scale is usually a three-sided ruler with 11 scales on it: 3/32, 3/16, 1/8, 1/4, 3/8, 1/2, 3/4, 1, 1 1/2 and 3 inches per foot, plus a plain ruler in sixteenths. Pick the edge that matches the scale on the drawing.

  1. Find the edge whose number matches the drawing, such as 1/4.
  2. Find zero. It is set in from the end. Past zero, toward the end, is one foot cut into inches.
  3. Put a whole foot mark on one end of the line, so the other end falls inside the subdivided foot.
  4. Read the feet at the foot mark. Then count inches in the subdivided foot, starting at zero.
1/46012345Scale 1/4" = 1'-0". Each small mark = 2 in.
The line ends on the 5 foot mark and runs 8 inches past zero: 5'-8".
1/260123Scale 1/2" = 1'-0". Each small mark = 1 in.
On the 1/2 edge each small mark is 1 inch. This line reads 3'-4".

Our on-screen scale says what each small mark is worth. Real scales vary by edge and by maker, so check yours: count the marks between zero and the end of the foot.

Common mistakes

  • Counting inches from the end of the scale instead of from zero. That gives 12 minus the right answer.
  • Reading the numbers of the other scale on the same edge, which run the other way.
  • Measuring a reduced copy. See written dimensions and reduced prints.

Questions

Why does an architect's scale not start at the end?
Zero is set in from the end so there is room for one foot cut into inches. Whole feet are counted on one side of zero and the inches on the other.
How many marks are in the subdivided foot?
It depends on the scale and the maker. Large scales such as 1" or 3/4" usually show every inch. Small scales such as 1/8" may only show every 2 inches. Count the marks between zero and the end of the foot before you read.
Why do two numbers run in opposite directions on one edge?
Each edge carries two scales, one twice the size of the other, such as 1/4 and 1/8. One is read from the left and the other from the right. Use the numbers that match the scale number at the end you started from.
What is the difference between an architect's scale and an engineer's scale?
An architect's scale is in inches per foot, for buildings. An engineer's scale is in feet per inch, such as 1" = 20', for site plans and civil work. See our common scales page for both.

Explained in our own words and checked against trade school textbooks. Metric scales follow the ISO 5455 series.