Lesson 2.5
Converting & Checking: Never Trust a Scaled Line
How to move a dimension between metric and imperial without error — and the single most important habit in reading any drawing: read the written number, never the measured one.
Start hereA contractor scales a wall off a printed plan with a ruler. The print was reduced slightly to fit the paper. The wall gets built 40 mm short. Expensive lesson.
The fix is a rule every professional lives by: figures govern. Read the dimension that's written, not the one you measure.
01 — Converting between systems
One number you must know cold
Almost all metric–imperial conversion rests on one exact fact: 1 inch = 25.4 mm. Everything else follows. A foot is 12 inches, so 1 foot = 304.8 mm. From those two, you can convert any architectural dimension. (And in reverse: 1 m ≈ 3.28 ft ≈ 39.4 in.)
Type any dimension into the flipper below and pick its unit. The same real length, spoken in three dialects.
Type any dimension, pick its unit. The same real length, spoken in three dialects — all resting on 1 inch = 25.4 mm.
02 — Dimensions to know by feel
The same things, both systems
You'll internalise a handful of everyday dimensions in both systems, so you sense when a number is wrong. These recur in every project, anywhere.
Notice many round metric numbers map to clean imperial ones (900 mm ≈ 3 ft) because both systems evolved around the same human body — the theme you'll meet head-on in Module 3.
| Element | Metric | Imperial (approx) |
|---|---|---|
| Single door width | 900 mm | 3'-0" |
| Counter height | 900 mm | 3'-0" |
| Counter depth | 600 mm | 2'-0" |
| Ceiling height (typical) | 2700–3000 mm | 9'-0" to 10'-0" |
| Stair riser | ~175 mm | ~7" |
| Corridor (residential) | ~1000 mm | ~3'-3" |
03 — The golden rule
Figures govern. Always.
This is the most important habit in this entire module: when a drawing shows a written dimension, that figure is the truth — not whatever you measure off the paper with a ruler. Prints get reduced, paper stretches, photocopies distort. The written “3600” is exact; the line you measure might be 3580 after the print shrank. Build to the figure, every time.
It's why drawings are dimensioned so thoroughly (Module 5's job) — so nothing important ever has to be scaled off. If a dimension you need isn't written, the correct move is to ask, not to measure.
Why do drawings carry a “do not scale” note at all? Because a print is a fragile copy: a plotter set to “fit to page”, a photocopier, even humidity in the paper can shift every measured line by a percent or two. The written dimension is immune to all of that — it's the designer's exact intent, transcribed once. “Figures govern” and “do not scale” are two ways of saying the same discipline: trust the number, never the line. It is the habit that separates a careful reader of drawings from a costly one.
12 minutes
- Convert with the flipper: a 2400 mm counter to feet-inches; an 8-foot ceiling to mm; a 4.2 m room to feet.
- Without looking, estimate a single door in feet, then in mm. Check against the table.
- A printed plan's “3000 mm” wall measures 2950 mm with your ruler. Which do you build to, and why?
- Memorise the one fact: 1 inch = ______ mm.
Key terms — added to the Drawing Atlas
- Conversion factor
- The exact relationship between unit systems. The key one: 1 inch = 25.4 mm (1 foot = 304.8 mm).
- Figures govern
- The rule that a written dimension is the truth, never a length measured off a (possibly distorted) print.
- Do not scale
- A standard drawing note reinforcing 'figures govern' — build only to written dimensions, never to scaled measurements.
- Unit system
- The family of units a drawing uses — metric (mm/m) or imperial (feet/inches). A regional dialect; the grammar of scale is shared.
Check yourself
2 quick questions — pick an answer to see why.
Q1One inch equals exactly how many millimetres?
Q2A printed plan's '3000 mm' wall measures 2950 mm with your ruler. What do you build to?
- All metric–imperial conversion rests on 1 inch = 25.4 mm (1 foot = 304.8 mm).
- Learn everyday dimensions in both systems so wrong numbers feel wrong.
- Many round metric values map to clean imperial ones — both track the human body.
- Figures govern: build to the written dimension, never to a measured line.
You can scale and dimension in any system. But what decides that a door is 900 mm and a counter 600 mm in the first place? The answer is the human body.