How many bags of concrete do I need?
Short answer: bags = cubic feet of concrete ÷ the bag's yield, rounded up. An 80 lb bag of QUIKRETE Concrete Mix yields about 0.60 cubic feet. A 10 × 10 ft slab 4 inches thick holds 33.3 cubic feet, so it takes 56 bags of 80 lb mix before any extra.
1. What are you pouring?
The extra starts at 0% because the bag maker does not publish a waste allowance. Uneven ground, a thick edge or a spill all use more. Round up, and buy a bag or two more than the count below.
2. How much concrete you need
- Cubic feet
- 33.33
- Cubic yards
- 1.23
- 80 lb bags
- 56
no extra added
27 cubic feet per yard
0.60 cu ft each
| Bag size | Yield per bag | Bags needed |
|---|---|---|
| 40 lb | 0.3 cu ft | 112 |
| 50 lb | 0.375 cu ft | 89 |
| 60 lb | 0.45 cu ft | 75 |
| 80 lb | 0.6 cu ft | 56 |
How this is calculated
- Slab: cubic feet = length × width × (thickness in inches ÷ 12).
- Round hole or tube: cubic feet = 3.1416 × (diameter in inches ÷ 24)² × depth in feet × count.
- Cubic yards = cubic feet ÷ 27.
- Bags = cubic feet × (1 + extra %) ÷ yield per bag, rounded up to a whole bag.
- Yields are QUIKRETE Concrete Mix figures: 0.30, 0.375, 0.45 and 0.60 cu ft for 40, 50, 60 and 80 lb bags. Other brands can differ, so check your bag.
- For post holes, the post itself takes up some space, so the count runs slightly high. That gives you a small buffer.
The formula
- Slab: length (ft) × width (ft) × thickness (in) ÷ 12 = cubic feet.
- Round hole or tube: 3.1416 × (diameter in inches ÷ 24)² × depth in feet = cubic feet per hole. Multiply by the number of holes.
- Cubic yards: cubic feet ÷ 27.
- Bags: cubic feet ÷ yield per bag, then round up to a whole bag.
How much one bag makes
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Yields are from the QUIKRETE Concrete Mix data sheet. Bags per cubic yard is 27 cubic feet divided by the yield. Other brands and mixes can yield a little more or less, so check the bag you buy.
Worked examples
| Project | Cubic feet | Cubic yards | 40 lb | 50 lb | 60 lb | 80 lb |
|---|---|---|---|---|---|---|
| 10 × 10 ft slab, 4 in thick | 33.33 | 1.23 | 112 | 89 | 75 | 56 |
| 10 post holes, 10 in wide × 36 in deep | 16.36 | 0.61 | 55 | 44 | 37 | 28 |
No extra is added here. Each 10 × 36 in hole holds about 1.64 cubic feet, or 2.7 bags of 80 lb mix. If you mix one hole at a time and open whole bags, plan on 3 bags per hole, which is 30 bags rather than 28.
Tips for post holes
QUIKRETE's data sheet says to dig the hole about 3 times the post's diameter. It also says the hole should be 1/3 of the post's total height. Wait 24 hours before putting any strain on the post. For load-bearing work, follow your local building code for footing size.
Safety
The QUIKRETE label warns that wet concrete causes severe skin burns and serious eye damage. Wear impervious gloves, such as nitrile, plus eye protection and a dust mask. Breathing the dry dust over time can cause lung damage or cancer. Read the Safety Data Sheet before you start, and wash wet concrete off skin right away.
Common questions
How many 80 lb bags are in a cubic yard?
45 bags. A cubic yard is 27 cubic feet, and each 80 lb bag yields about 0.60 cubic feet.
Should I add extra for waste?
The bag maker does not publish a waste figure, so the calculator starts at 0%. Uneven ground and spills use more, so round up and keep a spare bag or two.
Are bigger bags a better deal?
Yield scales with weight, so 80 lb bags mean fewer bags to carry and open. Smaller bags are easier to lift and handle alone.
Does the post take up space in the hole?
Yes. The calculator treats the hole as full of concrete, so it slightly overestimates. That small buffer helps with uneven hole walls.