Project guide
Concrete & masonry: volume first, then bags, tonnes, and bricks
Concrete and masonry are ordered by volume and weight, and both are expensive to get wrong in either direction. These calculators turn your dimensions into cubic metres or yards, then into the bags or loads you will actually buy.
Which calculator to use when
What the project involves
Most small concrete jobs follow the same pattern. You dig out, lay and compact a granular sub-base, set up formwork, then place concrete to a planned depth. Volume is simply length times width times depth, but the depth must be the finished concrete thickness, not the depth you dug, and an allowance for uneven ground and spillage is normal. For small pours, premix bags are practical; for larger slabs, ready-mix by the cubic metre or yard is usually cheaper and gives a more consistent result.
Post holes are a special case: the hole is a cylinder, and the post takes up part of it, so the concrete you need is the hole volume minus the post. Brickwork is counted per square metre of wall face, with the joint thickness changing how many bricks fit, and mortar estimated from a per-area rule of thumb that you can adjust for your bricks and bond.
Order of work
- Decide the finished size and thickness of the slab, wall, or post holes, following your plans or local guidance.
- Size the compacted sub-base under slabs and paths with the Gravel calculator, using its own depth.
- Estimate slab concrete with the Concrete Slab calculator and compare premix bags against a ready-mix delivery.
- For fence, deck, or gate posts, use the Post Concrete calculator with your hole diameter, depth, and post size.
- For a brick wall, use the Mortar & Brick calculator with your brick size and joint thickness, and order a few spare bricks.
Common mistakes
- Entering the excavation depth instead of the finished concrete thickness, or forgetting the sub-base takes up part of the dig.
- Using a bag count without checking the yield printed on the bag; yields differ widely between products.
- Ordering ready-mix with no margin: running short mid-pour leaves a cold joint that is hard to hide.
- Calculating post concrete as if the hole were empty, which overstates bags for wide posts.
- Treating these estimates as structural design. Thickness, reinforcement, and footings belong to plans or a qualified engineer.
Common questions
FAQ
Should I buy premix bags or order ready-mix?
Bags suit small pours and places a truck cannot reach. Once the slab needs well over a cubic metre (roughly 1.3 cubic yards), ready-mix is usually cheaper per unit and pours in one continuous batch. The Concrete Slab calculator shows both the volume and the bag count so you can compare quotes.
How much extra concrete should I allow?
An allowance of around 5–10% is common for small slabs to cover uneven ground, formwork flex, and spillage. Rough or soft ground can need more. The calculators let you set the percentage instead of hiding it.
Do I need gravel under a concrete slab?
A compacted granular sub-base is usual under patios, paths, and shed bases to give a stable, free-draining layer. Its depth depends on ground conditions and the load; use the Gravel calculator once you know the depth from your plans or supplier.
Does the mortar calculator work for block walls?
It is set up for bricks by default, but you can enter block dimensions and adjust the mortar-per-square-metre rule of thumb. Check the mortar quantity against the block manufacturer guidance.