How to Calculate and Pour a 20x20 Concrete Slab

A 20x20 concrete slab is the absolute standard footprint choice for residential detached garages, double-car parking driveways, and large backyard entertainment patios. Spanning exactly 400 square feet, this size of slab supports significant load distribution, which makes correct thickness and steel reinforcement preparation absolutely vital.

Need helper estimates for alternative sizes? Try our dynamic Concrete Slab Calculator. For backyard patios, check our specialized Concrete Patio Calculator.

Concrete Yards & Bag Estimates for 20x20 Slabs

Because 20x20 slabs are typically used for vehicle parking or heavy structures, a **6-inch depth** is highly recommended over the standard 4 inches. At 6 inches of thickness, a 20x20 slab requires **7.41 cubic yards** of wet concrete. Applying the standard 10% safety buffer increases the needed purchase volume to **8.15 cubic yards**.

If you are mixing concrete using dry pre-packaged bags by hand (not recommended for this volume unless completed in stages), you will require:

• 80 lb Bags: You will need 367 bags (each yielding 0.60 cu. ft.).
• 60 lb Bags: You will need 489 bags (each yielding 0.45 cu. ft.).

Important Tips for Pouring a 20x20 Slab

1. Incorporate Steel Rebar Grid: Do not rely on light wire mesh. Lay a structural grid of #3 (3/8-inch) or #4 (1/2-inch) steel rebar spaced 12 to 16 inches apart. Elevate the rebar on concrete "chairs" to ensure it sits in the middle of the slab's depth during the pour.
2. Focus on Subbase Compaction: Vehicles put heavy stress on the slab edges and joints. Remove all organic topsoil and lay 4 inches of crushed gravel. Use a mechanical plate vibrator to compact it until fully solid.
3. Create Control Joints: Concrete shrinks as it cures. Install joints at least every 10 feet. For a 20x20 slab, divide it into four equal 10x10 squares by cutting control joints down both center lines.