How to use the concrete calculator
Set the thickness, extra and stone base in the top bar; they apply to every slab. Add each slab in feet under How much to get cubic yards and bags, and click a slab for its own sheet of forms, stakes and joints. Lay it out walks through each step, and Order it tells you what to say to the plant.
How it's figured
Concrete is sold by volume. The calculator multiplies the area by the thickness and converts the answer. Area is in square feet, thickness is in inches, and one cubic yard is 27 cubic feet.
A 10 by 10 foot patio at 4 inches is 100 square feet. Times 4 divided by 12 gives 33.3 cubic feet. Divide by 27 and you get 1.23 cubic yards. With 5 percent extra that is 1.30 cubic yards, and a plant that sells in quarter yards would deliver 1.5. A circle uses π × radius squared, so a 10 foot circle is 78.5 square feet.
Bags come from each bag's yield. An 80 pound bag makes about 0.60 cubic feet, a 60 pound bag 0.45 and a 40 pound bag 0.30. The calculator divides your volume, extra included, by the yield and rounds up. It does not allow for compaction or settling, because concrete is poured and screeded, not compacted. The extra covers spillage, bowed forms and low spots. The stone base figure is compacted volume, so order it with the gravel calculator.
| Bag size | Yield | Bags in one cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Imperial or metric
The Imperial | Metric switch at the top of the calculator sets the whole page. It opens in the system your browser's region uses and remembers your pick. In metric, slab sizes are in meters (or centimeters or millimeters, under Measure in), thickness and base are in centimeters, and the answers are in cubic meters, square meters and kilograms. A typed unit always wins, so 12 ft and 3.5 m both work either way. Numbers you already typed keep their size when you switch.
The math is the same in both. One inch is exactly 2.54 centimeters, so a foot is exactly 0.3048 meters and a cubic yard (27 cubic feet) is exactly 0.764554857984 cubic meters; a pound is exactly 0.45359237 kilograms. A 3 by 3 meter patio at 10 centimeters is 0.9 cubic meters, or 0.95 with 5 percent extra. Bags keep the size they are sold in: an 80 pound bag (about 36 kg) makes about 17 liters, a 60 pound bag (about 27 kg) about 12.7 liters and a 40 pound bag (about 18 kg) about 8.5 liters. In metric the page counts the bags sold by the kilogram instead: 20 kg (the UK, Australia and New Zealand), 25 kg (the UK and Europe) and 30 kg (Canada), at the same yield per kilogram as the US bags. Metric bag counts assume about 0.0094 m³ per 20 kg bag (0.0117 per 25 kg, 0.0140 per 30 kg); check your bag's label. A cubic meter takes about 107 of the 20 kg bags, 86 of the 25 kg or 72 of the 30 kg. Code figures stay in inches as the IRC writes them, with the metric size in brackets.
How thick and how strong
The International Residential Code, section R506, sets a minimum of 3½ inches for a slab on the ground and calls for a 4 inch base of clean sand or stone under a slab that sits below grade. Four inches is standard for patios and walkways. Driveways that carry cars are usually 5 to 6 inches. Strength is ordered separately. Table R402.2 lists 2,500 psi for basement and inside slabs and, in its severe weathering column, 3,500 psi for garage floors, porches and steps exposed to the weather, with 5 to 7 percent air where freezing and thawing is expected. The code does not cover patios and driveways that are not part of the house, and many suppliers suggest 4,000 psi with air for them in cold climates. Your building department decides which edition applies.
Forms, steel, joints and slope
Forms are lumber set on stakes with the top edge at the finished surface, so the form must be at least as deep as the slab: a 2×4 holds 3½ inches, a 2×6 holds 5½ and a 2×8 holds 7¼. Drive a stake about every 3 feet and at each corner. Steel, if you use it, sits on chairs near the middle of the slab, not on the ground. This page does not size reinforcement, so ask your supplier or building department.
Concrete shrinks as it cures, and control joints tell it where to crack. Space joints, in feet, at 2 to 3 times the slab thickness in inches, so a 4 inch slab gets joints every 8 to 12 feet, and cut them one-quarter of the thickness deep. Keep panels close to square, with the long side no more than about 1½ times the short side. A hard surface within 10 feet of the house must slope at least 2 percent away from it, which is ¼ inch per foot (IRC R401.3).
Ordering ready-mix or bags
The Order it tab lists what to tell the plant. To compare with bags, an 80 pound bag costs its price ÷ 0.60 × 27 per yard. In metric, pick your bag size beside its price: a 25 kg bag costs its price ÷ 0.0117 per cubic meter. Past about a yard, ready-mix is usually worth it: 45 bags take hours to mix.
Where the numbers come from
- 27 cubic feet per yard. A cubic yard is 3 ft by 3 ft by 3 ft, so 27 is a definition.
- Bag yields and water. The 0.30, 0.45 and 0.60 cubic foot yields and about 3 quarts of water per 80 pound bag are from the bag makers' data sheets. Check your bag, because brands differ.
- Weight. Normal concrete weighs about 150 pounds per cubic foot, or about 4,050 pounds per cubic yard.
- Thickness, base, strength and slope. IRC R506, Table R402.2 and R401.3, as above.
- Joints. The 2 to 3 times rule and the one-quarter depth come from Portland Cement Association and American Concrete Institute guidance.
- Prices. Prices come from you. The calculator does not guess what concrete costs where you live.
What else it works for
The same math covers shed, hot tub and air conditioner pads, sidewalks, garage floors, a basketball court or a stair landing. Deck piers have their own calculator, linked below.
Tips for a good pour
- Compact the base in layers. Spread the stone 2 to 3 inches at a time and run a plate compactor over each layer. A soft spot becomes a crack.
- Dampen the base before the truck comes. Dry stone pulls water out of the bottom of the slab. Mist it so it is damp, with no puddles.
- Have the crew and tools ready before the truck arrives: helpers, a wheelbarrow, a screed board, a float, an edger and a groover.
- Screed, float, then wait. Strike it off with a straight board, float it once, and wait for the surface water to go away before you edge, groove and broom it.
- Don't add water to make it flow. Each extra gallon weakens the slab; ask about a plasticizer.
- Keep it damp while it cures. Mist it or cover it with plastic for the first week, and keep cars off a new driveway for about a week.
- Watch the weather. Below about 40°F or above about 90°F, ask your supplier about a cold- or hot-weather mix and how to protect the slab.
Common mistakes
- Measuring thickness at the thick edge of a sloped pour. If the base is level and only the top slopes, the average thickness is the thin edge plus half the drop. The Slope step shows that number.
- Forgetting the base. It is a separate material, ordered by the cubic yard.
- Ordering the exact amount. A short load costs a second trip and a cold joint, so keep the extra.
Questions
How many bags of concrete are in a cubic yard?
About 45 of the 80 pound bags, 60 of the 60 pound bags, or 90 of the 40 pound bags, at yields of 0.60, 0.45 and 0.30 cubic feet.
How thick should a patio slab be?
Four inches is standard for patios and walkways. Driveways carrying cars are usually 5 to 6 inches, and the IRC minimum for a slab on the ground is 3½ inches.
How much concrete do I need for a 10×10 slab?
At 4 inches thick, about 1.23 cubic yards, or 1.30 with 5 percent extra.
How much does a cubic yard of concrete weigh?
About 4,050 pounds, using 150 pounds per cubic foot. Normal concrete runs between about 140 and 150 pounds per cubic foot.
Can I use metric?
Yes. Press Metric at the top of the calculator: sizes go in meters, thickness in centimeters, and you get cubic meters, square meters and kilograms. One cubic yard is 0.7646 cubic meters. Bags are counted in 20, 25 and 30 kg sizes; metric bag counts assume about 0.0094 m³ per 20 kg bag, so check your bag's label.
Do I need rebar?
That depends on the use, the soil and your local code. The calculator does not size reinforcement. Ask your supplier or building department.