How this planner works
- Volume. Slab: area × thickness. Footing: length × width × depth. Post holes: the hole's circle area minus the post, times the depth above the gravel, times the number of holes.
- Waste. The volume is increased by the percentage you choose (10% by default) for uneven ground and spillage.
- Bags. Common premixed bags yield about 0.30 cu ft (40 lb), 0.45 cu ft (60 lb) and 0.60 cu ft (80 lb). Volume ÷ yield, rounded up.
- Ready-mix. Cubic yards rounded up to a quarter-yard increment for estimating. Confirm the supplier’s minimum order, increments and fees.
- Rebar. The estimate uses a 3-inch edge inset, grid spacing no greater than the spacing selected and 20 ft stock. It reuses suitable offcuts, allows for a ⅛-inch cut and adds the entered lap when a line needs more than one piece. Confirm cover, lap, joint placement and chair spacing with your design.
- Base and forms. Gravel base over the slab area with 15% for compaction. Rectangular forms use 8 ft stock with reusable offcuts and a ⅛-inch cut allowance, and stakes at corners and roughly every 3 ft. Circular forms use flexible material and are priced separately.
Reference guidance:QUIKRETE Concrete Mix yields·American Cement Association concrete guidance. Confirm material coverage and specifications with your supplier.
Worked examples
A 10 × 10 ft patio slab, 4" thick:
- Volume: 33.3 cu ft. With 10% extra, 36.67 cu ft = 1.36 cubic yards.
- Bags: 62 × 80 lb, or 82 × 60 lb. Ready-mix order: 1.5 yd.
- Rebar at 18": 152 ft of bar cut from 8 twenty-foot bars.
Ten fence posts (4×4) in 10" holes, 36" deep, with 6" of gravel in the bottom: 1.151 cu ft per hole, 22 × 80 lb bags in total (about 2.11 bags per hole).
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
An 80 lb bag yields about 0.6 cubic feet, so a cubic yard (27 cubic feet) takes 45 bags. With 60 lb bags (0.45 cubic feet) it is 60 bags, and with 40 lb bags (0.3 cubic feet) it is 90 bags. Check the yield printed on your brand.
When should I order ready-mix instead of bags?
Many people switch to a ready-mix truck at around 1 cubic yard. Mixing 45 or more 80 lb bags by hand is slow and hard to keep consistent across a slab. Small loads often carry a short-load fee, so compare a delivered quote against the bag price.
How thick should a concrete patio slab be?
A common thickness for patios and walkways is 4 inches. Pads for sheds and hot tubs are often 4 to 6 inches, and driveways that carry cars 5 to 6 inches. Local codes and the load you plan to put on the slab decide the final number.
Rebar or wire mesh for a patio slab?
Both control cracking rather than prevent it. Rebar laid in a grid (often #3 or #4 bars at 12 to 18 inches) is stiffer and easier to keep at mid-depth on chairs. Welded wire mesh is quicker to lay on small slabs but tends to get stepped down to the bottom during the pour.
How much extra concrete should I order?
Ordering 5 to 10 percent extra covers uneven ground, form bulges and spillage. Running short partway through a pour causes a weak joint, so it is better to have a little left over. The planner adds 10% by default.