Concrete Calculator
Estimate concrete volume, waste, bag quantities and project cost for slabs, walls, footings, columns and post holes.
Add one or more areas, then select Calculate.
Recommended concrete
Includes 10% waste across all project areas.
Area Breakdown
| Area | Exact Volume |
|---|
Premixed Bag Estimate
| Bag Size | Approx. Bags |
|---|---|
| 40 lb | 0 |
| 50 lb | 0 |
| 60 lb | 0 |
| 80 lb | 0 |
| 20 kg | 0 |
| 25 kg | 0 |
| 30 kg | 0 |
| 40 kg | 0 |
Use a custom bag yield
Cost Comparison
Ready-Mix
Bagged Concrete
How we calculated it
Bag yields are approximate defaults and vary by manufacturer. Verify bag yield, concrete specifications and local project requirements before purchasing materials.
About the Concrete Calculator
Planning a concrete pour starts with one practical question: how much mixed concrete will the finished shape actually occupy? This calculator turns slab, footing, wall, column or post-hole dimensions into cubic volume, converts that volume into cubic yards or cubic metres and can add a realistic waste allowance before you order.
Concrete is expensive to run short on once a pour has started, but ordering far too much is also costly. Use the calculator as a quantity-planning tool, then compare the result with the supplier’s minimum order, delivery increments and the actual site conditions.
How to Use the Concrete Calculator
- Choose the concrete shape that matches the work you are pouring, such as a slab, wall, footing, column or post hole.
- Enter the finished dimensions using the units shown. For slabs, make sure the thickness is the actual concrete thickness rather than the excavation depth.
- Add a waste allowance only after the basic dimensions are correct. Uneven subgrade, spillage and over-excavation can increase the amount needed.
- Calculate the volume, then compare the cubic-yard or cubic-metre result with the quantity your ready-mix supplier sells or the yield printed on bagged concrete.
How the Calculation Works
For a rectangular slab or footing, the base volume is length × width × thickness. If thickness is entered in inches while length and width are in feet, thickness is first converted to feet by dividing by 12. Other shapes use the appropriate geometric formula before all sections are added together.
After the net volume is calculated, cubic feet can be converted to cubic yards by dividing by 27. A waste allowance is then applied as volume × (1 + waste rate). Bag estimates use the selected bag yield rather than assuming every bag size produces the same volume.
Worked example
Concrete Calculator Worked Example
20 ft × 30 ft slab, 4 inches thick
A 4-inch thickness is 4 ÷ 12 = 0.333 ft. The slab volume is 20 × 30 × 0.333 ≈ 200 ft³. Divide by 27 to convert to cubic yards: 200 ÷ 27 ≈ 7.41 yd³. With 10% extra, 7.41 × 1.10 ≈ 8.15 yd³. The planning quantity is therefore about 8.15 cubic yards before supplier rounding.
Result guide
Understanding Your Concrete Calculator Result
Treat the calculated volume as the mathematical quantity required by the dimensions. If the number is close to a supplier break point, consider the consequences of being slightly short versus slightly over. For bagged concrete, round up to whole bags and use the manufacturer’s stated yield.
Before you rely on the answer
Assumptions and Limitations
- The dimensions describe the finished concrete shape and use compatible units.
- The waste percentage is an allowance, not a substitute for measuring an irregular excavation.
- Bag yields and ready-mix delivery rules vary by product and supplier.
Practical tip
Using This Result in Practice
Measure thickness in several places if the subgrade is uneven. For larger pours, confirm the quantity with the ready-mix supplier and keep separate volumes for stairs, thickened edges or footings instead of hiding them inside one rectangle.
Related calculations
Useful Calculations to Continue With
If you only know the floor dimensions, use the Square Footage Calculator first to confirm the plan area before adding concrete thickness.
If the pour sits on a compacted stone base, estimate that layer separately with the Gravel Calculator rather than including it in the concrete volume.
Common questions
Concrete Calculator FAQ
How much extra concrete should I order for waste?
There is no universal percentage. Small straightforward slabs may need only a modest allowance, while irregular excavation, pumping, spillage or difficult access can justify more. Measure carefully and confirm the final order with the supplier.
Why does the calculator convert cubic feet to cubic yards?
Ready-mix concrete in the United States is commonly sold by the cubic yard. One cubic yard equals 27 cubic feet, so the calculator divides the cubic-foot volume by 27.
Should I round a concrete order up?
For a real pour, running short is usually more disruptive than having a small excess. Check the supplier’s order increments and discuss borderline quantities with them before ordering.