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Mortar Calculator

Work out how much mortar a brick wall needs — volume and premix bags — from the wall size and brick size, using the exact wall-minus-brick method.

Mortar is usually guessed — a rule of thumb per square metre, or “a few bags”. This calculator does not guess: it subtracts the volume of the bricks from the volume of the wall, and what is left is the mortar.

The assumptions are stated plainly: a single-skin wall, joints fully filled, and a 10% mixing allowance. For a double-skin wall, calculate each leaf separately and add the two results.

Single skin: the wall is one brick wide. US modular: 3.75 in. UK standard: 102.5 mm.
Standard joint is 10 mm (3/8 in). Enter 9.5 for a 3/8 in joint.
10% mixing and spillage allowance.
Premix mortar yields about the same as concrete per bag: 60 lb ≈ 0.45 ft³.
Enter your local price — the calculator never guesses prices.

Formula

wall volume = wall length × wall height × brick width bricks in wall = wall area ÷ ((brick length + joint) × (brick height + joint)) brick volume = bricks × (brick length × brick width × brick height) mortar volume = wall volume − brick volume mortar to mix = mortar volume × (1 + wastage ÷ 100) bags needed = ⌈ mortar to mix ÷ yield per bag ⌉

All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.

How to use this calculator

  1. Measure the wall length and height.
  2. Enter the brick size, including its width (the wall thickness for a single skin).
  3. Set the joint size and wastage (10%).
  4. Enter a premix bag size to get a bag count.

Worked example

Wall (US)10 ft × 8 ft, brick 8 × 3.75 × 2.25 in, 3/8 in joint
Wall volume80 ft² × (3.75 ÷ 12) ft = 25 ft³
Bricks in wall80 ÷ 0.1527 = 524 bricks
Brick volume524 × (8 × 3.75 × 2.25 ÷ 1,728) = 20.47 ft³
Mortar volume25 − 20.47 = 4.53 ft³
With 10% wastage4.53 × 1.10 = 4.98 ft³
60 lb premix bags⌈4.98 ÷ 0.45⌉ = 12 bags

Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.

Frequently asked questions

How is mortar quantity calculated here?

Exactly: wall volume minus brick volume. The wall is a solid block of length × height × thickness; the bricks fill most of it, and the rest is mortar. No rule-of-thumb rate per square metre is used — the assumptions (single skin, joints fully filled, 10% allowance) are stated on this page.

How much mortar per square metre of brickwork?

Roughly 0.03 m³ per m² for a single-skin UK wall, but it varies with brick size and joint width. This calculator works it out for your exact bricks instead of using that rough rate.

What mix should I use for mortar?

A common general-purpose mix is 1 part cement to 4 parts soft sand (1:4), or 1:5 for internal walls. For structural or exposed work, follow the specification for the job — the mix affects strength and weather resistance.

Do I need to add water to premix mortar?

Yes — premix bags contain cement and sand only. Add clean water gradually until the mortar holds its shape on the trowel. Mix only as much as you can use in about two hours.

Related calculators

Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.