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Concrete slab calculator

Slabs fail in the shopping list when thickness is guessed. A patio at 3½ inches and the same footprint at 5 inches are different truck tickets. This page locks the shape to a rectangle (or simple pad), converts thickness from inches, applies waste, and shows yards plus bag counts. It will not tell you whether your soil needs a gravel base, vapor barrier, or control joints — only how much concrete fills the prism you measured.

ft
ft
in
10%
0510152025

+10% so you don't come up short

ORDER ESTIMATESlab
Cubic yards1.36
80 lb bags62
Waste applied10%
How it calculates
L × W × (T ÷ 12) → cu ft; ÷ 27 → yd³; × (1 + 10% waste); bags = ceil(order cu ft ÷ 80 lb yield).

Estimate only — not a contractor bid. Confirm bag yields and local codes before you pour or order.

When to use

Shed pad, AC pad, sidewalk rectangle, or patio pour where you can tape four sides and pick a uniform thickness.

When not to

Stepped footings, thickened edges with a different stem width, or stamped curves you cannot approximate as rectangles — break the job into pieces on the main concrete calculator or talk to whoever is forming it.

Assumptions

Single rectangular prism; thickness uniform; default waste 10%; same bag yields as the main concrete page (80 lb ≈ 0.60, 60 lb ≈ 0.45, 40 lb ≈ 0.30 cu ft). No slope/drainage pitch in the volume.

Worked examples

Example 1

12×14 ft patio, 4 in thick, 10% waste
→ ≈ 2.07 yd³ → ≈ 2.28 yd³ ordered; 80 lb bag count shown

Over 2 yards, price a short-load truck vs an afternoon of bags.

Example 2

4×4 ft AC pad, 6 in thick, 15% waste
→ ≈ 0.30 yd³ ordered — usually bags

Many pads sit on compacted gravel; concrete volume is only the slab, not the base.

How it calculates

Measure the space the concrete fills, enter length, width, and thickness in inches, set waste, and read ordered yards plus bags.

  1. Enter length and width in feet.
  2. Enter thickness in inches (not feet).
  3. Choose bag size and waste percent.
  4. Order the yardage or bag count on the ticket — confirm yield on the package.
cu ft = L_ft × W_ft × (thickness_in / 12)
yd³ = cu ft / 27
order_yd³ = yd³ × (1 + waste)
bags = ceil(order_cu_ft / bag_yield)

Common traps

  1. Inside form vs outside form. Measure the space the concrete fills, not the lumber outside the boards.
  2. 4 inch “minimum” myths. Local frost, load, and code beat blog rules of thumb.
  3. Gravel base is separate. Order stone with the gravel calculator; do not mix that tonnage into concrete yards.

Next tool

Multiple shapes / holes / columns → concrete calculator.

You already know yards and only want bags → concrete bag calculator.

FAQ

At 4 inches: about 1.23 yd³ before waste; with 10% waste, order roughly 1.36 yd³. Change thickness and the number moves fast.

Related

Estimate only — not a contractor bid or engineered takeoff. Confirm bag yields and local codes before you pour or order.