Most jobs need sand for two different reasons at once, and the two are usually ordered together. A paving or screed bed is a straightforward layer: area multiplied by bed depth gives the volume of sharp sand. Mortar is different, because the sand is only part of the mix and the volume you need depends on both the mortar quantity and the mix ratio.
Bedding depth is a real decision rather than a constant. Slabs laid on a full mortar bed sit on 30 to 40 millimetres, block paving on a screeded sand bed uses 40 to 50, and a sand blinding over a sub-base might be only 25. For mortar, one square metre of single-skin brickwork swallows about 0.02 cubic metres, and in a 1:4 mix four fifths of that is sand.
A 25 square metre patio on a 40 millimetre bed needs 1.00 cubic metres of sharp sand. Adding 10 square metres of brickwork at 0.02 cubic metres of mortar per square metre gives 0.20 cubic metres of mortar, of which 0.16 is sand at 1:4. The total of 1.16 cubic metres weighs 1.86 tonnes at 1600 kilograms per cubic metre.
The formula
bed depth- Thickness of the bedding layer under the paving, in mm
mortar rate- Mortar per m² of brickwork, about 0.02 m³
mix- Sand parts per one part cement; 1:4 is general purpose
density- Bulk density in kg/m³, about 1600 for sharp sand
How it works, step by step
- Enter the paving area and the depth of the bedding layer.
- Enter the area of brickwork or blockwork you are bedding in mortar.
- Choose the mortar mix; the sand fraction follows from the ratio.
- The gauge shows tonnes; bulk bags to order are in the breakdown.
Worked examples
A 25 m² patio with 10 m² of walling
The bed is 25 × 0.04 = 1.00 m³ of sharp sand. The walling takes 0.20 m³ of mortar, four fifths of which is sand at 0.16 m³. Together 1.16 m³ weighs 1.86 tonnes, or 3 bulk bags with waste.
The mix ratio moves the sand
The same 0.20 m³ of mortar in a strong 1:3 mix contains 0.150 m³ of sand rather than 0.160, and a soft 1:5 mix contains 0.167. The mortar volume is unchanged; only the split between cement and sand moves.
How to read your score
Frequently asked questions
What is the difference between sharp sand and building sand?
Sharp sand is coarse and angular, which makes it stable underfoot and right for bedding, screeds and concrete. Building or soft sand is fine and rounded, which makes mortar workable and buttery on the trowel. Using soft sand as a paving bed gives a bed that never firms up.
Why is bedding depth so variable?
Because it depends on the paving and the base beneath it. A full mortar bed under slabs is 30 to 40 millimetres, block paving is screeded to 40 to 50 before it is vibrated down to around 30, and a blinding layer over a compacted sub-base can be 25. The paving thickness itself does not come into the sand calculation.
Does sand compact like gravel?
Screeded bedding sand loses roughly ten to fifteen per cent of its loose depth when the paving is vibrated in, so a 50 millimetre screed finishes nearer 40. Adding that to the waste allowance rather than to the depth keeps the two effects separate and easier to reason about.
How much mortar does brickwork actually use?
About 0.02 cubic metres per square metre of single-skin brickwork with a standard 10 millimetre bed and perp. Blockwork uses less because there are far fewer joints per square metre, and frogged bricks laid frog up use more. It is an input here for exactly that reason.
Should I buy sand by weight or by volume?
Loose sand is sold by the tonne and bagged sand by the bag, which is a weight too. Volume is what you consume, so the conversion runs through bulk density. Wet sand can be several per cent heavier than the same volume dry, so a weight-based order after heavy rain buys you slightly less sand.
Sand quantity reference
| Paving area | 25 mm bed | 30 mm bed | 40 mm bed | 50 mm bed |
|---|---|---|---|---|
| 5 m² | 0.20 | 0.24 | 0.32 | 0.40 |
| 10 m² | 0.40 | 0.48 | 0.64 | 0.80 |
| 15 m² | 0.60 | 0.72 | 0.96 | 1.20 |
| 20 m² | 0.80 | 0.96 | 1.28 | 1.60 |
| 25 m² | 1.00 | 1.20 | 1.60 | 2.00 |
| 40 m² | 1.60 | 1.92 | 2.56 | 3.20 |
| 60 m² | 2.40 | 2.88 | 3.84 | 4.80 |
| 100 m² | 4.00 | 4.80 | 6.40 | 8.00 |
Bedding sand only. Mortar sand is separate: about 0.016 m³ of sand per m² of brickwork in a 1:4 mix.