Concrete Calculator
Estimate slab concrete in cubic feet and cubic yards, plus planning equivalents for common 40, 60, and 80 lb bags.
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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.
- Use the product-specific QUIKRETE No. 1101 comparisons or enter the approximate bag yield printed on another concrete product.
Formula and methodology
Slab volume = length × width × thickness. Divide cubic feet by 27 for cubic yards. The purchase estimate adds the selected waste allowance. QUIKRETE Concrete Mix No. 1101 comparisons use its approximate 0.30, 0.45, and 0.60 cu ft yields for 40, 60, and 80 lb bags; a custom count instead uses the product yield entered.
Important: This is a volume estimate, not structural, engineering, code, or placement advice. Product yields and field conditions vary.
How accurate is this estimate?
Slab volume comes directly from the entered dimensions and fixed unit conversions. The purchase estimate adds your selected waste allowance. QUIKRETE No. 1101 comparisons and custom-product bag counts depend on approximate product yields, while uneven subgrade, form variation, and spillage can change real usage; confirm the yield printed on your selected concrete product before buying.
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.
- The QUIKRETE comparisons use approximate product-specific yields from the Concrete Mix No. 1101 data sheet; other products may differ and the label on the product you purchase controls.
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 product-specific or custom-yield 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.