Concrete Measurement Calculator

Enter the three measurements you actually take on site — length, width, and thickness — and get cubic yards, bag counts, and cost. Planning quantities only; it does not replace a structural engineer or building code.

Units:

Length (ft)

Width (ft)

Thickness (in)

Price per cubic yard ($)

Average ready-mix concrete costs $120-$200 per cubic yard.

Enter dimensions above to see your concrete estimate.

From tape measure to cubic yards

Concrete is sold by the cubic yard in the United States. Your tape measure gives feet and inches. The conversion that most DIY orders get wrong is thickness: a 4 inch slab is not “4” in the formula — it is 4 ÷ 12 = 0.333 feet.

Cubic feet = Length (ft) × Width (ft) × Thickness (ft)
Cubic yards = Cubic feet / 27

27 is 3 × 3 × 3: one cubic yard is a 3-foot cube. The calculator above does the unit conversion for you. If you work in metric, switch to meters and centimeters; the result still shows cubic meters and an equivalent cubic-yard total.

Worked measurement: 10 × 12 ft patio, 4 inches thick

Thickness: 4 / 12 = 0.333 ft
Cubic feet: 10 × 12 × 0.333 = 40
Cubic yards: 40 / 27 = 1.48 cubic yards
10% waste: 1.48 × 1.10 = 1.63 cubic yards
80-lb bags (0.60 cu ft each): 40 / 0.60 ≈ 67 bags, or about 74 bags with waste

At 1.63 cubic yards you are already past a comfortable bag mix. Ready-mix is usually the better buy above 1–2 cubic yards, even after a short-load fee.

Irregular layouts

Break L-shapes, driveways with aprons, and thickened edges into rectangles (or a rectangle plus a round pad). Run the calculator once per section, write down each cubic-yard result, then add them. Do not enter an “average” thickness for a slab that is 4 inches in the field and 12 inches at the garage door.

Waste, bags, and ready-mix

Add 5–10% to the mathematical volume. Uneven subgrade, form bow, and the last of the chute account for the extra. Order the plant’s next increment (often a quarter yard) rather than the exact 1.48.

Bag size	Yield	Bags per cubic yard
40 lb	~0.30 cu ft	~90
60 lb	~0.45 cu ft	~60
80 lb	~0.60 cu ft	~45

Use bags for fence posts, small pads, and repairs under about 1 cubic yard. Use a truck for slabs, driveways, and foundation pours. Mix strength (PSI), slump, and air entrainment still come from the project specs — this page only converts measurements to volume.

Limits

The calculator does not check bearing capacity, frost depth, rebar, or permit rules. If the pour is structural, have the dimensions confirmed on drawings before you call the plant.

Measurement FAQ

Why divide by 27?

A cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. Ready-mix dispatchers take the order in cubic yards, so cubic feet from the tape measure must be divided by 27.

Do I enter thickness in inches or feet?

In imperial mode the slab thickness field is inches. The calculator divides by 12 internally. Length and width stay in feet. Mixing those units in a spreadsheet is the usual source of a 12× error.

How much extra should I order?

Add 5–10% for waste. On a 1.48 cubic yard patio, 10% is 1.63 cubic yards; round up to the supplier’s increment so the chute does not run empty.

When are bags cheaper than a truck?

Usually under 1 cubic yard, or when you cannot get a truck to the pour. Above 1–2 cubic yards, bag count, mixing time, and cold-joint risk all favor ready-mix.

Can I use this for footings as well as slabs?

Yes. The same length × width × depth / 27 formula applies. Switch to the footing calculator for pier and strip examples, or keep this page if you only need the unit conversion.

Related calculators

How much concrete do I need? — step-by-step formula guide
Slab calculator — patios, driveways, and garage floors
Footing calculator — strips and piers
10x12 slab — pre-filled 10 × 12 × 4 in example
Slab cost guide — 2026 pricing