Concrete Calculator

How much concrete do I need?

Concrete is sold by volume, and in the US that volume is the cubic yard — not the cubic foot, and never the bag:

area (ft²)        = length (ft) × width (ft)
volume (ft³)      = area × (thickness in ÷ 12)
volume (yd³)      = volume (ft³) ÷ 27
bags              = volume (ft³) ÷ yield per bag, rounded up

Worked example, the page default — a 10 × 10 ft patio, 4 inches thick:

1. Area: 10 × 10 = 100 ft²
2. Volume: 100 × (4 ÷ 12) = 33.33 ft³
3. Cubic yards: 33.33 ÷ 27 = 1.23 yd³
4. Add 10% waste: 36.67 ft³ ÷ 27 = 1.36 yd³ → 62 bags of 80 lb mix

A cubic yard is 27 cubic feet because it's a 3 × 3 × 3 ft cube — worth remembering when a supplier quotes per yard and your tape measure gives feet.

Why you order more than the math says

The geometric volume is the floor, not the order. Three things push the real number up:

• Sub-grade isn't flat. Over-excavation and low spots are the biggest single cause of a short pour.
• Forms spread and the surface is never level. A 4" slab finished slightly thick across 100 ft² quietly eats another few bags.
• Trucks don't deliver partial yards cheaply. Ordering 1.23 yd³ usually means paying for 2 yd³ anyway, plus a short-load fee on small pours.

That's why the default waste allowance here is 10% and why the headline number is the order volume, not the exact one. Drop it to 5% for a tightly formed, well-compacted pad; raise it to 15% for a rough grade or a first-time pour.

Bags or ready-mix?

Both cost columns use the page's placeholder prices — $150 per cubic yard delivered and $5.50 per 80 lb bag — and they exclude sub-base, reinforcement, and your labor. Replace them with a local quote before you commit; the comparison is only as good as the prices you type in. Rough rule of thumb: below about 1 yd³, bags usually win on money; above it, the truck wins on money and on your weekend.

The three inputs people get wrong

• Thickness. 4" is right for a patio or walkway; 6" is the usual minimum where a vehicle will park. Going 4" → 6" on the default slab is 1.23 → 1.85 yd³, a 50% jump in concrete for a 2-inch change.
• Units. Inches become feet by dividing by 12 — writing 6 inches as 0.6 ft instead of 0.5 ft overstates the thickness by 20%, and 10 ft 6 in is 10.5 ft, not 10.6 ft.
• Shape. A round patio isn't the square that encloses it: a 10 ft circle is 78.5 ft², not 100 ft² — 21% less concrete. Pick "circle" in the calculator and enter the diameter.

Reinforcement doesn't change the volume, but it changes the job: wire mesh or rebar sits inside the pour, so plan the thickness around it, not after it.

How to use this calculator

1. Pick the shape — rectangle for slabs, driveways, and walkways; circle for round patios and piers.
2. Enter the dimensions in feet and the thickness in inches. The defaults are a 10 × 10 ft slab at 4 inches.
3. Set the waste allowance (10% by default).
4. Read the order volume in cubic yards, then the bag counts for 80/60/40 lb mix.
5. Enter your local prices to compare a delivered truck against buying bags.

Frequently asked questions

How many bags of concrete do I need for a 10×10 slab?

At 4 inches thick, 56 bags of 80 lb mix exactly, or 62 with a 10% waste allowance — about 1.36 cubic yards. With 60 lb bags it's 82; with 40 lb bags, 123.

How much does one cubic yard of concrete cover?

One cubic yard is 27 cubic feet: 81 ft² at 4 inches thick, or 54 ft² at 6 inches. Coverage drops fast as thickness rises.

Should I pour 4 or 6 inches?

4 inches is standard for patios, walkways, and shed floors. 6 inches is the common minimum for driveways and anything carrying vehicle loads. On the default 10 × 10 slab, that choice moves you from 1.23 to 1.85 cubic yards.

Is it cheaper to buy bags or order a truck?

With the placeholder prices here, bags are cheaper for small pours — roughly under a cubic yard — because trucks charge a short-load fee on small orders. Above that, delivered ready-mix wins on price and saves the mixing. Put your real quotes into the calculator to check.

How much waste should I add?

10% is the normal allowance for a formed slab on a prepared base. Use 5% on a tight, well-compacted pad and 15% on rough grade, odd shapes, or a first pour.

Does rebar or mesh change how much concrete I need?

No — reinforcement displaces a negligible volume. It changes the thickness you should plan for, since mesh and bar need to sit inside the slab rather than on the ground.

How do I estimate a round patio or a pier?

Use the circle option: area = π × radius², which the calculator applies as diameter² × π/4. A 10 ft round patio is 78.5 ft² and needs 0.97 yd³ at 4 inches (1.07 yd³ with waste) — about 48 bags of 80 lb mix.

Data sources: Volumes are pure geometry from the dimensions you enter (27 ft³ = 1 yd³). Bag yields are the manufacturers' nominal figures — 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³ (Quikrete/Sakrete published yields). The prices shown are placeholders, not market quotes; replace them with your supplier's numbers. Every figure on this page was produced by this calculator and can be reproduced with the same inputs. This is an estimating tool, not a structural specification — check load, thickness, and reinforcement requirements with your local building code. Funding a larger project? Compare the numbers with the refinance calculator. Related tools: all home project calculators, percentage increase calculator, mm to inches, all finance calculators.