How Much Concrete Do I Need?
To find how much concrete you need, multiply length by width by thickness to get the volume, convert it to cubic yards by dividing by 27, and add about 10% for waste. Here is the formula in plain terms, worked examples for slabs, footings and columns, and a calculator that does it for you.
Key takeaways
- Volume in cubic yards = (length x width x thickness in feet) divided by 27.
- Always add 5 to 10% for waste and round ready-mix up to the nearest quarter yard.
- A 10x10 ft slab at 4 inches thick needs about 1.36 cubic yards, or 62 bags of 80 lb mix.
- One cubic yard takes roughly 45 bags of 80 lb mix, 60 bags of 60 lb, or 90 bags of 40 lb.
The simple formula
Concrete is measured by volume. For any rectangular pour the volume in cubic feet is length times width times thickness, with everything in feet. Since thickness is usually given in inches, divide it by 12 first. Then convert cubic feet to cubic yards, the unit ready-mix is sold in, by dividing by 27.
Volume (ft³) = Length(ft) × Width(ft) × (Thickness(in) ÷ 12)Cubic yards = Volume(ft³) ÷ 27Always add a waste allowance of 5 to 10%, because the ground is never perfectly flat and you cannot top up a pour once the truck leaves. Rounding ready-mix up to the nearest quarter yard is normal too, since that is how it is delivered.
How much concrete for a slab
A slab is the most common pour. Take the area in square feet and multiply by the thickness in feet. A 10 by 10 foot slab at 4 inches is 100 times 0.333, which is about 33.3 cubic feet, or 1.23 cubic yards before waste and 1.36 with 10%. The concrete slab calculator handles slabs directly, including the bag counts.
| Slab (4 in) | Cubic yards (+10%) | 80 lb bags |
|---|---|---|
| 8 × 10 ft | 1.09 | 49 |
| 10 × 10 ft | 1.36 | 62 |
| 12 × 12 ft | 1.96 | 88 |
| 20 × 20 ft | 5.43 | 245 |
Footings and round columns
A strip footing is a long prism: length times width times depth, with the width and depth usually in inches. A 40 foot footing 16 inches wide and 8 inches deep is about 35.6 cubic feet, or 1.45 cubic yards with waste. The footing calculator does this for you.
A round column or pier is different: its volume is pi times the radius squared times the height. A 12 inch tube 4 feet tall holds about 3.14 cubic feet, roughly 6 bags of 80 lb mix. Use the sonotube calculator for round forms.
How many bags make a cubic yard
A cubic yard is 27 cubic feet. The yield printed on pre-mixed concrete is about 0.60 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag, and 0.30 for a 40 lb bag. So it takes roughly 45 bags of 80 lb mix to make a cubic yard, 60 bags of 60 lb, or 90 bags of 40 lb.
That is a lot of mixing, which is why anything over about a cubic yard is usually ordered as ready-mix.
Tips to avoid running short
The most common reason people come up short is forgetting the waste, or measuring the thickness wrong. A slab poured a half inch over its nominal thickness across a large area is a surprising amount of extra concrete, and a soft or uneven subgrade quietly swallows more. Add 5 to 10% and round up, every time.
The second mistake is mixing units. Keep length and width in feet and thickness in inches, and convert the thickness by dividing by 12 before you multiply. If you would rather not juggle the conversions, the calculator keeps them straight for you.
If your pour is not a simple rectangle, break it into rectangles, find each volume, and add them. A round pad or column is different: its area is pi times the radius squared, so use the round-shape calculator rather than forcing a circle into a rectangle.
Let the calculator do it
Doing this by hand is fine for one simple shape, but it is easy to slip a decimal. Enter your length, width and thickness in the concrete calculator and it returns the cubic yards, the number of 40, 60 and 80 lb bags, whether to order ready-mix, and an optional cost. For an L-shaped or stepped pour, split it into rectangles, run each, and add the results.