Start from volume, not from bags
Every concrete estimate begins as a volume, and every mistake begins with a depth. Measure length and width carefully, then treat the depth as the number that decides the job: a 100 mm slab poured 20 mm deep on average — because the sub-base dipped — is 20% more concrete than you planned. Break the pour into rectangles, footings, columns and steps, calculate each separately and add them. Only once you have a defensible total volume should you convert to bags or to a ready-mix order. Bag yields vary widely: a 25 kg bag of dry mix gives roughly 0.012 m³, a 40 kg bag around 0.017 m³, and the printed yield on the sack is the number to trust over any rule of thumb.
Waste, over-order and the ready-mix threshold
Add 5–10% to any concrete volume. The lower end suits a formed pour on a level, compacted base; the upper end suits footings dug in soft ground, where the trench spreads and swallows material invisibly. Below roughly 1 m³ (about 1.3 cubic yards) it is usually cheaper and easier to mix on site from bags. Above that, ready-mix wins on cost, consistency and your back — but a truck typically bills a part-load fee, so it pays to round up to a sensible whole number rather than order 1.9 m³ and hope. If you are pouring in stages, plan the joints before the truck arrives, not while it is turning.
Mix ratios and what the grade actually means
Nominal mixes such as 1:2:4 or 1:1.5:3 give the proportions of cement to sand to coarse aggregate by volume, and map roughly onto grades: 1:3:6 ≈ M10, 1:2:4 ≈ M15, 1:1.5:3 ≈ M20, 1:1:2 ≈ M25. Higher grades mean more cement per cubic metre, not more sand. Because the dry ingredients pack into the voids between each other, wet volume is about 1.5–1.55× the sum of the dry volumes — which is why a cubic metre of finished concrete needs about 1.52 m³ of dry material. Water is the ingredient nobody measures and everybody gets wrong: extra water buys workability and costs strength, permanently.
Bricks, blocks and mortar
Masonry is a per-square-metre problem, not a per-wall problem. A standard UK brick laid in a single-leaf wall with a 10 mm joint works out at about 60 bricks per m²; a double-leaf wall is simply twice that. Standard 440 × 215 mm concrete blocks run about 10 per m². Mortar is the quantity people forget: budget roughly 0.02–0.03 m³ of mortar per m² of single-leaf brickwork, and remember that a 1:4 cement–sand mortar needs about 6 bags of cement per cubic metre. Always add 5% for cuts and breakage — more if the wall has many openings, because every reveal, corner and closer means a cut brick and an off-cut you cannot use.
