Concrete Calculator

Quick reference for the most common DIY concrete projects. All values include the standard 10 percent waste factor. For other dimensions, use the calculator above.

Bags-only is fine up to about 1 cubic yard (30 bags of 80 lb). Above that, order ready-mix from a truck — usually $150-200/yard plus delivery.

Concrete is measured by volume. For a flat slab, the formula is length × width × thickness. The hard part is keeping the units straight: in the US and Canada, contractors order in cubic yards (1 cu yd = 27 cu ft = 0.765 cu m). In the UK, EU, and most of the world, it is cubic metres. Bags of pre-mixed concrete are sold by weight, but each weight covers a specific volume.

Our concrete calculator does all the math for you. Enter your dimensions, pick a project type (slab, footing, or round column), and we instantly show cubic yards, cubic feet, cubic metres, and exactly how many 40, 60, and 80 lb bags you need. We also include a waste factor (the industry standard 10%) so you do not run short mid-pour.

For a flat slab or footing

volume = length × width × thickness. Convert thickness from inches to feet by dividing by 12, then divide the cubic feet result by 27 to get cubic yards. Example: a 10 ft × 10 ft × 4 in slab is 10 × 10 × (4/12) = 33.33 cu ft = 1.23 cu yd. Add 10% waste and you should order 1.36 cubic yards.

For a round column or pier

volume = π × radius² × height. A 12 in diameter column 6 feet tall is π × 0.5² × 6 = 4.71 cu ft = 0.17 cu yd. Sonotube post holes for fence and deck footings use this formula.

Bag yield

Pre-mix concrete bags are standardized:

• 40 lb bag yields about 0.30 cubic feet
• 60 lb bag yields about 0.45 cubic feet
• 80 lb bag yields about 0.60 cubic feet

For projects under 1 cubic yard (about 27 cu ft), bags are usually cheapest. Above 1 cubic yard, ready-mix delivery from a truck almost always beats bagged on price and is far less labor.

Rule of thumb: if your project needs more than 30 bags of 80 lb concrete, call your local ready-mix supplier for a quote. The truck delivery almost always pays for itself in time saved alone, even before the price difference.

1. 1

   Prep and form

   Excavate 2–4 in deeper than your slab thickness, set 2x4 forms to the perimeter, and check level. Add 2 in of gravel as a base and tamp it down.
2. 2

   Reinforce

   Lay wire mesh or rebar grid 2 in above the gravel using rebar chairs. This dramatically extends the slab's lifespan.
3. 3

   Order or mix

   Order ready-mix the day before, or pre-position bags within 10 ft of your mixer. Have a backup mixer or extra hands ready.
4. 4

   Pour and screed

   Pour from one corner and work backward. Use a 2x4 longer than the form width to screed the surface flat in 3 ft passes.
5. 5

   Float and finish

   Bull-float when the surface bleeds (sheen disappears). Edge with a concrete edger, broom finish for non-slip, and let cure.
6. 6

   Cure

   Keep moist for 7 days with plastic sheeting or daily misting. Full strength reaches in 28 days.

How many bags of 80 lb concrete in a yard?

45 bags of 80 lb concrete equals one cubic yard. That is why bagged mix becomes uneconomical above 30 bags or so — the labor of mixing each one starts to dwarf the cost.

How thick should a concrete slab be?

4 inches for patios, walkways, and shed pads. 5 inches for driveways and structural slabs. 6 inches for heavy loads (trucks, RVs).

Why is ready-mix sold by the yard?

Historical: most US batch plants set up around the cubic yard decades ago and never changed. In countries that switched to metric (UK, EU, Canada in metric mode, Australia), concrete is ordered by cubic metre. 1 cubic yard = 0.765 cubic metres.

Can I mix bagged concrete in a wheelbarrow?

One or two bags at a time, yes. For three or more bags at once, rent a portable mixer ($40–60 per day). The mixer pays for itself in better consistency and saves your back.