Slabs and flatwork

Concrete Slab Calculator

Use slab mode on the homepage for patios, sidewalks, shed pads, garage floors, and small pads. Enter length, width, thickness, number of sections, waste buffer, and ready-mix price.

Estimate a slab

Common slab thickness

Use	Typical thickness	Notes
Walkway	4 in	Common for pedestrian traffic on a compacted base.
Patio	4 in	Use thicker edges or reinforcement when loads are heavier.
Driveway	5 to 6 in	Vehicle loads usually need more thickness and a strong base.
Garage floor	4 to 6 in	Confirm local code, reinforcement, and load requirements.

Slab formula

Convert thickness from inches to feet, then multiply length x width x thickness x sections. Divide cubic feet by 27 for cubic yards.

For multiple rectangles, either enter the quantity if they are identical or calculate each section separately and add the order notes.

Before ordering

Measure inside the forms, not just the planned outside footprint.
Check whether the grade is level or if low spots need extra concrete.
Confirm the base depth, reinforcement, control joints, and finish access.
Add waste before calling the supplier, then ask about minimum loads and short-load fees.

Decision guide

Turn slab geometry into an order conversation

The calculator handles geometric volume; it does not design the slab. Thickness, reinforcement, base preparation, drainage, joints, strength, air entrainment, and curing depend on loads, exposure, soil, and local requirements. Confirm those details before converting the result into a ready-mix order.

12 ft x 16 ft patio at 4 in

The base volume is 12 x 16 x (4/12) = 64 cubic feet, or about 2.37 cubic yards. A 10% planning allowance produces about 2.61 yards. Measure the formed excavation at several points because a low subgrade or bowed form can add more than a generic percentage.

Before you decide

Measure inside completed forms.
Confirm design thickness and edge details.
Plan control joints and placement access.
Ask the producer about minimum loads and delivery timing.

Method and limitations

NRMCA's ordering guidance says mixture requirements should match the application, loads, exposure, placement, and finishing needs. Give the supplier project specifications when they exist. Avoid adding uncontrolled water at the jobsite because it can increase cracking risk and reduce strength and durability.