How to use this concrete calculator
- Choose Slab, patio or footing for anything rectangular, or Round column or post hole for cylinders such as fence posts and sonotubes.
- Enter the measurements. Each one has its own unit, so you can enter length in feet and thickness in inches, or use metres and centimetres.
- Set How many? if you are filling several identical holes or pads, such as 8 fence post holes.
- Keep the 10% extra unless you have a reason to change it. Running short in the middle of a pour is much worse than having a bag left over.
- Open Cost estimate and enter local prices to compare bags against ready-mix.
Concrete volume formulas
Convert every measurement to the same unit first. Most mistakes come from multiplying feet by inches.
Round column / hole: Volume = π × (Diameter ÷ 2)² × Depth
Cubic yards = Cubic feet ÷ 27
Bags = Volume ÷ bag yield (round up)
Worked example: a 10 × 10 ft patio, 4 inches thick
- Convert the thickness: 4 in ÷ 12 = 0.333 ft
- Volume: 10 × 10 × 0.333 = 33.3 cu ft
- In cubic yards: 33.3 ÷ 27 = 1.23 yd³ (0.94 m³)
- Add 10% extra: 33.3 × 1.10 = 36.7 cu ft
- 80 lb bags (0.6 cu ft each): 36.7 ÷ 0.6 = 61.1, so buy 62 bags
Bags of concrete per cubic yard and cubic metre
These figures use the yield printed on common pre-mixed bags. Metric bag yields are approximate and vary by product, so always check the label.
| Bag size | Yield per bag | Bags per yd³ | Bags per m³ |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 118 |
| 50 lb | 0.375 cu ft | 72 | 95 |
| 60 lb | 0.45 cu ft | 60 | 79 |
| 80 lb | 0.60 cu ft | 45 | 59 |
| 20 kg | ≈ 0.0093 m³ | ≈ 83 | ≈ 108 |
| 25 kg | ≈ 0.0116 m³ | ≈ 67 | ≈ 87 |
Typical concrete thickness
| Project | Common thickness |
|---|---|
| Walkway or garden path | 4 in (10 cm) |
| Patio | 4 in (10 cm) |
| Shed base | 4 in (10 cm) |
| Car driveway | 4–6 in (10–15 cm) |
| Garage floor | 4–6 in (10–15 cm) |
| Hot tub pad | 4–6 in (10–15 cm) |
These are common starting points, not engineering advice. Building codes, soil, frost depth and loads vary, so check local rules for structural work.
Concrete for fence post holes
A common rule of thumb is a hole about three times the width of the post, and deep enough to hold about a third of the post’s length, going below the frost line where the ground freezes. Remember that the post itself takes up space. For a precise figure, subtract the post’s volume from the hole volume, or simply treat the extra 10% as your buffer.
Example: 8 holes, each 10 in wide and 24 in deep, need about 1.09 cu ft each, or 8.7 cu ft in total before subtracting the posts. That is 15 bags of 80 lb concrete, or 16 bags with 10% extra.
Tips for a successful pour
- Prepare the base. A compacted gravel sub-base helps drainage and stops the slab from settling unevenly. Our gravel calculator works out how much base stone to order.
- Check your forms are level and braced. Forms that bow outward quietly add volume.
- Watch the weather. Avoid pouring in freezing conditions or heavy rain, and keep fresh concrete damp while it cures.
- Have help ready. Bagged concrete is heavy. An 80 lb bag is a lot to lift dozens of times, so line up a helper or a mixer for bigger jobs.
- Buy a bag or two extra. Most stores accept unopened returns, which is far easier than a mid-pour trip.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
About 45 bags. A typical 80 lb bag of pre-mixed concrete makes 0.6 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45 bags.
How many 60 lb or 40 lb bags make a cubic yard?
About 60 bags of 60 lb concrete (0.45 cu ft each) or 90 bags of 40 lb concrete (0.30 cu ft each). Yields can vary slightly by brand, so check the bag.
How much concrete do I need for a 10x10 slab?
A 10 ft × 10 ft slab that is 4 inches thick needs 33.3 cubic feet, or about 1.23 cubic yards. That is 56 bags of 80 lb concrete, or about 62 bags once you add 10% extra for spillage and uneven ground.
How thick should a concrete slab be?
Around 4 inches (10 cm) is common for patios, walkways and shed bases. Driveways and slabs carrying heavier loads are often 5–6 inches (12–15 cm). Local building codes and the load on the slab decide the right thickness.
Should I use bags or order ready-mix concrete?
Bags are handy for small jobs such as post holes, steps and small pads. For larger pours, often anything over about 1 cubic yard (0.75 m³), mixing dozens of bags gets slow and tiring, so it is worth getting a quote from a local ready-mix supplier.
How much does a cubic yard of concrete weigh?
Roughly 4,000 lb (about 1,800 kg). In metric terms, normal concrete weighs about 2,400 kg per cubic metre.
How much extra concrete should I order?
Most people add 5–10% to cover spillage, an uneven sub-base and forms that bow slightly. The calculator uses 10% by default; set it to 0 if you only want the exact volume.
Last reviewed November 5, 2026.