A drawing shown on a screen has no fixed size, so its dimension figures are the only reliable source of measurement. This lesson shows you how to read a size from the figure and how to find a missing one.
It sits in drawing conventions and scale. Read interpreting line types first if dimension lines still look like ordinary lines.
Why does the screen size not matter?
Your phone, laptop and a printed sheet all show the same drawing at different sizes. The dimension figure does not change, because it states the real size of the object in millimetres.
So the rule is simple: read the figure, not the picture. The picture may be stretched, shrunk or cropped without any warning.
How is a dimension read?
A dimension has three parts: two thin projection lines from the object, a thin dimension line between them with arrowheads, and a figure. The figure tells you the distance between the two projection lines.
Read each figure in the direction the drawing states. Horizontal sizes usually sit above the line and vertical sizes are read from the right, but follow the layout given in your question.
Worked example: a missing dimension
An original plate is drawn with a chain of three horizontal dimensions along the top: 25, a missing size marked x, and 18. A total dimension of 90 sits above the chain.
The parts must add to the total, so:
25 + x + 18 = 90
x = 90 − 25 − 18 = 47
Check: 25 + 47 + 18 = 90, which matches the total.
Now a second question on the same plate. A slot is 12 wide and is centred on the segment x. How far is each edge of the slot from each end of that segment? The space left is 47 − 12 = 35, and centring splits it equally, so each side is 35 ÷ 2 = 17.5 mm.
The mistake: measuring the picture
A student sees that the segment marked x looks about twice as long as the segment marked 25. They guess x is about 50 and write that down.
| Approach | Wrong | Right |
|---|---|---|
| Source of x | Ruler on the screen | Subtraction from the total |
| Result | About 50 | 47 |
| Risk | Changes with screen size | Same on every device |
Here the guess happened to land near the answer, but on another device the same method would give a different number. Only the subtraction is repeatable, and it earns the working marks.
Check yourself
A rod is shown with three horizontal dimensions in a chain: 35, y, and 28. The overall length is 110. Find y, then find the distance from the left end of the rod to the start of the 28 section.
Answer
35 + y + 28 = 110, so y = 110 − 35 − 28 = 47. Check: 35 + 47 + 28 = 110.
The distance from the left end to the start of the 28 section is 35 + 47 = 82. The last section then runs from 82 to 110, which is 28, as stated.
What to study next
Dimensions change meaning when the drawing is reduced or enlarged, so continue with converting between stated scales. Then test yourself with the cluster practice set.
If you want a teacher to check your reasoning on dimension chains, see online one-to-one Engineering Drawing tuition.