Mortar Calculator

About the Mortar Calculator

Mortar is the part of a brick or block wall that nobody estimates carefully, and it shows: half-used bags of cement going hard in the shed, or a second trip to the builders' merchant for sand halfway through a wall.

The volume of mortar in a wall is easy to reason about once you see it the right way round. A wall is a solid block of volume; the bricks or blocks fill most of it, and the mortar is everything left over. So the mortar per square metre is simply the wall's volume per square metre less the volume of the bricks in it.

This mortar calculator works that out for UK and US bricks and blocks, then turns the mortar into what you actually buy — bags of cement and tonnes of sand — at the mix ratio you choose. On the way it applies the one factor most home estimates miss: dry sand and cement shrink when they are mixed.

How to Use the Mortar Calculator

Choose metric or imperial units.

Pick what the wall is laid with: UK or US bricks, or UK or US blocks.

Enter the wall area — the face area, after subtracting doors and windows.

Choose single or double skin.

Set the mix ratio as parts of sand to one part of cement: 4 for a 1:4 mix, 5 for 1:5.

Adjust the waste allowance — 10% is typical for mortar.

How the Mortar Is Worked Out

  mortar per m²   =  wall thickness − volume of units in 1 m²
  wet mortar      =  mortar per m² × area × skins × (1 + waste %)
  dry materials   =  wet mortar × 1.3
  cement          =  dry ÷ (1 + ratio)          × 1,440 kg/m³
  sand            =  dry × ratio ÷ (1 + ratio)  × 1,600 kg/m³

Step-by-Step Example

10 m² of single-skin UK brickwork, 1:4 mix, 10% waste.

  Wall volume per m²:    0.1025 m³
  Bricks in it:          59.3 × 0.215 × 0.065 × 0.1025  =  0.0849 m³
  Mortar per m²:         0.1025 − 0.0849                =  0.0176 m³
  Wet mortar:            0.0176 × 10 × 1.10             =  0.194 m³
  Dry materials:         0.194 × 1.3                    =  0.252 m³
  Cement (1 part in 5):  0.050 m³ × 1,440               =  73 kg  → 3 bags
  Sand (4 parts in 5):   0.202 m³ × 1,600               =  0.32 tonnes

Why Dry Materials Exceed the Mortar Volume

If you fill a bucket with dry sand, there is air between the grains. Add cement and the fine cement particles settle into some of those gaps. Add water and the whole mix slumps and packs down further.

The result is that about 1.3 cubic metres of dry sand and cement makes 1 cubic metre of mortar. Estimating sand from the mortar volume alone, without that factor, leaves you about a quarter short — one of the commonest reasons for a mid-job trip back to the merchant.

Choosing the Mix

Mortar mixes are given as parts of cement to parts of sand, by volume.

Stronger is not always better. Mortar should be weaker than the bricks it joins, so that any movement cracks the joints — easily repointed — rather than the bricks themselves. That is why soft or old bricks are laid in weaker, often lime-based, mortars.

Plasticisers or a proportion of lime are commonly added to make mortar easier to work; they do not change the quantities much.

Bricks Versus Blocks

Blocks need far less mortar per square metre than bricks, because there are so many fewer joints: about 7 litres per m² for solid UK blocks against about 18 litres for UK bricks. A block wall of the same area therefore takes well under half the cement and sand.

Buying and Storing

Cement is sold in 25 kg bags in the UK, and as masonry cement in 70 or 75 lb bags in the US. It goes hard if it gets damp, so store it off the ground and covered, and buy close to when you need it.

Sand is cheapest in bulk bags. Keep it covered: rain washes out fines and wet sand bulks up, changing the mix if you measure by bucket.

Mixing Mortar Consistently

The ratio only helps if every batch is mixed the same way.

Measure by volume with the same container. A bucket or gauge box for the cement and the same one for the sand keeps proportions right; shovelfuls vary too much.

Mix dry first. Turn the sand and cement together until the colour is even, then add water gradually.

Aim for the right consistency. Mortar should hold its shape on the trowel and spread easily, without being sloppy. Too wet and it squeezes out of the joints and weakens; too dry and it will not bond.

Keep colour consistent. Different sands and different amounts of water change the colour of dried mortar. Using the same sand and mixing method across a whole wall avoids patchy joints.

Understanding Your Result

Materials is the headline: bags of cement and tonnes of sand.

Mortar gives the volume of mixed mortar, including waste.

Cement gives the weight in kilograms and pounds.

Sand gives cubic metres, tonnes and cubic yards.

Mortar per area shows the litres per square metre for your bricks or blocks.

Worth knowing covers mix strength and handling sand.

When Should You Use This Calculator?

Alongside the brick or block calculator, once you know the wall area.

Before ordering sand and cement, to avoid a second delivery.

When comparing bricks and blocks, to see the mortar difference.

When choosing a mix, to see how the ratio changes the cement.

Common Mistakes

Forgetting the 1.3 dry factor. You will run short of sand.

Using too strong a mix. Mortar should be weaker than the bricks.

Adding water to stiffening mortar. Mix fresh instead.

Storing cement on the ground. It absorbs moisture and sets in the bag.

Measuring wet and dry sand the same way. Wet sand bulks up.