Skip to content
CalcSpectrum

Concrete Calculator

Calculate how much concrete a rectangular slab needs in cubic feet, cubic yards, and cubic meters, plus the number of 60lb or 80lb bags required.

Free to use · Instant results

Calculator Inputs

Choose the shape of one pour.

The unit system for the dimensions below.

The length of the slab, in feet.

The width of the slab, in feet.

The thickness (depth) of the slab, in inches.

For example 8 post holes of the same size. A whole number of 1 or more.

Extra for spillage, uneven ground and form flex. Many contractors add 5-10%. Leave at 0 for the exact volume.

Optional: the bag size you would buy, for the cost comparison.

Optional: your local price for one bag of the size above. Leave at 0 to skip the comparison.

Optional: your supplier's delivered price per cubic yard (per cubic meter if you chose metric units).

Optional: any flat delivery, small-load or minimum-order charge your supplier quotes.

Concrete Needed
1.23
Volume (cubic feet)
33.33
Volume (cubic meters)
0.94
60lb Bags Needed
75
80lb Bags Needed
56
40lb Bags Needed
112
50lb Bags Needed
89
Order incl. Waste (cubic yards)
1.23
Order incl. Waste (cubic meters)
0.94
Bagged Concrete Cost (your prices)
—
Ready-Mix Cost incl. Fee (your prices)
—
Bags vs Ready-Mix
Enter a bag price and a ready-mix price to compare
Your spreadsheet is generated in your browser.

About these results

CalcSpectrum calculators produce automated estimates using established formulas and the specific values you enter. Because real-world outcomes depend on input accuracy, mathematical assumptions, and rounding conventions, please verify critical figures with an appropriately qualified financial, tax, medical, legal, or other professional, as relevant.

How It's Calculated

Formula

Vft3={Lft×Wft×Tin12slabπ(Din24)2×Hftround×n,bags=⌈V×(1+waste%)yield⌉V_{ft^3} = \begin{cases} L_{ft} \times W_{ft} \times \frac{T_{in}}{12} & \text{slab} \\ \pi \left(\frac{D_{in}}{24}\right)^2 \times H_{ft} & \text{round} \end{cases} \times n, \quad \text{bags} = \left\lceil \frac{V \times (1 + \text{waste}\%)}{\text{yield}} \right\rceil

Concrete needed is the volume of the pour. For a rectangular slab, footing or wall it is length times width times thickness; for a round column or post hole it is pi times the radius squared times the depth. Thickness and diameter are usually measured in inches (or centimeters), so they are converted to feet (or meters) first; metric inputs are converted with the exact definition 1 ft = 0.3048 m. The result is multiplied by the number of identical pours and reported in cubic feet, cubic yards (cubic feet ÷ 27) and cubic meters, since ready-mix is ordered by the cubic yard in the US and by the cubic meter elsewhere. The optional waste allowance only affects the order quantity and bag counts, never the measured volume. Bag counts use the yields published on the QUIKRETE Concrete Mix data sheet — 0.30 cubic feet per 40 lb bag, 0.375 per 50 lb, 0.45 per 60 lb and 0.60 per 80 lb — and round up to whole bags, since partial bags cannot be bought; other brands publish similar but not identical yields. If you enter your own bag price and ready-mix price, the calculator totals both options and says which costs less at those prices. Prices are never filled in for you because they vary by region and supplier, and ready-mix suppliers often add minimum-load or short-load charges, which you can include as the delivery fee. This is a material quantity estimate only: it does not size footings, choose a mix strength or specify reinforcement.

Worked Examples

10ft x 10ft patio slab, 4 inches thick

  1. Thickness in feet: 4 / 12 = 0.3333 ft
  2. Volume: 10 x 10 x 0.3333 = 33.33 cubic feet
  3. Cubic yards: 33.33 / 27 = 1.23 cubic yards
  4. 60lb bags: ceil(33.33 / 0.45) = 75 bags

2ft x 2ft fence post footing, 6 inches thick

  1. Thickness in feet: 6 / 12 = 0.5 ft
  2. Volume: 2 x 2 x 0.5 = 2 cubic feet
  3. 60lb bags: ceil(2 / 0.45) = 5 bags
  4. 80lb bags: ceil(2 / 0.6) = 4 bags

8 post holes, 10 inches wide and 3 feet deep

  1. Radius: 10 / 2 = 5 in = 0.4167 ft
  2. One hole: 3.1416 x 0.4167 x 0.4167 x 3 = 1.636 cubic feet
  3. Eight holes: 1.636 x 8 = 13.09 cubic feet (0.48 cubic yards)
  4. 50lb bags: ceil(13.09 / 0.375) = 35 bags

10ft x 10ft x 4in slab with 10% waste: bags or ready-mix?

  1. Order volume: 33.33 x 1.10 = 36.67 cubic feet (1.36 cubic yards)
  2. 80lb bags: ceil(36.67 / 0.6) = 62 bags; at 6.50 per bag = 403.00
  3. Ready-mix: 1.36 x 150 per cubic yard + 60 delivery fee = 263.70
  4. At these example prices ready-mix costs less; enter your own local prices to compare

Frequently Asked Questions

Why is thickness divided by 12 in the formula?

Length and width are entered in feet, but thickness is typically measured in inches since slabs are usually only a few inches deep. Dividing by 12 converts the thickness to feet so all three dimensions are in the same unit before multiplying them together.

Should I order slightly more concrete than the calculated amount?

Usually, yes. The measured volume is exact geometry; many contractors add 5-10% for uneven subgrade, spillage and form flex. Enter that percentage as the waste allowance and the order volume and bag counts include it, while the measured volume stays unchanged.

Where do the bag yields come from?

From the QUIKRETE Concrete Mix product data sheet: a 40 lb bag yields about 0.30 cubic feet, 50 lb about 0.375, 60 lb about 0.45 and 80 lb about 0.60. Other brands publish similar but not identical yields, so check the bag you buy. Bag counts are rounded up to the next whole bag.

Should I use bags or order ready-mix concrete?

It depends on your local prices and the size of the job. Enter the price of one bag and your supplier's ready-mix price (plus any delivery or short-load fee) and the calculator compares the two totals. Bags tend to suit small pours such as post holes, and a truck tends to suit larger slabs, but the comparison uses only the prices you enter.

Can I use this for footings, sidewalks, or driveways?

Yes — the rectangular shape works for any rectangular concrete pour, and the round shape covers post holes, tube forms and columns. This calculator estimates material quantity only; it does not tell you how big a footing must be or what strength or reinforcement a structure needs.