Concrete Slab Calculator
Yards and bag count for slabs of any size and thickness
What is concrete slab calculator?
A concrete slab is a flat, on-grade pour — the workhorse of residential concrete. Patios, garage floors, basement floors, walkways, and driveways are all slabs. The math is length × width × thickness, but slabs have stricter requirements than pads because they’re large enough to crack from thermal movement and flex from edge loads.
The thickness rules: 4 in for patios, walkways, sidewalks, and any pedestrian-only surface. 6 in for driveways, garage floors, and anywhere a vehicle drives. 8+ in for parking pads, equipment yards, and surfaces under loaded trucks. These minimums assume a properly compacted gravel base; on poor soil, add 2 in.
Reinforcement is required for anything over ~80 sq ft. Patios and walkways get fiber mesh in the mix or a single layer of 6 × 6 wire mesh. Driveways and garages need #3 or #4 rebar in a 16-18 in grid, chair-supported in the middle of the slab. The rebar doesn’t prevent cracks — it holds the cracks tight after they form.
Control joints are not optional. A slab will crack; the only question is whether it cracks where you want it to. Saw-cut or tool joints at 24-36 times the slab thickness — that’s every 8-12 ft for a 4 in slab. Cut within 24 hours of pouring, before the random cracks form.
Pour-day logistics: for slabs over 1 cu yd, ready-mix is faster, cheaper, and yields a more consistent finish. The truck dumps in 10 minutes; you’d spend 4 hours mixing 50 bags and the cold joints would be visible forever.
When to use a concrete slab calculator
- Backyard patio — A 12 × 16 ft patio at 4 in is the canonical residential pour. The Slab tab gives you 2.4 cu yd — call ready-mix and skip the bag math.
- Garage floor or workshop — Two-car garages run 20 × 20 to 24 × 24. At 6 in thick (vehicle-bearing), a 22 × 22 ft floor is 8.9 cu yd — definitely a ready-mix pour.
- Front walkway — A 4 ft wide × 30 ft long walkway at 4 in is 1.5 cu yd. Right at the bag-vs-ready-mix breakeven, but most pours benefit from a continuous truck delivery.
How to use the Concrete Slab Calculator
- Measure length, width, thickness — Length and width in feet; thickness in inches. The Slab tab handles rectangles directly — break L-shapes into two rectangles and add.
- Pick the right thickness — 4 in for patios and walkways, 6 in for driveways and garages, 8+ in for parking pads or under heavy machinery.
- Apply waste factor — 10% is the default — covers spillage, uneven subgrade, and the form-board absorption. Bump to 15% if the subgrade is rough or you have lots of rebar in the way.
- Decide pour method — Under 1 cu yd: bagged. 1-3 cu yd: ready-mix short load. 3+ cu yd: standard ready-mix delivery. The bag count tells you which range you're in.
Worked examples
Patio: 12 × 16 ft × 4 in
Input: Slab tab: 12 ft × 16 ft × 4 in
Output: 2.37 cu yd / 108 of 80-lb bags — ready-mixGarage: 22 × 22 ft × 6 in
Input: Slab tab: 22 ft × 22 ft × 6 in
Output: 8.96 cu yd — ready-mix onlyWalkway: 4 × 30 ft × 4 in
Input: Slab tab: 4 ft × 30 ft × 4 in
Output: 1.48 cu yd / 67 of 80-lb bags