How Much Concrete Do I Need? A Practical Guide to Ordering

The basic formula

Concrete is sold by volume: cubic yards in the US and cubic metres almost everywhere else. Multiply length × width × depth with all measurements in the same unit. In feet, divide cubic feet by 27 to get cubic yards. For example, a 10 × 12 ft patio that is 4″ (0.333 ft) deep needs 10 × 12 × 0.333 = 40 cu ft, or 1.48 cu yd.

Always order extra

Sub-bases are never perfectly flat, forms bow slightly and some concrete is always left in the chute and the wheelbarrow. Order 5–10% more than the calculated volume. Running short mid-pour can leave a cold joint, which is a weak line where fresh concrete meets concrete that has already started to set.

Bags or ready-mix?

An 80 lb bag of concrete makes about 0.6 cu ft, so one cubic yard takes about 45 bags. That's a lot of mixing. As a rule of thumb, bags make sense for post holes, small pads and repairs under about 1 cu yd (0.75 m³). Above that, a ready-mix truck is faster, more consistent and often no more expensive once you count your time.

Typical thicknesses

Common minimum thicknesses for residential work:

• Sidewalks, patios, shed bases: 4″ (100 mm)
• Driveways and garage floors: 5–6″ (125–150 mm)
• Strip footings: often 8–12″ deep (200–300 mm) and 16–24″ wide, but check your local building code
• Hot tub pads: 5–6″ (125–150 mm) with reinforcement

Footings and post holes

For a strip footing, treat it as a long, narrow slab: length × width × depth. For round post holes, the volume is π × radius² × depth for each hole. A 10″ diameter hole 3 ft deep holds about 1.6 cu ft, which is roughly three 60 lb bags.

Calculate it in seconds

Our concrete calculator handles imperial and metric units, multiple identical pours, waste allowance, bag counts and rebar, and gives you a cost estimate so you can compare a bagged-mix job with ready-mix delivery.