Ordering a slab right starts with three measurements and one decision. Get those, and the concrete volume falls out of a simple calculation. This guide walks through each step, gives two worked examples, and lists the mistakes that lead to a short or wasted order.
Step 1: measure the footprint
Measure the length and width of the slab in feet. For an irregular shape, split it into rectangles and add the areas, the same way you would for any surface. Measure to the inside of your forms, since that is the actual space the concrete fills, and check the diagonals to confirm the form is square.
Step 2: choose the thickness
Thickness depends on the load the slab will carry. Four inches suits patios, walkways and shed floors. Driveways for cars are 4 to 5 inches, and garage floors or anything taking heavy vehicles are 5 to 6 inches. A thickened edge or a non level subgrade adds volume, so measure the realistic deepest case rather than the design thickness alone.
Step 3: work out the volume
Multiply length by width by thickness in feet, then divide by 27 for cubic yards. Convert the thickness from inches to feet by dividing by 12.
Step 4: add waste and convert to bags
Add about 7 percent for spillage and an uneven base, then round up. If you are using bags, an 80 pound bag yields about 0.60 cubic feet, a 60 pound bag 0.45, and a 40 pound bag 0.30.
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | about 45 |
| 60 lb | 0.45 cu ft | about 60 |
| 40 lb | 0.30 cu ft | about 90 |
Worked example one: a patio
A 10 by 10 foot patio at 4 inches: 10 times 10 is 100 square feet, times 0.333 feet is 33.3 cubic feet, divided by 27 is about 1.24 cubic yards before waste. That is roughly 56 of the 80 pound bags. With a 7 percent allowance, round up to about 60 bags, or order one and a third cubic yards of ready mix.
Worked example two: a garage floor
A 20 by 22 foot garage floor at 5 inches: 440 square feet times 0.417 feet is about 183 cubic feet, divided by 27 is about 6.8 cubic yards. With waste, order about 7.3 cubic yards. At this size you would never bag it, since that is over 300 bags. This is firmly ready mix territory.
When to switch to ready mix
Mixing 56 bags by hand is a long, hard day and risks cold joints between batches. Once you approach a cubic yard, ready mix delivered by the truck is usually cheaper and far easier, and it gives one continuous pour with consistent strength.
Common mistakes
- Measuring the thickness as the design figure when the subgrade dips, which leaves you short.
- Forgetting a thickened edge, common around the perimeter of a slab.
- Ordering with no waste, then running out part way through the pour.
- Bagging a job that is really a ready mix job, turning one delivery into a punishing day of mixing.