Formula
How the concrete estimate works
- Volume = slab length × slab width × slab depth
- Adjusted volume = volume × (1 + extra allowance %)
- Premix bags = adjusted volume ÷ yield per bag, rounded up
Free project planning tool
Concrete calculator
Find concrete volume, premix bag quantity, bag cost, and an optional ready-mix estimate.
✓ Metric & imperial✓ Transparent assumptions✓ No signup
Slabs & padsInstant estimate
Example calculation
A 12 × 10 ft slab at 4 in deep is 40 cubic feet before extra allowance. Add 5% and the order volume is about 1.56 cubic yards. Bag quantity depends on the yield printed on the bag.
Planning tips
- Always enter slab depth in inches or centimetres as shown beside the field.
- Replace the default bag yield with the value printed on your premix bag.
- For structural work or ready-mix delivery, confirm volume and specifications with the supplier.
Common questions
Concrete calculator FAQ
How much extra concrete should I order?
Five to ten percent is often used for minor grade variation, spillage, and measurement error, but the project conditions should guide the allowance.
Why is bag yield editable?
Premix yield changes by bag weight and product. Using the manufacturer’s stated yield produces a more reliable bag count.
Can I use this for footings or columns?
This version calculates rectangular slabs and pads. Break a project into rectangular sections and add the results, or have structural quantities verified professionally.
Planning guides
Understand the estimate
01Estimating checklistA repeatable checklist for measuring a project, verifying product coverage, rounding packages, pricing the order, and reviewing the result.→02Material waste allowanceChoose a waste allowance based on cuts, breakage, patterns, package sizes, and project complexity instead of one universal percentage.→03Methodology & assumptionsHow BuildWise calculators handle measurements, units, product coverage, waste, package rounding, cost, examples, and limitations.→