The calculation
Length × width × depth gives slab volume. The allowance is added before volume is divided by the bag yield and rounded up.
Concrete / slab estimate
Concrete calculator
Estimate slab volume and full premix bags from finished dimensions, product yield, waste, and local shelf price.
Volume is simple. Real yield belongs on the bag.
Use the product yield
The 0.60 ft³ default matches a common 80 lb concrete mix, but bag sizes and products vary. Replace it with the label. Source
Before you pour
Check the form, base, and batch size.
Measure finished depth
Confirm excavation, compacted base, drainage, reinforcement, and local requirements before calculating concrete.
Read the yield line
Bag weight alone is not yield. Use the mixed-volume figure from the exact product data sheet.
Compare delivery
A high bag count may be slower or more expensive than ready-mix. Use the order-volume result when requesting quotes.