Concrete Calculator: How Much Concrete Do I Need?

How to measure for a slab, pad, or walkway

1. Measure length and width in feet. Measure the inside of the forms, since that's the space the concrete fills. Convert inches to decimal feet by dividing by 12, so 10 ft 6 in is 10.5 ft.
2. Choose the thickness in inches. Most patios and walkways are 4 inches. Driveways for heavy vehicles and small slabs that carry weight go thicker. See the thickness table below.
3. Check the base for low spots. Stretch a string across the forms and measure down to the compacted base in several places. If the depth varies, use the average, or raise the extra percentage.
4. Split odd shapes into rectangles. An L-shaped patio is two rectangles, so run each one through the calculator and add the results. For a round pad, the area is 3.1416 × radius × radius.
5. Set the extra percentage. The default 10 percent matches Sakrete's advice to add 10 percent to every estimate. Quikrete lists its bag yields as approximate, so a little extra covers spills and a base that isn't perfectly flat.

Bag yields and bags per cubic yard

Yields are from the Quikrete Concrete Mix data sheet. Sakrete lists its 90 lb bag a little lower, at 0.66 cu ft, so always check the yield printed on the bag you buy. Bags per cubic yard is simply 27 ÷ the yield.

How thick should the concrete be?

Thickness drives volume fast. Here's what 100 sq ft takes at common thicknesses, before any extra:

Anything structural, such as footings, a garage slab, or a pad under a building, follows your local building code. Get the thickness, base, and reinforcement from your plans or your building department, not from a rule of thumb.

Bags or ready-mix?

Sakrete's rule of thumb is that a pallet of bags is about a cubic yard, and it takes 45 bags of 80 lb mix to make one. Concrete Network recommends bags for small jobs such as walkways, small patios, and footings, and ordering by the yard for larger jobs such as driveways.

Put the default example in physical terms: 62 bags × 80 lb is 4,960 lb of dry mix to haul, mix, and place in one continuous pour. If the calculator shows more than about a cubic yard, get a ready-mix quote and compare. Ready-mix is sold by the cubic yard, so give the supplier the cubic yards from the calculator along with the slab size and thickness.

Tips and common mistakes

• Measuring the outside of the forms. The form boards take up space. Measure inside them.
• Assuming a perfectly flat base. Concrete Network suggests budgeting as if the slab were 1/4 inch thicker than planned. For the 10 × 10 example, 4.25 inches with 10 percent extra comes to 1.44 cubic yards, or 65 bags of 80 lb mix.
• Forgetting thickened edges. Concrete Network notes that driveway edges are sometimes thickened by 1 to 2 inches for 4 to 8 inches in from the edge. Figure that strip as its own rectangle and add it.
• Skipping control joints. Quikrete recommends control joints about 1/4 of the slab's depth, spaced every 6 to 8 ft in a 4 inch slab, and says patios larger than 10 × 10 ft should have them.
• Pouring dead level. Quikrete generally recommends a slope of 1/4 inch per foot for walks. Its patio plan uses 1/8 inch per foot. Either way, slope away from the house.
• Adding too much water. Quikrete calls for about 6 pints of water per 80 lb bag to start, up to about 9 pints. Stay within the range on the bag.
• Curing too briefly. Quikrete's data sheet calls for 5 days of curing in warm weather (70°F and up) and 7 days between 50 and 70°F. Sakrete says to keep the concrete moist for 3 to 5 days, protect it from freezing for 48 hours, and wait 24 hours for foot traffic and 72 hours for vehicles.
• Running short mid-pour. A few spare bags cost far less than stopping partway through a slab. Round up, not down.