Mortar Calculator UK | Bricks, Blocks & Paving Beds

How to use the mortar calculator

Under Work it out from choose a number of bricks, a number of blocks, the area of a wall, or a mortar bed for paving or slabs.
Enter the amount: Number of bricks, Number of blocks or Area. For a bed, also enter the Thickness of the bed. 30–40 mm is normal under paving slabs.
Pick the Wall thickness for bricks (single or double skin) or the Block thickness for blocks.
Choose your Mix (cement : sand). 1:4 is a strong general mix and the usual choice for paving beds. 1:5 and 1:6 are weaker.
Leave Extra for waste at 10%, or change it. Blockwork always gets at least 20%.
Read the cement and sand, then tap Add to shopping list to keep them or Share link to send them to your builder.

Why use this calculator

It starts from what you know. You can give it a number of bricks, a number of blocks, a wall area or a paving area, so you don't have to convert anything first.

It covers paving beds as well as walls. A patio bed is worked out from its area and thickness, and uses sharp sand instead of building sand.

It tells you how far a bag goes. You get the number of bricks or blocks one bag of cement will lay, or the area it will bed, which is handy for checking a quote or your own guess.

It shows its working. Under How we worked it out you can see every step with your own numbers.

How we worked it out

Your numbers, step by step. This updates as you change the calculator.

A single-skin wall takes about 0.53 m³ of mortar per 1,000 bricks (this includes filling the frogs and the bedding).
Mortar = 1,000 ÷ 1,000 × 0.53 = about 0.53 m³
Add 10% for waste: 0.53 × 1.1 = 0.583 m³
At 1:4, 1 m³ of mortar needs about 14.4 bags of cement, so 0.583 m³ needs about 8.4, round up to 9 × 25 kg bags
Sand = 0.583 m³ × 1 m³ of sand per m³ of mortar = 0.583 m³, × 1.6 t/m³ = about 0.93 tonnes of building sand (2 bulk bags of 850 kg)
One bag of cement lays about 119 bricks: 1,000 bricks ÷ 8.4 bags

The formula

First we work out the volume of wet mortar, then the cement and sand to make it.

From bricks: number of bricks ÷ 1,000 × 0.53 m³ for a single skin wall. For a one-brick-thick wall the figure is 0.08 m³ a m² ÷ 120 bricks a m² × 1,000, which is about 0.667 m³ per 1,000 bricks.
From blocks: number of blocks ÷ 10 gives the wall area in m², then we multiply by the mortar a m² for your block thickness: 0.0049 m³ (75 mm), 0.0066 (100 mm), 0.0092 (140 mm) or 0.0141 (215 mm).
From a wall area: area × 0.032 m³ (single skin brick), 0.08 m³ (one brick thick) or the block figure above.
For a bed: area × thickness. A 10 m² bed at 40 mm is 10 × 0.04 = 0.4 m³.
Waste: the volume × (1 + your waste percentage). Blockwork uses at least 20%.
Cement and sand: mortar volume × the figures for your mix, per m³ of mortar. 1:3 takes 18 bags and 0.94 m³ of sand. 1:4 takes 14.4 bags and 1.0 m³. 1:5 takes 12 bags and 1.04 m³. 1:6 takes 10.3 bags and 1.07 m³. We round the bags up.

What we assume

The brick mortar figures include filling the frogs and the bedding. They come from a mortar maker’s data sheet (1 m³ lays 1,800 to 2,000 bricks).
Block mortar is worked out from the joint sizes, so it needs the extra allowance we add.
Cement and sand quantities are worked out from standard volumes, not from a maker’s table. Treat them as good to about 8% either way.
Building sand weighs about 1.6 tonnes a m³ and sharp sand (used for beds) about 1.7. A bulk bag holds 850 kg.
Damp sand bulks up. Lime mortars, plasticisers and old soft bricks use different mixes and are not covered.

Tips

Count the bricks first with the brick calculator, then bring the number here. It gives you the same mortar figures in one step.
Use the same bucket or gauge box for every batch. The mixes are by volume, so 1:4 means 1 bucket of cement to 4 of sand.
Sand from a merchant is often damp. Damp sand takes up more room and weighs differently from dry sand, so treat the tonnes as a guide and ask your merchant for their own figure.
For a patio, check the bed your paving maker wants. Some want a full mortar bed of 30–40 mm. Work out the slabs or blocks with the block paving calculator.
Keep a spare bag of cement on site. The bag counts are already rounded up, but a trip to the merchant mid-job still costs you time.
1:6 is a weak mix. Use it only for sheltered, low-load work, and ask your bricklayer if you are not sure.

FAQs

How much sand and cement do I need for 1,000 bricks?

About 0.53 m³ of mortar, which is 8 bags of 25 kg cement and 0.85 tonnes of building sand at 1:4, before waste. With the default 10% waste it is 9 bags and about 0.93 tonnes.

What is the best mortar mix ratio for bricklaying: 1:4, 1:5 or 1:6?

1:4 is a strong general mix and the usual choice for outdoor walls and paving beds. 1:5 and 1:6 are weaker and suit sheltered, low-load walls. For anything exposed or structural, ask your builder or the brick maker.

How many bricks will a 25 kg bag of cement lay?

About 120 bricks at a 1:4 mix with 10% waste. That becomes about 125 with 5% waste, and about 130 if none is wasted.

How much mortar do I need for a patio or paving slabs?

A 10 m² patio on a 40 mm bed needs 0.4 m³, or 0.44 m³ with 10% waste. At 1:4 that is 7 bags of cement and about 0.75 tonnes of sharp sand.

How much mortar do I need per m² of blockwork?

About 0.0066 m³ per m² for 100 mm blocks, before waste. We add at least 20% because block joints are measured exactly and real mortar gets dropped or laid thicker, so 100 blocks (10 m²) need about 0.08 m³.

Do I need lime in my mortar mix for old or soft bricks?

Old, soft bricks are normally laid in lime mortars, which use different mixes. This calculator only does cement and sand. Ask a bricklayer who works on older buildings, or your council's conservation officer.

How much extra mortar should I allow for waste?

5–10% is usual. The calculator starts at 10%, and for blockwork it never goes below 20%.