Concrete slab calculator
Work out the concrete volume, bags and cost for a flat rectangular slab pour.
Concrete slab calculator
Work out the concrete volume, bags and cost for a flat rectangular slab pour.
Enter your slab dimensions to see the concrete estimate.
Concrete needed
Project summary
How it works
These are volume estimates for planning. Actual concrete requirements depend on form dimensions, subgrade and waste.
Concrete slab calculator
Volume is the whole calculation
A slab is the simplest concrete shape to estimate: length times width times thickness gives the volume directly. The part people get wrong is thickness, which is almost always entered in feet by mistake when it should be inches. This calculator takes thickness in inches specifically so a 4 inch slab does not turn into a 4 foot one.
Bags versus ready-mix
Small slabs, a shed floor or a stepping stone pad, are often poured from bagged concrete rather than a ready-mix truck. The bag count here assumes a standard yield for each bag size, which is close enough for planning but always buy a few extra bags: partial bags do not mix well and running short mid-pour is worse than having leftovers.
Frequently asked questions
How much concrete do I need for a 10 by 10 foot slab?
At a standard 4 inch thickness, a 10 by 10 foot slab (100 square feet) needs about 33 cubic feet of concrete, or 1.24 cubic yards. With a 10 percent waste allowance that is closer to 1.36 cubic yards, or about 82 bags of 60 lb concrete mix. Enter your own dimensions above for an exact figure.
How thick should a concrete slab be?
Four inches is the common thickness for patios, walkways and shed floors. A garage floor or a slab that carries vehicle weight typically needs more, often 5 to 6 inches with reinforcement. Check local code and the manufacturer or engineer specification for your specific use before pouring.
How much does it cost to pour a 20 by 20 foot concrete slab?
A 4 inch thick slab of that size takes about 6 cubic yards. At 130 dollars per cubic yard delivered, the concrete alone costs around 780 dollars. Labour, rebar, forms and finishing add to the total. The calculator gives you the volume so you can get accurate quotes.
How thick should a concrete slab be for a shed?
A 4 inch slab is adequate for most small sheds storing light items. A 6 inch slab is better for larger sheds or those holding heavy equipment. The calculator lets you enter any thickness and shows the exact concrete volume needed.
Do I need gravel under a concrete slab?
Yes, a 4 to 6 inch layer of compacted gravel provides drainage and a stable base. Without it, moisture wicks up through the slab and causes cracking. The calculator shows concrete volume only, but plan to order gravel separately for the base layer.
How long before I can build on a concrete slab?
Concrete reaches about 70 percent strength in 7 days and full strength in 28 days. You can frame walls after about 7 days for light loads. Heavy structural loads should wait until the full 28 day cure. The calculator helps you plan the pour so you can schedule the rest of your project.
What is the difference between a slab and a foundation?
A slab-on-grade foundation is a single pour that serves as both the floor and the foundation. A traditional foundation has footings, foundation walls and a separate floor. Slabs are simpler and cheaper for single story structures. The calculator estimates concrete for slab-on-grade pours.
Does a concrete slab need expansion joints?
Yes, expansion joints prevent random cracking as the slab expands and contracts with temperature changes. Place joints at intervals roughly 2 to 3 times the slab width in feet. For a 20 foot slab, place joints every 8 to 10 feet. The calculator gives you the slab area to help plan joint placement.