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Concrete Bag Calculator
Cubic yards of concrete and 40, 60 and 80 lb bags for a slab or footing.
Bagged concrete is the DIY default for small slabs, footings and post holes — but the bag count for even a modest slab surprises almost everyone. This calculator works out the volume of your pour in cubic feet and yards, then converts it into the three standard bag sizes so you can compare prices per bag fairly.
Each bag yields a fixed volume once mixed: an 80 lb bag gives about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. Bigger bags are cheaper per cubic foot but heavier to lift and mix — a very real consideration when the count passes fifty.
Calculate
Enter your measurements — the estimate updates as you type.
Estimates for planning and buying only. Product coverage varies by brand — check the label of the exact product you buy.
How to use this calculator
- Measure the pour and set the thickness honestly — measure the excavation, including any low spots.
- Add at least 10 percent for an uneven base and spillage; concrete cannot be stretched on the day.
- Compare bag sizes by cost per cubic foot, and be honest about how many heavy bags you can mix in a session.
- If the count passes roughly 80 to 100 bags, price a ready-mix delivery — it is often cheaper and far less brutal.
How concrete quantities are calculated
Volume in cubic feet = length x width x (thickness in inches / 12), increased by your waste percentage. Cubic yards = cubic feet / 27. Bags = volume / yield per bag: 0.60 cu ft for 80 lb, 0.45 for 60 lb and 0.30 for 40 lb bags.
Common mistakes to avoid
- Measuring thickness at the edges only, while the middle of the excavation is an inch deeper — that extra inch on a big slab is another dozen bags.
- Planning to mix a huge count of bags solo. Concrete waits for no one; a slab must be placed and finished while it is workable.
- Forgetting water, tools and curing in the plan: you need mixing water on site, a way to level (screed) and a way to keep the slab damp while it cures.
Frequently asked questions
How many 80 lb bags are in a cubic yard?
About 45 bags: a yard is 27 cubic feet and each 80 lb bag yields 0.60 cubic feet. For 60 lb bags the figure is about 60 bags per yard, and for 40 lb bags about 90.
When is ready-mix cheaper than bags?
Roughly beyond one cubic yard, once you count your labour honestly. Ready-mix also arrives as one consistent batch, which matters for visible slabs. Bags win for small, staged jobs like setting posts.
How thick should a driveway slab be?
Four inches is the minimum commonly used for cars over a well-compacted base, and many builders prefer 5 to 6 inches with reinforcement mesh for longevity and heavier vehicles.
How long before I can walk or drive on new concrete?
Walking is usually fine after 24 to 48 hours. Keep vehicles off for at least 7 days while the slab gains strength — full cure takes about 28 days.
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