How Much Concrete Do I Need?
A clear, accurate method for estimating concrete for any slab, footing, column, or set of stairs — with the exact formula, a worked example, and a reference table of common slab sizes.
To find how much concrete you need, multiply length × width × thickness (all converted to feet) to get cubic feet, divide by 27 for cubic yards, then add about 10% for waste. A 10 × 10 ft slab poured 4 inches thick works out to 33.3 cubic feet — roughly 1.36 cubic yards with waste, or about 62 × 80 lb bags. Above about one cubic yard, ready-mix delivery is almost always cheaper and easier than mixing bags by hand.
Concrete is unforgiving of a bad estimate. Order too little and the truck leaves before the slab is full, leaving a weak cold joint where the fresh batch meets the set concrete. Order too much and you’ve paid for material that gets shoveled into a wheelbarrow and wasted. The good news is that estimating concrete is straightforward arithmetic once you know the shape you’re pouring and one conversion factor. This guide walks through the method the pros use, then gives you a worked example and a reference table you can copy.
If you’d rather skip the math, the concrete calculator does every step below and hands you both a bag count and a ready-mix figure. Understanding the formula, though, lets you sanity-check any quote a supplier gives you.
Everything is volume × waste
No matter the shape, a concrete estimate is two steps: find the volume, then add a waste margin. Volume is what fills the form; waste covers spillage, subgrade that’s dug a little deep, and the reality that forms are never perfect. The only thing that changes between a slab, a footing, a column, and a staircase is how you calculate the volume.
Concrete is sold by the cubic yard (ready-mix, delivered) or the bag (dry mix you add water to). So the whole job is: get your volume into cubic feet, convert to cubic yards, and decide whether to buy yards or bags.
Step 1: Find the volume
For a slab or footing, volume is simply length × width × thickness — with every dimension in feet:
Volume (cu ft) = length × width × thickness, all in feet
Thickness is the trap here: slabs are measured in inches. A 4-inch slab is 4 ÷ 12 = 0.333 ft thick, not 4. For other shapes the volume formula changes but the idea is identical:
- Round column or post hole:
π × (diameter ÷ 2)² × height - Stairs:
width × run × rise × [ n(n+1) ÷ 2 ], where n is the number of steps
Step 2: Convert cubic feet to cubic yards
There are 27 cubic feet in a cubic yard (3 × 3 × 3). This single conversion is where most DIY estimates go wrong — people order “36 yards” when they meant 36 cubic feet, which is a wildly different truck.
Cubic yards = cubic feet ÷ 27
Step 3: Add waste
Multiply by roughly 1.10 to add a 10% cushion for spillage and over-dug forms:
With waste = volume × (1 + waste %)
Bump that to 15% if the subgrade is rough or hand-dug. This is the number you actually order.
A fully worked example
Say you’re pouring a 10 ft × 10 ft slab, 4 inches thick:
- Volume:
10 × 10 × 0.333 = 33.3 cu ft - Cubic yards:
33.3 ÷ 27 = 1.23 cu yd - With 10% waste:
1.23 × 1.10 = 1.36 cu yd
That 1.36 cubic yards is your order. If you priced it in bags instead, an 80 lb bag yields 0.60 cu ft, so 36.7 ÷ 0.60 ≈ 62 bags. Sixty-two bags is a brutal day of mixing — which is exactly why anything over about a cubic yard should be ready-mix delivery. Round ready-mix up to the next quarter yard and order 1.5 cubic yards.
Reference: concrete for common slab sizes
These assume a 4-inch slab with 10% waste, the most common patio and shed-floor pour. Confirm any real job with the concrete calculator, which also handles footings, columns, and stairs.
| Slab size | Volume | Cubic yards (with waste) | 80 lb bags | Order as |
|---|---|---|---|---|
| 8 × 8 ft | 21.3 cu ft | 0.87 cu yd | 40 bags | Bags |
| 10 × 10 ft | 33.3 cu ft | 1.36 cu yd | 62 bags | Ready-mix (1.5 yd) |
| 10 × 12 ft | 40.0 cu ft | 1.63 cu yd | 74 bags | Ready-mix (1.75 yd) |
| 12 × 12 ft | 48.0 cu ft | 1.96 cu yd | 89 bags | Ready-mix (2 yd) |
| 16 × 20 ft | 106.7 cu ft | 4.35 cu yd | 196 bags | Ready-mix (4.5 yd) |
| 20 × 24 ft | 160 cu ft | 6.52 cu yd | 294 bags | Ready-mix (6.75 yd) |
Bags or ready-mix?
The crossover is roughly one cubic yard. Below it, bagged mix is convenient and there’s no delivery minimum to worry about. Above it, the bag count explodes and hand-mixing becomes impractical — a single cubic yard is already 45 bags of 80 lb mix. For the full breakdown of how many bags each size yields and when a short-load delivery beats bags, see Concrete Bags per Cubic Yard.
Don’t let thickness fool you
The most expensive estimating error isn’t the area — it’s the depth. Bumping a slab from 4 inches to 5 adds 25% more concrete, and a form that bows outward or a subgrade dug an inch too deep quietly inflates the volume. Those mistakes, plus the classic cubic-feet-vs-cubic-yards mix-up, are covered in Concrete Slab Thickness & Estimating Mistakes.
Once you’ve got your number, plug your real dimensions into the concrete calculator for a waste-adjusted estimate in both bags and ready-mix yards that you can save or share.