How to use this calculator
- Enter slab dimensions. Use rectangular slab length, width and thickness.
- Set slab count and waste. Enter the number of identical slabs and an overage allowance.
- Choose buying basis. Use the bag yield, ready-mix truck capacity and optional entered price that match your supplier.
- Read the takeoff. Use total volume, cubic yards, bags to buy, truckloads and concrete weight as a material estimate.
How it works
This concrete slab calculator treats the slab as a rectangular prism:
V_base = L x W x t x N
Waste or overage is added before buying quantities are rounded:
V_total = V_base x (1 + waste / 100)
bags = ceil(V_total / bag_yield)
yards = V_total / 0.764554858
Concrete weight and optional cost use the entered density and entered price:
weight = V_total x density
cost = yards x entered_price_per_yard
Worked example
Verified against the live calculator
A 12 ft by 10 ft slab at 4 in
thick has 40 ft^3 of base concrete volume. With
10% waste, the takeoff is 44 ft^3, or
about 1.63 yd3. With 0.6 ft^3 per bag,
that rounds up to 74 bags.
Frequently asked questions
How do you calculate concrete for a slab?
For a rectangular slab, multiply length by width by thickness, then multiply by slab count. Apply any waste allowance before converting to cubic yards, bags or truckloads.
How many bags of concrete do I need for a slab?
Divide the total slab volume after waste by the yield printed on the bag, then round up to the next whole bag. Bag yield varies by product, so use the bag label for final buying.
Does this calculator choose slab thickness or rebar?
No. It is a material takeoff only. Slab thickness, reinforcement, joints, subbase, drainage, frost protection and local code requirements need project-specific review.
Is the concrete cost a live price?
No. The cost output uses only the price per cubic yard you enter. It does not look up current ready-mix, bag, delivery or short-load prices.
Method & assumptions
- Rectangular flat slabs only. Irregular shapes, thickened edges, turndowns, grade beams and steps need separate volume math.
- Waste is applied before bag and truckload rounding.
- Cost uses the entered price per cubic yard only. It does not include delivery, short-load charges, pump time, tax, labor, forms, subbase, reinforcement or finishing.
- This is not a structural, code or mix-design calculator. It does not choose slab thickness, reinforcement, control joints, subgrade compaction, vapor barrier, drainage or frost protection.
- Use rebar spacing, rebar lap splice length, deck footing concrete and deck footing size for adjacent planning checks.