How to measure for sand and cement
Pick the job first, because it changes what you measure. For slabs on a bed, measure the area you are paving: the length (A) and the width (B). Awkward patios split into rectangles, triangles and L-shapes, and Add another area adds them up.
For a block wall or a brick wall, measure the face of the wall: the length (A) along the ground and the height (B) from the top of the foundation to the top course. A wall 5 m long and 1 m high is 5 m² of face. If there is a gateway or a pier, measure the wall in sections either side of it.
Then choose the mix and, for a bed, its depth: the thickness of mortar you spread before the slabs are tapped down. If you measure in feet and inches, switch the units at the top. The answer still comes out in bags of cement and tonnes of sand, the way UK merchants sell them.
Typical mixes and quantities
Each job uses a different sand and a different amount of mortar. These are the mixes UK cement makers and paving guides give, with the mortar each job takes per square metre. The Use this buttons set the calculator to that job.
| Use | Sand | Mix (sand : cement) | Mortar | Apply |
|---|---|---|---|---|
| Slabs on a mortar bedPatios and paths. | Sharp sand | 3–4 parts sand to 1 | A bed 30–60 mm deep | |
| Block wallStandard concrete blocks. | Building sand | 4–6 parts sand to 1 | 0.0067–0.01 m³ per m² | |
| Half-brick wallOne skin of bricks, 102.5 mm. | Building sand | 4–6 parts sand to 1 | 0.03–0.036 m³ per m² | |
| One-brick wallTwo skins bonded, 215 mm. | Building sand | 4–6 parts sand to 1 | 0.066–0.086 m³ per m² |
Mixes are parts by volume, measured with the same bucket or shovel. For blockwork the calculator takes the higher of the two published mortar figures, and for brickwork a figure in the middle of the range, so you are less likely to run short. A one-brick wall is two skins bonded together, so it takes twice the bricks and a little more than twice the mortar of a half-brick wall.
For walls, step 1 also shows a note on height: a freestanding half-brick wall can only go so high before it needs piers or a one-brick wall, and the calculator warns you if yours is taller. The mortar calculator explains wall thickness, piers and mortar strengths in more detail.
Sand and cement per m² of blocks
Xella’s pack guide puts standard concrete blocks at 10 per m², laid with normal joints. For the mortar, Cemex’s and Tarmac’s data for 100 mm blocks work out at 0.0067–0.01 m³ per m² of wall. Wider blocks have wider bed joints and take more, so raise the extra allowance if you are building with them.
Turned into dry materials, the sand and cement per m² of blocks is modest: a single bag of cement covers several square metres of blockwork, and the blocks themselves are by far the heaviest part of the order. For a quick figure per square metre, enter a wall 10 m long and 1 m high and divide the results by ten; a 1 m × 1 m wall works too, but everything is rounded up to whole bags.
Why the dry materials come to more than the mortar
Mortar is measured wet, in the joints or under the slabs, but you buy sand and cement dry. The dry materials take up more room than the mortar they make, because once water is added the fine cement fills the gaps between the grains of sand.
No UK source we found publishes a wet-to-dry factor for mortar. What B&Q does publish is a concrete table: a cubic metre of concrete takes a full cubic metre of ballast plus the cement on top, so the dry materials come to 1.25 times the finished volume. The calculator uses that same factor for mortar. It is an honest approximation, not a measured figure, which is one reason the extra allowance is there.
The cement is then weighed at a density worked out from B&Q’s concrete table, which sits inside the wide range cement makers’ safety sheets give for loose cement, 0.9–1.5 t/m³. Treat the bag count as a close estimate and round up rather than down.
Worked example: a low brick wall
Here is the full calculation for a low garden wall built half a brick thick, using the Half-brick wall preset at its usual mix, with the standard extra allowance. Every line comes from the same code that runs the calculator above.
- Wall 5 m × 0.45 m = 2.25 m²
- Bricks 2.25 m² × 60 per m² + 10% = 149
- Wet mortar 2.25 m² × 0.033 m³ per m² of half-brick wall = 0.074 m³
- Plus extra 0.074 m³ + 10% = 0.082 m³
- Dry materials 0.082 m³ of wet mortar × 1.25 = 0.102 m³
- Cement (1 part in 5) 0.102 m³ ÷ 5 × 1.4 t/m³ = 29 kg → 2 25 kg bags
- Building sand (4 parts in 5) 0.102 m³ × 4 ÷ 5 × 1.65 t/m³ = 0.13 t
On the list:
- 149bricks (215 × 65 mm face, half-brick wall)60 per m² + 10%
- 225 kg bags of cement29 kg needed
- 625 kg bags of building sand0.13 t needed
Notice how little sand and cement a low brick wall needs next to the number of bricks: the cement goes only just over a whole bag, so the list rounds up. Notice also that the sand comes in small bags, because the job needs far less than half a bulk bag. The wall is low enough to build half a brick thick without the height warning; a taller one would need piers or a one-brick wall.
UK cement and sand sizes
Cement is the simple part of any sand and cement calculator. UK merchants sell general purpose cement in 25 kg bags and you buy whole bags. Sand comes in small bags of 25 kg, easy to carry but a lot of bags on a big job, or in bulk bags of around 850 kg dropped on the drive.
| Product | Size | Good for |
|---|---|---|
| Cement, 25 kg bag | 25 kg | General purpose cement. |
| Small bag, 25 kg | 25 kg | Merchant “handy bag” or trade pack. |
| Bulk bag, 850 kg | 850 kg | The most common merchant bulk bag. About 10 m² of gravel at 50 mm. |
Bulk bags of sand vary a little in weight with moisture, so the calculator works in kilograms and rounds up. When the list shows one bulk bag topped up with a few small bags, that is usually easier than a second bulk bag that would be mostly left over.
Common mistakes when buying sand and cement
- Using building sand for a slab bed. Soft sand is for walls. Patio beds use sharp sand, which the calculator switches to when you pick Slabs on a bed.
- Buying for the wet volume. Mortar is measured wet but bought dry, and the dry materials take up more room. A quick sum that skips this step comes up short.
- Reading the mix by weight. Mix ratios are parts by volume, measured with the same bucket or shovel, not kilograms of each.
- Measuring only the length of the wall. The calculator needs the face area, so measure the height too, from the foundation to the top course.
- Entering the settled bed depth. Marshalls notes the bed ends up thinner once the slabs are tapped down. Enter the depth you spread, not the depth you expect to end with.
- Ordering exactly what the sum says. Some mortar is always dropped or left in the bucket. The calculator adds 10%, the allowance B&Q gives for concrete.
What else you'll need
- The mortar volume, wall thickness and the right mix: the mortar calculator works in cubic metres of mortar and explains one-brick walls, piers and mortar strengths.
- Slabs, sub-base and jointing for a whole patio: the patio calculator does the bed as part of the full job.
- Sand on its own, for a laying course or a bigger order: use the sand calculator.
- Ballast for concrete, for a footing or a base: the aggregate calculator works out the tonnes.
- Plasticiser for wall mortar, which cement makers recommend in leaner mixes, and a bucket to measure the parts by volume.
Last checked 28 October 2026