Concrete Slab Calculator
Enter the length, width and thickness of a slab to get the volume in cubic yards and cubic feet, and how many 40, 60 or 80 lb bags of concrete you may need, with a waste allowance included. Everything runs in your browser.
Enter your measurements and press Calculate.
How to use this concrete slab calculator
- Choose imperial or metric. Imperial uses feet for length and width and inches for thickness. Metric uses meters for length and width and centimeters for thickness.
- Enter the length, width and thickness of the slab. For an odd shape, split it into rectangles and add the results.
- Keep the 10% waste allowance or change it. Waste covers spills, uneven ground and a little extra.
- Press Calculate or Enter. You get the volume with and without waste, and bag counts for 40, 60 and 80 lb bags. Reset clears everything.
The concrete slab formula
A slab is a flat box, so its volume is length x width x thickness. In imperial units the thickness is in inches, so it is divided by 12 to get feet:
- Cubic feet = length x width x (thickness / 12)
- Cubic yards = cubic feet / 27
- Metric: cubic meters = length x width x (thickness / 100), then cubic feet = cubic meters x 35.3147 and cubic yards = cubic meters x 1.30795
- With waste: multiply the volume by (1 + waste / 100)
- Bags = the volume with waste in cubic feet divided by the bag yield, rounded up to a whole bag
How many bags of concrete do I need?
The calculator uses common bag yields of 0.30 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag and 0.60 for an 80 lb bag. Bag yields differ between brands, so check the label on the bag you plan to buy. The table shows how many bags make up one cubic yard at those yields.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cubic feet | 90 |
| 60 lb | 0.45 cubic feet | 60 |
| 80 lb | 0.60 cubic feet | 45 |
Worked example
A slab is 10 ft long, 10 ft wide and 4 in thick. Cubic feet = 10 x 10 x (4 / 12) = 33.33, which is 33.33 / 27 = 1.23 cubic yards. With 10% waste the volume is 36.67 cubic feet, or 1.36 cubic yards. Dividing 36.67 by the bag yields and rounding up gives 123 bags of 40 lb, 82 bags of 60 lb or 62 bags of 80 lb concrete.
When is ready-mix the better choice?
Mixing bags by hand is practical for small jobs such as a pad or a few steps. As the volume grows, the number of bags, the mixing time and the lifting add up quickly. Many people start to consider ready-mix delivery once a job gets to roughly a cubic yard or more. Talk to a local supplier about minimum loads, prices and what suits your project.
Assumptions & limits
- This is an estimate only. Real slabs vary, and the ground, the base, the thickness you choose and any reinforcement change how much concrete you need. Ask a local professional before you order or pour.
- The calculator treats the slab as one flat rectangle with even thickness. It does not account for footings, edges, slopes or forms.
- Bag yields shown are common figures only. Check your bag label.
- Local rules and requirements differ, so confirm them with your local authority or supplier. Results are rounded for display only.
Frequently asked questions
How many bags of concrete are in a cubic yard?
At the yields used here, a cubic yard is about 90 bags of 40 lb, 60 bags of 60 lb or 45 bags of 80 lb concrete.
Why add a waste allowance?
Spills, uneven ground and measuring errors all use extra concrete. Ten percent is a common starting point, and you can change it.
Why are bag counts rounded up?
You cannot buy part of a bag, and running short mid-pour is a bigger problem than having a little left over.
What thickness should my slab be?
That depends on the use, the ground and local requirements, so ask a local professional. The calculator works with whatever thickness you enter.
Can I use metric?
Yes. Pick metric, enter meters and centimeters, and the results also show cubic meters.
Does this include rebar or gravel?
No. It only estimates the concrete volume of the slab itself.
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Last updated: October 2026