Free Concrete Calculator
Work out how much concrete a slab needs in cubic yards, cubic feet and 60 or 80 lb bags. Free, fast, and works entirely in your browser with no sign-up required.
Updated
Concrete slab
Extra material for cuts, spillage and breakage.
Concrete needed
1.36
cubic yards
- Volume
- 36.7cubic feet
- 80 lb bagsEach yields ~0.60 ft³
- 62bags
- 60 lb bagsEach yields ~0.45 ft³
- 82bags
How this is calculated
volume ft³ = length × width × (thickness ÷ 12); cubic yards = ft³ ÷ 27
- An 80 lb bag of pre-mixed concrete yields about 0.60 ft³; a 60 lb bag about 0.45 ft³.
- Bag counts are rounded up — you cannot buy part of a bag.
- Ready-mix is usually sold in ¼-yard increments, so round the cubic-yard figure up when ordering.
Frequently Asked Questions
How does the Concrete Calculator work?
It applies the formula volume ft³ = length × width × (thickness ÷ 12); cubic yards = ft³ ÷ 27. Enter your measurements and the result updates instantly in your browser — nothing is uploaded and no sign-up is needed.
Why should I include a waste allowance?
Offcuts, spillage, breakage and an uneven sub-base all consume material that the raw geometry does not account for. The default allowance is deliberately modest; increase it for complex layouts, and remember that running short usually costs more in a second delivery than the surplus did.
Are the results exact?
They are a close estimate, not a guarantee. An 80 lb bag of pre-mixed concrete yields about 0.60 ft³; a 60 lb bag about 0.45 ft³. Real materials vary by supplier, moisture content and compaction, so confirm the rate with your supplier before placing a large order.
Is this tool free?
Yes, completely free with no registration. All calculations happen client-side in your browser.
Does it work on mobile?
Yes. The calculator is fully responsive and works on phones and tablets in any modern browser — useful when you are measuring on site.
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About the Concrete Calculator
The Concrete Calculator estimates the concrete volume a slab needs from its length, width and thickness, then converts that into cubic yards for ready-mix and into 60 lb and 80 lb bag counts for smaller pours. Enter your measurements and the result updates as you type — there is nothing to submit and nothing to wait for.
Everything runs in your browser. Nothing you enter is sent to a server, there is no sign-up, and there is no usage limit.
Who it's for
Homeowners pouring a patio, shed base or walkway, and contractors sanity-checking an order before the truck arrives.
How the calculation works
volume ft³ = length × width × (thickness ÷ 12); cubic yards = ft³ ÷ 27
The result depends on a few standard constants, which are stated on the page rather than hidden:
- An 80 lb bag of pre-mixed concrete yields about 0.60 ft³; a 60 lb bag about 0.45 ft³.
- Bag counts are rounded up — you cannot buy part of a bag.
- Ready-mix is usually sold in ¼-yard increments, so round the cubic-yard figure up when ordering.
Getting an accurate estimate
Measure twice. Small errors in a linear dimension compound, because material volume scales with two or three of them at once — a 6-inch error across a 20-foot run is over a cubic foot of material on a 4-inch pour.
Keep the waste allowance. The default is deliberately conservative: offcuts, spillage, uneven sub-base and breakage are all real, and running short mid-job usually costs more in a second delivery than the surplus did.
Round up when you order. Suppliers sell in fixed increments — quarter yards, whole bags, full boxes — so treat the figure here as the minimum you need rather than the exact amount to buy.