How to Use
Select the shape that matches your pour: slab, cylinder, tube, stairs, or curb and gutter.
Choose your unit system and enter all dimensions in that same unit.
Set a waste factor (10 percent is standard) and select your preferred bag size.
Optionally enter a price per bag to get a cost estimate.
Click Calculate to see volume, weight, and bag count instantly.
How Volume Is Calculated per Shape
Slab and rectangle pours use length multiplied by width multiplied by depth. Solid cylinders and posts use the standard circle area formula where volume equals pi times the radius squared times the depth, with the diameter halved to get the radius. Hollow tubes subtract the inner circle area from the outer circle area before multiplying by depth. Stairs use rise multiplied by run multiplied by width multiplied by the number of steps multiplied by 0.5, which treats each step as a triangular prism. Curb and gutter sections multiply the rectangular cross section area (height times width) by the total run length.
Example: Calculating a Backyard Slab
- Slab dimensions: 12 ft long, 10 ft wide, 4 inches thick.
- Convert thickness to feet: 4 inches / 12 = 0.333 ft.
- Volume = 12 x 10 x 0.333 = 40 ft³.
- Add 10 percent waste: 40 x 1.10 = 44 ft³.
- Bags needed at 80 lb (0.60 ft³ each): ceil(44 / 0.60) = 74 bags.
- At $6.50 per bag: estimated cost = $481.
When to Add Extra Waste
- Uneven or unprepared subgrade that may absorb more material
- Complex forms with internal angles where concrete can pool
- Stair pours where triangular approximation may undercount
- Long curb runs where form flex can increase cross section
- Any pour where a second batch mid job would cause a visible cold joint
Results Are Estimates
Calculated volumes are geometric approximations. Actual concrete needed depends on subgrade conditions, form tightness, mixing water ratio, and contractor practice. Always verify quantities with your supplier or a licensed contractor before purchasing materials for large pours.
Premix Bags vs. Ready Mix Delivery
Key Features
Five Shape Types
Covers the most common concrete pours: rectangular slabs, solid cylinders, hollow tube forms, stair stringers, and curb with gutter sections.
Imperial and Metric
Switch between inch/foot and millimetre/centimetre/metre input. Results display in both cubic feet and cubic metres regardless of input unit.
Bag Count by Size
Calculates bags needed for 80 lb, 60 lb, 40 lb, and 50 kg premix bags so you can compare what is available at your supplier.
Waste Factor
Adds a configurable percentage to account for spillage, uneven substrate, and form leakage. Industry standard is 5 to 10 percent.
Cost Estimate
Enter a price per bag to instantly see your projected material cost based on bag count.
Privacy
All calculations run entirely in your browser. No dimensions or project data are sent to any server.
Frequently Asked Questions
Why does the stair formula use 0.5?
Each step is approximated as a triangular prism where the cross section is a right triangle formed by the rise and run. The 0.5 factor reflects that a triangle is half the area of the enclosing rectangle. This is an industry standard estimate and slightly underestimates volume, so adding a 10 percent waste factor is especially recommended for stairs.
What unit should I enter dimensions in?
Use any single unit consistently within a calculation. For imperial, enter everything in inches or everything in feet. For metric, enter everything in millimetres, centimetres, or metres. The calculator converts internally based on the unit system you select, not a specific subunit.
How accurate is the bag count?
Bag counts are based on manufacturer stated yields: 0.60 ft³ for 80 lb bags, 0.45 ft³ for 60 lb bags, 0.30 ft³ for 40 lb bags, and approximately 0.033 m³ for 50 kg metric bags. Real yield varies slightly with water ratio and mixing method, so the waste factor accounts for this.
Should I order more than the calculator says?
The waste factor handles most overordering needs. For large pours or ready mix deliveries, contractors typically add 8 to 10 percent. For small DIY projects with premix bags, buying one or two extra bags is common practice since unused sealed bags can often be returned.