How to measure a wall for mortar
Mortar for a wall is worked out from the face of the wall, so you need two measurements: the length (A) and the height (B). Measure the height from the top of the foundation to the top of the last course, including any coping laid in mortar. A wall 4 m long and 600 mm high has 2.4 m² of face.
A wall that steps down a slope, turns a corner or has a gateway is easier in sections. Each straight section is a rectangle; add them with Add another area. A sloping top can go in as a triangle on top of the rectangle below it.
For a bed of mortar under slabs, measure the paved area instead and enter the bed depth in step 3. The mortar is then simply area × depth.
How much mortar per square metre
Mortar makers publish how many bricks or blocks a given amount of their mortar lays. Turned into square metres of wall, that gives the figures below: for standard bricks in a half-brick wall (one skin, 102.5 mm thick) and a one-brick wall (two skins bonded together, 215 mm thick), and for 100 mm concrete blocks. The Use this buttons set the calculator to each job.
| Use | Mortar per m² | Bricks or blocks per m² | Apply |
|---|---|---|---|
| Half-brick wallOne skin of standard bricks. | 0.03–0.036 m³ per m² | 60 per m² | |
| One-brick wallTwo skins bonded together. | 0.066–0.086 m³ per m² | 120 per m² | |
| Block wall100 mm concrete blocks. | 0.0067–0.01 m³ per m² | 10 per m² | |
| Slab bedFull bed under paving. | A bed 30–60 mm deep | – |
A half-brick wall takes several times as much mortar per square metre as a block wall, because there are so many more joints. A one-brick wall takes twice the bricks and a little more than twice the mortar, because of the vertical joint between the two skins. For a slab bed, every 10 mm of depth is 0.01 m³ of mortar per square metre.
How much mortar do I need?
For a wall, multiply the face area by the mortar per square metre. Tarmac and Cemex both say how many bricks a given amount of their mortar lays; for a half-brick wall, with bricks at 60 per m², that works out at 0.03–0.036 m³ per m², and the calculator takes a figure in the middle. For a one-brick wall the range is 0.066–0.086 m³ per m²: twice the half-brick figure at the bottom, Tarmac’s own figure for a 215 mm wall above it, and the calculator uses the lower end of Tarmac’s figure. For blocks it takes the higher of the two published figures.
For a slab bed, multiply the area by the bed depth. UK guides give 30–60 mm: Marshalls works at the thinner end, Wickes and B&Q at the thicker. Then add 10% for mortar dropped or left in the bucket, the same allowance B&Q gives for concrete.
The result at the top of the calculator is the volume of mortar to mix. To buy it, the calculator turns that wet volume into dry sand and cement; the sand and cement calculator explains that step and puts the bags first.
Which mortar mix for a garden wall?
Mortar mixes are written as parts of sand to one part of cement, by volume. The cement data sheets from Heidelberg Materials and Hanson group them by strength class, from M12, with the least sand, through M6 to M4, with the most. For garden walls the presets here use 4–6 parts sand to one of cement, and the Mix box lets you choose another.
Stronger is not automatically better. The Brick Development Association warns that a very strong mortar is also brittle, and recommends lime or a plasticiser to keep a mix workable. B&Q’s garden wall guide moves to a stronger mix, still with plasticiser, as the wall gets more exposed to the weather. The BDA also suggests sulfate-resisting cement for freestanding walls. The right mix depends on your bricks and your site, so check it with your brick supplier.
Changing the mix does not change the volume of mortar, only how it splits: a richer mix uses more cement and less sand.
Half-brick or one-brick wall?
A half-brick wall is a single skin of bricks laid lengthways, 102.5 mm thick. A one-brick wall is two skins bonded together, 215 mm thick, with some bricks laid across the wall to tie the skins. The calculator counts 60 per m² for a half-brick wall and 120 per m² for a one-brick wall, figures B&Q and Beesley & Fildes give. The count carries the same extra allowance as the mortar; B&Q and Beesley & Fildes suggest 5–10% extra for bricks, so the allowance sits at the top of their range.
Thickness matters for how high a freestanding wall can go. A half-brick garden wall can only be built so high before it needs piers, thicker columns of brickwork at intervals along it, or needs to be built one brick thick instead. The note in step 1 gives the height, and the calculator warns you if the half-brick wall you enter is taller. The calculator does not count the extra bricks for piers, so ask your supplier how many your piers need.
Height, piers and footings are a guide only. Wind, soil, slopes and anything the wall holds back all change the design, so check yours with your supplier or a professional before you build.
Worked example: a one-brick garden wall
Here is the full calculation for a garden wall built one brick thick, using the One-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 6 m × 0.9 m = 5.40 m²
- Bricks 5.40 m² × 120 per m² + 10% = 713
- Wet mortar 5.40 m² × 0.075 m³ per m² of one-brick wall = 0.405 m³
- Plus extra 0.405 m³ + 10% = 0.446 m³
- Dry materials 0.446 m³ of wet mortar × 1.25 = 0.557 m³
- Cement (1 part in 5) 0.557 m³ ÷ 5 × 1.4 t/m³ = 156 kg → 7 25 kg bags
- Building sand (4 parts in 5) 0.557 m³ × 4 ÷ 5 × 1.65 t/m³ = 0.74 t
On the list:
- 713bricks (215 × 65 mm face, one-brick wall)120 per m² + 10%
- 725 kg bags of cement156 kg needed
- 1bulk bag of building sand (approx. 850 kg each)0.74 t needed
Notice how many bricks a one-brick wall takes: twice as many per square metre as a half-brick wall, with more than twice the mortar. At this height a half-brick wall would bring up the warning about piers, which is why the example is built one brick thick. Once turned into dry materials the mortar is mostly sand, several times the weight of the cement, so the sand decides whether a bulk bag is worth ordering.
UK cement and sand for mortar
You mix mortar from general purpose cement, sold in 25 kg bags, and building sand, sold in bags of 25 kg or bulk bags of around 850 kg. For a slab bed the sand is sharp sand, sold the same way.
| 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. |
Ready-mixed mortar in bags, which only needs water, is handy for a small repair. The calculator does not count those bags, because each brand makes a different amount per bag: divide the volume of mortar in the result by the yield printed on the bag, and round up.
Common mistakes when working out mortar
- Measuring only the length of the wall. Mortar depends on the face area, so the height matters as much as the length.
- Forgetting the courses below ground. Brickwork that starts below ground level still needs mortar. Measure the height from the top of the foundation, not from the lawn.
- Choosing the strongest mix by default. A very strong mortar is brittle. Use the mix your brick supplier recommends for how exposed the wall is.
- Building a tall half-brick wall without piers. A freestanding half-brick wall can only go so high on its own. If the calculator warns you, plan piers or a one-brick wall and check the design with your supplier.
- Leaving out the plasticiser. Cement makers recommend a plasticiser in leaner mixes to keep them workable. It does not change the quantities here.
- Picking the wrong wall thickness. A half-brick wall is one skin; a one-brick wall is two skins bonded together and needs twice the bricks and more than twice the mortar. Choose the right one in step 1.
- Mixing it all at once. The totals are for the whole job. Mix in batches you can lay while the mortar is still workable.
What else you'll need
- The bags to buy, with the wet-to-dry step explained: the sand and cement calculator puts the cement and sand first.
- Ballast for a concrete footing under the wall: the aggregate calculator works out the tonnes.
- Slabs and sub-base when the mortar is for a patio bed: the patio calculator does the whole job.
- A spot board, a bucket and plasticiser, so you can measure each batch by volume and keep the mortar workable.
Last checked 28 October 2026