How Much Concrete Do I Need? Complete Guide
From a shape and a measurement to an order quantity, step by step
Start with the shape
Every concrete estimate starts by identifying the shape you're pouring: a rectangular slab, a round column (a sonotube), a footing with a different width than the wall above it, or a fence-post hole. The shape determines the formula — a slab is length × width × depth, a round column is π × radius² × height, and so on.
Get the exact volume
With the shape identified, measure carefully and calculate the raw geometric volume in a single consistent unit. This is the 'exact' figure — the amount of concrete the shape mathematically contains, with no allowance for anything that happens on the actual pour day.
Add a waste allowance
Real pours lose material to an uneven subgrade, spillage, over-excavation, and formwork that isn't perfectly plumb. A waste allowance of 5-10% is standard practice for a straightforward slab or footing, and it's added on top of the exact volume, not baked into it silently. Rougher ground or a first-time DIY pour often justifies the higher end of that range.
Round to what you can actually order
Ready-mix suppliers typically sell in fractions of a cubic yard (often ¼-yard increments) and usually charge a short-load fee below a minimum order (often around 1 yard, though this varies by supplier). If your waste-adjusted figure is 1.63 yd³, most suppliers round that up to 1.75 yd³ — check with your specific supplier rather than assuming a number.
Worked Example
A 10 ft × 12 ft slab, 4 inches thick, with a 10% waste allowance.
Exact volume: 10 × 12 × (4/12) = 40 ft³ = 1.48 yd³
With 10% waste: 1.48 × 1.10 = 1.63 yd³
Rounded to the supplier's ¼-yard increment: 1.75 yd³
Assumptions & Limitations
- Waste-allowance ranges here are general estimating practice, not a fixed rule any supplier enforces.
- Short-load fees and order increments vary by region and supplier — confirm both before finalizing an order.
FAQ
- Should I always add the same waste percentage?
- No — a smooth, formed slab on graded ground can use less (5%), while a footing poured into a hand-dug, irregular trench often needs more (10%+). BuildFactor's calculators let you change the percentage and always show the exact figure alongside it.
- Is it cheaper to under-order and get a second small load if I run short?
- Usually not. A second delivery typically costs more per yard than ordering slightly extra the first time, on top of the delay to the pour. Rounding up to the next order increment is standard practice for exactly this reason.
Related Calculators
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From the Blog
Sources
NIST SP 811
Unit conversion reference (cubic feet, cubic yards, cubic metres).
NIST SP 811 — reference