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Concrete Calculator — Bags vs Ready-Mix Cost
Turn slab measurements into concrete volume, whole bags and a ready-mix order. Compare supplier quotes including delivery, short-load fees and minimum order volume.
All prices are editable examples, not local quotes. The dimensions describe a rectangular slab or footing; this tool estimates material quantity, not structural design or safe slab thickness.
How to calculate concrete bags
Multiply length × width × thickness after converting to the same unit. Add the selected allowance, then divide by the mixed yield printed on the bag and round up. Dry bag weight alone does not define its mixed yield.
Worked example: a 10 × 10 ft slab at 4 inches is 33.333 ft³. With 10% extra it needs 36.667 ft³. A bag yielding 0.60 ft³ gives 62 whole bags. The example ready-mix increment rounds the order to 1.5 yd³.
What bag yield should I enter?
The 0.60 ft³ default matches the approximate yield stated for an 80 lb bag of QUIKRETE Concrete Mix No. 1101. Other mixes differ. For UK products, select litres and enter the manufacturer’s mixed yield.
Why can the ready-mix order exceed calculated volume?
A supplier may impose a minimum billed quantity or sell in increments. Both are editable here. A short-load fee is included only when you enter one; check your quote.
Does a lower price mean I should mix the bags myself?
No. It compares the entered costs only. Mixing time, placing crew, access, equipment and continuous placement requirements can change the practical choice.
Sources and assumptions
Bag-yield example: QUIKRETE Concrete Mix No. 1101 data sheet. Confirm your exact product, required strength, dimensions and supplier terms before ordering. Reviewed 30 October 2026. Calculations run in your browser; this calculator does not send your entered measurements to a server.
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