Length, Area, Volume & Falls: Reading mm, m2, m3 and 1:40 on Your Drawings

These are the units you already half-know: millimetres, metres, square metres, cubic metres. The reason they still trip people up on a building site is that each one belongs to a different job, and the gradient notation on a drainage run or a flat roof (1:40, 1:80, or 25mm per metre) is written three different ways for the same thing. Get the four straight and you can read a drawing, check a quote line by line, and spot when a delivery does not match the order.

Length: why the drawings are all in millimetres

Open any architect's or structural drawing for an extension and almost every dimension is in millimetres. A door opening is 2100, a wall is 3600 long, a cavity is 100. There are no decimal points and no unit letters, because on UK building drawings the millimetre is the assumed unit. Working in whole millimetres keeps the numbers exact: 2100 leaves no room for the rounding errors that creep in once you start writing 2.1m and someone reads it as 2.10 or 2.01.

Metres still appear, but for different work. Room sizes, setting-out distances and quote line items are usually in metres: a quote might read "supply and fit 5 linear metres of worktop" or "dig footings, 8m run". Lengths of timber and pipe come in metres too (a 4.8m length of 47 x 100 C24, where the section size flips back to millimetres). Centimetres are the odd one out. They are almost never used in UK construction, so if a measurement reaches you in centimetres, convert it and write it back in millimetres before it causes confusion on site.

1m = 1000mm = 100cm

On a drawing, no unit letter means millimetres. Multiply metres by 1000 to get millimetres, or move the decimal point three places. A 2.4m ceiling is 2400mm.

Area: the square metre (m2)

Area is length times width, measured in metres, and written m2 (square metres). It is the unit almost all finishing trades price by, because the cost tracks the surface they have to cover. Painting, plastering, rendering, tiling, plasterboarding and most flooring are all quoted per square metre, so a 3m by 4m room has 12 m2 of floor and roughly 30 to 34 m2 of wall once you take off the openings.

Area is also how materials that cover a surface are counted. A wall takes about 10 blocks/m² of standard blockwork or 60 bricks/m² of facing brick in the usual stretcher bond, and a bag of tile adhesive or a tub of paint will state the m2 it covers on the label. When a builder's quote, a merchant's order and a paint tin all talk in m2, you can cross-check them: the wall area on the drawing should roughly match the number of blocks ordered divided by ten.

Volume: the cubic metre (m3)

Volume is length times width times depth, all in metres, written m3 (cubic metres). It is the unit for anything you buy by the bulk: ready-mix concrete for foundations and slabs, the spoil dug out of the ground, hardcore and aggregate, and the capacity of a skip. A structural engineer or a groundworker will talk in m3 because that is how the concrete plant sells and how the muckaway lorry charges.

The number matters because a small change in depth moves a lot of material. A trench 10m long and 0.6m wide filled to 1m deep is 6 m3 of concrete, and ready-mix is ordered in part-loads to the nearest fraction of a cubic metre. The same logic sizes your waste: a typical builder's skip holds around 6 m3, so knowing the volume of spoil tells you how many skips the dig will fill before the first one is even ordered.

1 m3 of concrete ≈ 2.4 tonnes

A cubic metre is a cube one metre on every side. Concrete weighs roughly 2.4 tonnes per cubic metre, which is why a full ready-mix load needs proper access and why over-ordering is expensive in more ways than one.

Falls and gradients: the same slope, written three ways

A fall is a deliberate slope built into a pipe or a surface so that water runs off under gravity. It turns up on every drainage run and every flat roof, and it is written in whichever of three forms the drawing prefers: as a ratio (1:40), as a drop in millimetres per metre (25mm per metre), or occasionally as a percentage. They all describe the same slope. A ratio of 1:40 means the surface drops one unit for every forty it travels, which works out at exactly 25mm of drop per metre of length.

1:40 = 25mm per metre, 1:80 = 12.5mm per metre

To turn a ratio into a drop per metre, divide 1000 by the second number. 1000 divided by 40 is 25mm per metre; 1000 divided by 80 is 12.5mm per metre. To go the other way, divide 1000 by the drop.

For drainage, the slope has to sit in a narrow band. Too steep and the water outruns the solids and leaves them behind; too shallow and the solids settle and block the pipe. For the 110mm pipe that serves a kitchen extension, the standard drainage gradient is 1:40, that same 25mm per metre. There is a builder's shortcut for it, Maguire's rule: take the pipe diameter and divide by 2.5 (100 / 2.5 = 40), which gives the 1:40. The starting height for the whole calculation is the invert level, the inside bottom of the existing drain you are connecting into.

For a flat roof, the fall stops rainwater sitting in pools. BS 6229, the flat-roof code, sets a finished fall of at least 1:80 across the roof, but tells the designer to draw it steeper, at 1:40, so that the slope still holds once timber deflection and site tolerances have eaten into it. That fall is usually created with tapered firring strips on top of the joists. Get it wrong and you get too little fall, which leads to ponding: standing water that ages the covering early and finds any weak seam. The same mm per metre logic also sets the fall on a floor screed laid to a wet room or an external threshold.

Reading it all at a glance

Why getting these right pays off

These four units are where a quote and a delivery quietly stop matching. A floor priced as 12 m2 that turns up measured as 15 m2 is a quarter more tiling than budgeted; a foundation drawn to take 6 m3 of concrete that arrives as a single 4 m3 load is a part-pour with a cold joint; a drainage run laid at 1:80 where the drawing said 1:40 is a pipe that blocks every winter. None of it needs you to do the trade's job. It needs you to read the unit on the drawing, check it against the order and the ticket, and ask the question before the concrete sets or the screed goes down.

The mixed imperial and metric trade sizes that sit alongside these units (why a 4 x 2 is neither four inches nor two, and why pipe and timber sizes never quite read as you expect) are decoded in the blog post on UK building measurements, imperial versus metric. For the rest of the notation on a quote, a datasheet or a set of structural calculations, see the master units and symbols decoder.