Slabs & footings
Concrete Calculator
Estimate a rectangular slab, pad, or footing and convert volume into complete premix bags using the yield on your product.
How to calculate it
- Measure finished length and width.
- Enter compacted slab depth—not excavation depth.
- Use the yield printed on the exact bag size you plan to buy.
What the calculator is doing
Volume = length × width × finished depth; required volume = volume × (1 + waste %); bags = ceil(required volume ÷ yield per bag).
- Imperial depth is converted from inches to feet and cubic feet to cubic yards.
- Bag yield is a cured-volume estimate, not the dry volume of the package.
- The tool plans material only; it does not size a slab, reinforcement, joints, or subbase.
Buying checklist
- Verify finished dimensions from the plan or formwork.
- Read yield on the exact bag weight and mix.
- Check whether the supplier sells by cubic yard or cubic metre.
- Plan subbase, reinforcement, joints, water, access, and placement separately.
Common mistakes
Using excavation depth
Aggregate subbase is not concrete; use only the specified finished concrete thickness.
Ignoring an uneven subgrade
Low spots consume real volume even when the average design thickness looks correct.
Choosing mix by bag price alone
Strength, exposure class, aggregate, working time, and specification come before yield cost.
Frequently asked questions
How much extra concrete should I allow?+
Five to ten percent is common for small slabs because excavation and subgrade are rarely perfectly uniform.
Can I use this for structural foundations?+
Use it only for quantity planning. Foundation dimensions, reinforcement, mix design, and bearing requirements must follow local plans and codes.