How the calculation works
The calculator converts every length into meters. Rectangular volume is length times width times depth, while cylindrical volume is pi times radius squared times depth. Repeated shapes are multiplied by quantity, declared post displacement is subtracted, waste is applied, and adjusted volume is divided by explicit bag yield and rounded up.
US example
A 10 ft by 10 ft slab at 4 in thick has an exact volume of 33.33 cu ft, or about 1.23 cu yd. With 10% waste, it needs 36.67 cu ft before dividing by bag yield.
Metric example
A 3 m by 3 m slab at 10 cm thick has an exact volume of 0.9 m³. With 10% waste, it needs 0.99 m³ before dividing by the yield stated for each bag.
Rectangular and cylindrical volume
Use a rectangle for a slab, footing, pad, or other rectangular pour. Use a cylinder for a circular pad or post hole. Advanced mode lets you change shape, repeat identical pours, add another rectangular pour, or subtract the volume occupied by an embedded post.
Thickness and bag yield are separate inputs
Thickness controls geometric volume. Bag yield controls purchase quantity. A correct slab calculation can still produce a wrong bag count if the entered yield does not match the selected mix and bag size. The product data sheet and bag label should take priority over generic assumptions.
Volume units and whole bags
The engine calculates in cubic meters, then displays cubic feet or cubic meters for the active unit system. It can also support cubic yards in detailed formula outputs. The purchase line always rounds up because partial bags are not normally a purchasable unit.
Before a structural pour, confirm design, excavation, base preparation, forms, reinforcement, joints, cure plan, local requirements, and delivery constraints with appropriate project professionals.
Limitations
- This is a quantity estimate and does not choose slab thickness, reinforcement, mix strength, joints, base, drainage, or structural design.
- Form accuracy, uneven excavation, compaction, spillage, pumping, weather, and site access can change the real order.
- Bag yield varies by product and mix, so the calculator intentionally leaves yield blank until the visitor enters it.
Frequently asked questions
Why is concrete bag yield blank?
Bag weight and stated yield differ among products and markets. Enter the yield from the exact bag or technical data sheet you plan to buy.
Can the calculator handle post holes?
Yes. Choose a cylindrical shape and enter its diameter, depth, and quantity. Advanced options can subtract an embedded rectangular post.
Does the result tell me how thick a slab should be?
No. Thickness is an input, not a recommendation. Loads, soil, reinforcement, climate, codes, and professional design can govern the required section.
Sources
Sources support unit conventions, product variability, and project limitations. They do not endorse this calculator.