How to use the concrete calculator
- Pick the shape. Use Slab, footing or wall for any rectangular pour — patios, driveways, sidewalks, shed bases, strip footings and walls. Use Round column for sonotubes, deck footings and fence post holes, and Steps for a solid flight of stairs.
- Enter the dimensions. Length and width go in feet (or meters); thickness goes in inches (or centimeters). For several identical slabs or holes, set the quantity instead of multiplying yourself.
- Keep a waste allowance. 5–10% covers uneven subgrade, form flex and spillage. Use the higher end when the ground hasn't been compacted or graded flat.
- Read the result. Order ready-mix by the cubic yard, or buy the number of pre-mixed bags shown for smaller jobs.
Concrete volume formula
Concrete is sold by volume, so every estimate comes down to length × width × depth, with all three in the same unit. Because thickness is usually measured in inches, divide it by 12 to get feet, then divide cubic feet by 27 to get cubic yards.
For round columns and post holes, the area of the circle replaces length × width:
Steps are treated as a stack of slabs: the first step is one rise tall, the second is two rises tall, and so on. The calculator adds those blocks together for you.
Worked example: a 12 × 16 ft patio
A patio 12 ft long and 16 ft wide at the standard 4 in thickness works out as 12 × 16 × 4 ÷ 12 = 64.00 cubic feet. Divide by 27 and you get 2.37 cubic yards. Adding 10% for waste brings the order to 2.61 cubic yards — so you would ask for 2.75 yards of ready-mix, or buy 118 bags of 80 lb mix.
How many bags of concrete do I need?
Pre-mixed bags list their yield on the label. The standard figures used by this calculator are:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
As a rule of thumb, once a job needs more than about one cubic yard (45 bags of 80 lb mix), ready-mix delivered by truck is usually less work and often cheaper than mixing bags by hand. Many suppliers charge a short-load fee for very small truck orders, so for pours under a yard, compare both options.
Concrete slab chart
Quick reference for common slab sizes, before any waste allowance:
| Slab size | 4 in thick (yd³) | 6 in thick (yd³) | 80 lb bags at 4 in |
|---|---|---|---|
| 10 × 10 ft | 1.23 | 1.85 | 56 |
| 10 × 12 ft | 1.48 | 2.22 | 67 |
| 12 × 12 ft | 1.78 | 2.67 | 80 |
| 12 × 20 ft | 2.96 | 4.44 | 134 |
| 16 × 16 ft | 3.16 | 4.74 | 143 |
| 20 × 20 ft | 4.94 | 7.41 | 223 |
| 24 × 24 ft | 7.11 | 10.67 | 320 |
| 30 × 30 ft | 11.11 | 16.67 | 500 |
Post holes and round columns
For a hole or tube 4 ft deep, these are the volumes per hole:
| Diameter | Cubic feet | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 8 in | 1.40 | 3 | 4 |
| 10 in | 2.18 | 4 | 5 |
| 12 in | 3.14 | 6 | 7 |
| 16 in | 5.59 | 10 | 13 |
| 18 in | 7.07 | 12 | 16 |
Holes dug by hand are rarely perfect cylinders — they flare out at the top. Add 10–15% when filling earth-formed holes rather than tubes.
How thick should a concrete slab be?
Typical thicknesses for residential work are:
- Sidewalks, patios and shed bases: 4 in.
- Driveways for cars: 4–5 in; 6 in where trucks, RVs or heavy equipment will park.
- Garage floors: 4–6 in, depending on the load.
- Footings: set by your local building code and the structure they carry — check with your building department.
Thickness has the biggest effect on volume: going from 4 to 6 in adds 50% more concrete, so measure the depth of your forms rather than guessing.
Tips for an accurate order
- Measure the forms after they're set, not the plan drawing — real forms are rarely exact.
- Check the subgrade depth in several places. A slab that is 4 in at the edges and 5 in in the middle needs about 25% more concrete than planned.
- Split complicated shapes into rectangles, calculate each one, and add them up.
- Running short mid-pour causes cold joints; ordering slightly too much is the cheaper mistake.
Frequently asked questions
How many cubic yards of concrete do I need for a 10×10 slab?
At 4 in thick, a 10 × 10 ft slab needs 1.23 cubic yards (33.33 cubic feet) before waste, or about 1.36 yards with a 10% allowance. At 6 in thick it needs 1.85 cubic yards.
How many 80 lb bags of concrete make a yard?
An 80 lb bag yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet, so you need 45 bags of 80 lb mix per cubic yard. With 60 lb bags you need 60, and with 40 lb bags you need 90.
How much concrete do I need for a fence post?
It depends on the hole size. A 10 in diameter hole 3 ft deep holds about 1.64 cubic feet — roughly 3 bags of 80 lb mix per post, minus the small volume of the post itself. Use the Round column option above with your hole size.
How much extra concrete should I order?
Order 5–10% more than the calculated volume. Use 10% or more if the ground is uneven, the forms are long and flexible, or you are filling earth-formed holes.
Is it cheaper to use bags or ready-mix concrete?
For small jobs under about one cubic yard, bags are usually more practical. Above that, ready-mix tends to be cheaper per yard and far less labor, though many suppliers add a short-load fee for small orders. Enter your local price per yard to compare.
Can I use this calculator for a concrete wall or footing?
Yes. A wall or strip footing is a rectangular block: enter its length, its height as the width, and its thickness. For a footing, enter its length, width and depth.