Why does a construction drawing say 4120, not 4.12 m?
Because construction drawings measure in millimetres without a unit, as a rule. It removes decimal points, which can vanish in a photocopy, and makes every figure on the drawing directly comparable.
Short answer: Construction drawings almost always give measurements in millimetres, with no unit written. "4120" means 4120 mm = 4.12 m, and "980" means 98 cm. The reason is that millimetres give whole numbers with no decimal points — and a decimal point is exactly the mark that vanishes in a photocopy, a fax, or a poor print, after which 4.12 can be misread as 412. With millimetres, a figure cannot be misread by a factor of ten. Levels (height figures), on the other hand, are normally given in metres with two decimals, e.g. +2.80.
The rule: whole numbers beat decimal points
A construction drawing uses one set of units, and it applies to the whole drawing. Millimetres are chosen because the unit is small enough that any building measurement can be written as a whole number.
A wall of 4.12 m becomes 4120. A door of 0.98 m becomes 980. A joint of 12 mm becomes 12. There are no decimal points, no fractions, and no way to misread a figure as ten times too large.
It sounds like a detail, but it is a fail-safe. A drawing gets copied, folded, sent as a PDF, printed on a building site and read with dirty hands. A decimal point on 0.3 mm of paper does not survive that journey. A whole number does.
This is an international convention (ISO), not a British or Danish habit, so it holds regardless of which country drew the plan. If you are more used to seeing feet and inches on a drawing, note that this whole-number trick is specifically a metric one — it does not translate directly into feet-and-inches notation. But the underlying discipline is the same everywhere: put the measurement where it cannot be misread, whatever unit the country happens to use.
How to translate a figure on a construction drawing
- Four digits such as 4120, 2350, 1200 are millimetres, and so metre-scale figures: 4.12 m, 2.35 m, 1.20 m. Move the decimal point three places to the left.
- Three digits such as 980, 600, 375 are also millimetres, but smaller: 98 cm, 60 cm, 37.5 cm. Typically door widths, cupboard depths and wall thicknesses.
- Two digits such as 12 or 45 are millimetres in a practical sense: joint widths, panel thicknesses, the thickness of floorboards.
- Figures with a decimal point and two decimals such as 12.85 or +2.80 are usually levels — heights in metres above a reference point — not lengths.
If you see a drawing where a wall is marked "4.12", it is not a construction drawing in the classic sense, but a home layout or a sales drawing. Those are more often given in metres, because they are made for buyers rather than tradespeople. Either is fine — just check which kind of drawing you are holding before you calculate anything further.
The dimension chains: where the figures sit, and what they measure between
The figures do not float freely on the drawing. They sit in dimension chains: thin lines running alongside the plan, with a tick mark or an arrow at each end of what is being measured. A figure always belongs to the stretch between two marks — not to whatever happens to be nearby.
Several chains run outside one another, coarsest first:
- Outermost: the overall dimensions of the building, corner to corner.
- Middle: the main divisions — where the partition walls sit.
- Innermost: the details — window widths and their distance to the nearest corner.
The check you can always run: the sum of an inner chain must equal the outer chain. If the innermost chain reads 1200 + 980 + 1940, and the outer chain reads 4120, then 1200 + 980 + 1940 = 4120 — and the drawing adds up. If it does not, there is an error, and you need to find out which figure is wrong before you order anything.
The trap: measured to the centre of the wall, or to its face?
This is the error that costs the most money, and it does not show up in the figure itself — only in where the marks land.
- Clear (internal) dimensions run from wall face to wall face. That is what you measure yourself with a tape measure standing in the room.
- Grid (structural) dimensions run from wall centreline to wall centreline. This is used in structural drawings, because load-bearing lines and foundations are set out from the centre.
Worked example: two 100 mm partition walls stand 4120 mm apart measured centre to centre. The room between them then measures 4120 − 50 − 50 = 4020 mm, so 4.02 m. That 10 cm gap is precisely enough for a wardrobe measuring 4.10 m not to fit, even though the drawing said 4120.
Look at the marks on the dimension chain: does the tick land in the middle of the wall, or on its face? That decides which of the two figures you are holding. In HouseSense, internal (clear) dimensions are the basis throughout, precisely because they can be checked: you can walk into the room with a tape measure and get the same number.
Grid dimensions and M-numbers: 7M, 8M, 9M
With doors and windows you will come across labels such as 8M × 21M. M stands for module and is 100 mm. An 8M door therefore has a wall opening 800 mm wide and 2100 mm high.
The system exists because building is organised around a 100 mm grid — it makes it possible to order standard components that fit standard openings. The M-notation itself is not used everywhere; elsewhere the opening's width and height are simply given in millimetres. The 100 mm grid underneath is the same. Note that the M-number describes the opening in the wall, not the door leaf. The leaf is narrower, because the frame takes up space, and the clear passage is narrower still than the leaf. If you are checking whether the sofa will fit through, it is the clear passage you need to measure — not the M-number.
The rule that saves you every time: the number beats the line
Whether the drawing is in millimetres or metres, the same basic rule applies: the written figure is the truth, not what you measure with a ruler on the paper.
Paper routinely lies. An A3 drawing printed on A4 with "fit to page" shrinks to about 70.7%. A PDF on a screen has no physical size at all. A photo of a drawing is both scaled and distorted. But the figure 4120 is still 4120 in all three cases.
So: use the dimension chain's figures wherever they exist. Only reach for a scale rule and a ruler once a measurement is missing — and write down whatever you measure your way to on the drawing in a different colour, so you can later see that it is a reading, not a stated figure.
What to put on the drawing if you draw it yourself
If you draw a floor plan yourself — on graph paper or in an app — there are four things that need to be on it before the drawing is usable to anyone else:
- The unit. Write "all dimensions in mm" or "all dimensions in cm" somewhere on the drawing. Then no one can get it wrong.
- What is being measured to. Write "clear dimensions" or "internal dimensions", if that is what you used.
- Wall thicknesses. Without them, no one can convert between internal and overall dimensions.
- Scale and north arrow. Even a drawing with every dimension marked becomes easier to use once you can see the scale and the orientation.
Those four lines take two minutes to write and remove almost every question a tradesperson would otherwise have to call and ask.