How the calculation works
Ready-mix concrete is sold by the cubic yard (or cubic metre). Bagged mix is sold by weight, and each bag lists how much volume it makes once mixed. The calculator finds the volume of your slab or holes, adds overage, and divides by the bag yield.
Worked example
A 12 × 10 ft patio is 120 ft². At 4 inches thick that is 40 ft³, or 1.48 cubic yards. Add 10% and you need 44 ft³: about 1.6 cubic yards of ready-mix, or 74 bags of 80 lb mix.
Bag yields
| Bag | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
| 30 kg | 0.50 ft³ (0.014 m³) | 54 |
These are typical yields for standard concrete mix. Fast-setting and high-strength mixes can differ, so check the yield printed on your bag.
How thick should concrete be?
| Project | Depth | Mix |
|---|---|---|
| Patio | 4 in · 10 cm | Concrete |
| Sidewalk | 4 in · 10 cm | Concrete |
| Shed base | 4 in · 10 cm | Concrete |
| Driveway | 4–6 in · 10–15 cm | Concrete |
| Hot tub pad | 4–6 in · 10–15 cm (check the tub manual) | Concrete |
| Fence posts | Hole about 3× the post width | Concrete |
Slabs go on a compacted gravel base, usually 4 inches deep. Work that out with the gravel calculator. Footings and anything structural should follow your local building code.
Post holes and other round pours
Choose "Circle", enter the hole diameter and depth, and put the number of holes in "How many". A common rule of thumb is a hole about three times the width of the post, and a third of the post's length deep, or below the frost line where the ground freezes.
Bags or ready-mix?
Bags make sense for post holes and small pads. Above about a cubic yard, mixing that many bags by hand or with a small mixer is slow, and a ready-mix truck is usually easier. Many suppliers charge extra for small loads, so ask about minimums when you call.
Common questions
How many bags of concrete make a yard?
About 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix. A typical 80 lb bag yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet.
How thick should a concrete patio be?
Four inches is standard for patios, sidewalks and shed bases. Driveways are usually 4 to 6 inches, thicker where heavy trucks will park.
How much concrete do I need for a fence post?
A 10 in hole 36 in deep holds about 1.6 cubic feet, a little under three 80 lb bags. The post takes up some of that space, so the estimate leaves you a small margin.
How much does a yard of concrete weigh?
About 4,000 lb, or 2 tons. Normal-weight concrete is roughly 150 lb per cubic foot.
Should I order extra concrete?
Yes. Add 5 to 10% for spills, form movement and uneven ground. Running short in the middle of a pour leaves a cold joint, which is harder to fix than a little leftover.
Sources: bag yields from bagged concrete product data such as QUIKRETE Concrete Mix; normal-weight concrete is about 150 lb/ft³. More on how we calculate.