Concrete & Masonry Calculators

Everything here is a volume problem

Concrete work is one calculation wearing different hats: area × depth, converted to cubic yards by dividing by 27. A slab, a footing, a post hole and a column are the same sum with different shapes. What changes between the calculators is the geometry and the rounding, not the maths.

The error that dominates is unit mixing. Depth is quoted in inches and area in feet, so 4 inches has to become 0.333 ft before you multiply. Forget it and the answer is twelve times too big, which at least announces itself; use 0.4 instead of 0.333 and it is 20% high, which does not.

Bags or ready-mix: where the line falls

This is the decision the calculators exist to settle, and the numbers make it unambiguous once you see them together.

Forty-five bags of 80 lb mix is 3,600 lb to carry, and each one needs mixing and placing before the last one starts to set. Around one cubic yard is where ready-mix stops being extravagant and starts being cheaper as well as easier — and a truck will usually have a minimum load charge below that, which is worth asking about before you commit to bags.

Masonry is counted per unit area

Brick, block and pavers work the other way around: rather than a volume, you need a count, and the count comes from the unit's face size plus its mortar joint. A standard modular brick with a 3/8 in joint runs about 7 per square foot of wall face; a nominal 8 × 8 × 16 in block is 1.125 per square foot.

The joint is not a rounding detail. It is deliberately included in the nominal size, which is why a "4 inch" brick is really 3 5/8 in of brick plus 3/8 in of mortar. Calculate on the actual unit plus joint and the count comes out right; calculate on the nominal size alone and it will not.

Pavers add one more layer underneath. A paver patio is really three calculations stacked: the pavers themselves, the bedding sand, and the compacted base beneath that, which is usually 4 to 6 inches of crushed stone for a walkway and more under a driveway.

Getting the order right

• Order 10% over on a slab, and more on uneven subgrade. A slab that runs out mid-pour gets a cold joint, which is a permanent weakness rather than a cosmetic one.
• Footings follow the code, not the calculator. Depth is set by your local frost line, and that is a number to get from the building department.
• Check the bag yield printed on the bag. Yields differ between mixes and manufacturers, and every calculator here lets you enter your own.
• Buy the whole pour at once. Cement colour varies between batches, and on exposed concrete it shows.