Concrete Calculator
Estimate slab concrete in cubic feet and cubic yards, plus planning equivalents for common 40, 60, and 80 lb bags.
Instant estimate · No sign-up · Your measurements stay in your browser
Project details
Enter your measurements
Enter your project measurements. Planning defaults are identified where used. After your first calculation, edits update the estimate automatically.
Project estimate
Results
Enter your measurements, then select Calculate.
How to use this calculator
Enter slab length, width, and thickness to compare a ready-mix cubic-yard estimate with common packaged-concrete bag counts.
- Measure the inside dimensions of the form.
- Enter slab thickness and its unit carefully.
- Set a waste allowance for grade and form variation.
- Verify bag yield on the chosen product or discuss the cubic-yard result with a ready-mix supplier.
Formula and methodology
Slab volume = length × width × thickness. Divide cubic feet by 27 for cubic yards. The recommendation adds waste; bag counts divide recommended cubic feet by centralized approximate yields and round up.
Important: This is a volume estimate, not structural, engineering, code, or placement advice. Product yields and field conditions vary.
Worked example
A 10 ft × 10 ft slab at 4 in thick needs about 33.33 cu ft, or 1.23 cu yd. With 10% waste, plan for about 1.36 cu yd.
Before you buy
- Confirm excavation, base, reinforcement, joints, and thickness with an appropriate project professional.
- Ready-mix suppliers may have minimum orders and delivery requirements.
- Check the exact yield on packaged concrete before purchasing.
Frequently asked questions
Why do concrete bag estimates vary?
Dry bag weight does not itself define finished volume. Mix design and manufacturer-stated yield can vary, so the product label is authoritative.
Are partial concrete bags included?
No. Each bag estimate rounds up to a whole bag after adding the selected waste allowance.
Is this suitable for footings or structural design?
It estimates rectangular volume only. It does not determine dimensions, reinforcement, strength, code compliance, or structural suitability.