How many cubic yards of concrete do I need for a 10x10 slab?
A 10×10 ft slab that is 4 in thick needs 1.23 cu yd net and 1.36 cu yd with the 10% allowance, which is 62 bags of 80-lb premix. At 6 in thick it needs 2.04 cu yd (92 bags).
Concrete Slab Calculator: Cubic Yards, Bags and Thickness
Enter the length, width and thickness of your slab. The calculator returns cubic yards including a 10% spill allowance, the number of 80-lb, 60-lb or 40-lb bags, rebar footage and the gravel base underneath, and it tells you whether bags or a ready-mix truck is cheaper.
1. Material Grade & Project Dimensions
Area: 144.0 sq ft (13.38 m²)Net Vol: 1.78 cu yd (1.36 m³)
2. Material Takeoff, Blueprint & Cost
Total Concrete Volume (incl. waste)
1.96 cu yd (1.50 m³)
52.8 cu ft · 7,814 lbs (3.91 US tons)
80-lb Concrete Bags (0.60 ft³)
89 bags
2.1 pallets (@ 42 bags/pallet)
60-lb or 40-lb Bag Option
118 × 60-lb
or 177 × 40-lb (0.30 cu ft)
Rebar Grid (#4 @ 18" O.C.)
209 lin ft
11 × 20-ft bars recommended
4" Compacted Gravel Sub-Base
2.52 US tons
Crushed stone base under slab
Cost Break-Even: Ready-Mix Truck vs. 80-lb Bags
Ready-Mix Truck saves $209
Ready-Mix Delivery (1.96 cu yd + $140 short-load fee)
$234
89 × 80-lb Pre-Mix Bags
$443
Show Step-by-Step Formula & Takeoff Math
Net Area = 144.00 sq ft (13.38 m²)
Net Volume = 48.00 cu ft ÷ 27 = 1.78 cu yd
Total Volume (+10% waste) = 52.80 cu ft = 1.96 cu yd (1.50 m³)
80-lb Bags = ceil(52.80 ÷ 0.60) = 89 bags
How to calculate the concrete for a slab
The volume of a slab is length × width × thickness. Work in feet, turn the thickness from inches into feet by dividing by 12, and divide the cubic feet by 27 to get cubic yards. A 12×12 ft patio slab at 4 in is 144 sq ft × 0.333 ft = 48 cu ft, or 1.78 cu yd net. With a 10% allowance for an uneven subgrade and spillage the calculator arrives at 1.96 cu yd, which is 89 bags of 80-lb premix.
Choose the thickness for the job. 4 in is the usual choice for patios, walkways and shed pads. 6 in is common for driveways and garage floors that carry vehicles. Going from 4 in to 6 in adds 50% to the volume: the 12×12 ft slab grows from 1.96 to 2.93 cu yd, or 132 bags.
The same dimensions also give you the supporting materials: 209 ft of #4 rebar in an 18 in grid and 2.52 US tons of gravel for a 4 in compacted base under the 12×12 ft slab. Fresh concrete weighs about 148 lb per cu ft, so one cubic yard is 3,996 lb.
Concrete needed by slab size (10% allowance included)
| Slab size | Area | 4 in thick | 6 in thick |
|---|---|---|---|
| 10×10 ft | 100 sq ft | 1.36 cu yd · 62 bags | 2.04 cu yd · 92 bags |
| 10×12 ft | 120 sq ft | 1.63 cu yd · 74 bags | 2.44 cu yd · 110 bags |
| 12×12 ft | 144 sq ft | 1.96 cu yd · 89 bags | 2.93 cu yd · 132 bags |
| 16×16 ft | 256 sq ft | 3.48 cu yd · 157 bags | 5.21 cu yd · 235 bags |
| 20×20 ft | 400 sq ft | 5.43 cu yd · 245 bags | 8.15 cu yd · 367 bags |
| 24×24 ft | 576 sq ft | 7.82 cu yd · 353 bags | 11.73 cu yd · 528 bags |
Frequently Asked Questions & Field Guide
How many bags of concrete do I need for a 12x12 slab?
For a 12×12 ft slab at 4 in thick plan on 89 bags of 80-lb concrete, or 118 bags of 60-lb or 177 bags of 40-lb. That is about 2.1 pallets of 80-lb bags at 42 bags per pallet.
How thick should a concrete slab be?
Use 4 in for patios, walkways and shed pads and 6 in for driveways and garage floors with vehicle loads. Enter the thickness in the Depth field of each zone; the volume scales linearly, so a 6 in slab needs 50% more concrete than a 4 in slab of the same size.
How much does a 20x20 concrete slab weigh?
A 20×20 ft slab at 4 in needs 5.43 cu yd including the allowance. At 148 lb per cu ft that is about 21,707 lb, or 10.9 US tons of fresh concrete. A load that size is delivered by a ready-mix truck rather than mixed from 245 bags.
Related calculators & tools
Concrete Slab & Bag Calculator
Concrete calculator for slabs, footings and post holes: cubic yards, 80-lb and 60-lb bag counts, rebar and ready-mix truck cost.
Concrete Slab Cost Calculator
Bagged concrete versus a ready-mix truck, priced for your slab size.
Concrete Yard Calculator
Cubic yards of concrete from feet and inches, for one pour or several sections.
Concrete Bag Calculator
How many 80-lb, 60-lb or 40-lb bags of premixed concrete a project takes.
Cubic Yards to Tons Converter
Convert cubic yards, cubic feet and cubic meters to tons for 24 materials, with a density table.