Lesson 5.1
Dimensioning: Telling the Builder the Numbers
A drawing shows shape; dimensions give it size. Done well, dimensioning is invisible — the builder finds every number instantly. Done badly, it's a maze. Here's the grammar of getting it right.
Start hereFrom Module 2 you know the golden rule: figures govern. So a drawing lives or dies by its figures — the written dimensions a builder actually works from.
But where do they go? How many? Dimensioning has conventions as precise as grammar, and they're the same the world over.
01 — The anatomy
The parts of a dimension
A dimension isn't just a number — it's a small assembly of lines, each with a job. Get the anatomy right and dimensions read cleanly; get it wrong and they clutter the drawing. Tap each part to see it.
The extension line is a thin line projecting from the object's edge, with a small gap, showing what's being measured. The dimension line runs between the extension lines, carrying the measurement. The terminator is the arrowhead, tick or dot at each end — a 45° tick or slash is common in architecture. And the figure is the number itself, placed above (or in a gap in) the dimension line, reading left-to-right or bottom-to-top.
Notice all the dimension lines are thin — they're reference lines (Module 1, Tier 3). They must never compete with the building's cut and seen lines. The drawing is the star; dimensions are stage directions.
The part
Extension lines: thin lines projecting from the object's edges (with a small gap) to show exactly what's being measured. They don't touch the object.
Every part is a thin reference line — the figure is the only thing that governs.
02 — The rules of placement
Where dimensions go, and in what order
Good dimensioning follows a clear hierarchy, usually in layered strings outside the plan. Reading from the building outward: small things first, then the big total. Each string lines up so a builder can add the small dimensions and check they equal the overall — a built-in error check. This layering is universal practice.
| String | Carries | Position |
|---|---|---|
| Overall | Total width / length of the building | Outermost |
| Intermediate | Room-to-room, wall centres, grid | Middle |
| Detail | Openings, offsets, small features | Closest to the object |
03 — Hand and digital
The same grammar, two tools
By hand, you draw each dimension's lines and letter the figure carefully — slow but total control. In CAD, dimensions are usually associative: you pick two points and the software draws the whole assembly and reads the length automatically; if you later stretch the wall, the dimension updates itself.
This is a real safeguard — the figure can't drift out of sync with the geometry. But it only works if you dimensioned to the right points, so the thinking is identical. The tool automates the drafting, not the judgement.
A handful of conventions exist purely to stop ambiguity. Dimension to faces or centrelines consistently — never mix (is that 3000 to the wall face or its centre?). Don't dimension the same thing twice — redundant dimensions can disagree after an edit, and then which is right? Keep the figure clear of other lines, breaking the dimension line if needed. Never measure off the drawing to supply a missing figure — add the figure instead. Each rule traces back to one principle: the dimensions must be complete, consistent and unambiguous, because the builder will build exactly what the figures say. A drawing that needs interpretation has already failed.
15 minutes, both hands
- By hand, dimension a simple rectangular room: one overall string and one detail string for a door opening. Use thin lines and tick terminators.
- Check: do your detail dimensions add up to the overall? If not, find the error.
- In CAD, dimension the same room using associative dimensions. Stretch one wall and watch the figure update.
- Write the four anatomy parts from memory: ___, ___, ___, ___.
Key terms — added to the Drawing Atlas
- Dimensioning
- Adding measured figures to a drawing so a builder knows exact sizes. The drawing's most important annotation.
- Extension line
- A thin line projecting from an object's edge (with a small gap) to show what a dimension is measuring.
- Dimension line
- The thin line between extension lines that carries a measurement, with terminators at each end.
- Terminator
- The mark at each end of a dimension line — an arrowhead, dot, or (common in architecture) a 45° tick.
- Dimension string
- A row of dimensions. Layered as detail (inner), intermediate, overall (outer) so they cross-check by adding up.
- Associative dimension
- A CAD dimension linked to the geometry, so its figure updates automatically when the object changes.
Check yourself
3 quick questions — pick an answer to see why.
Q1Which lineweight tier do dimension lines belong to?
Q2In a layered dimension string, the OVERALL dimension sits…
Q3Why should you avoid dimensioning the same thing twice?
- A dimension is an assembly: extension lines, dimension line, terminators, the figure — all thin (Tier 3).
- Layer dimensions in strings: detail (inner) → intermediate → overall (outer), so they cross-check.
- Dimension to faces or centres consistently; never duplicate; keep figures unambiguous.
- CAD dimensions are associative and self-updating, but the judgement of what to dimension is yours.
Numbers tell the builder how big. But a drawing also needs to say what things ARE — a door here, a power point there, north is that way. How do drawings pack in all that meaning?