How to Estimate Concrete for a Slab
A slab is the easiest concrete pour to estimate, because it is just length times width times thickness. The only things that trip people up are the thickness conversion and forgetting to add waste. Work through the example below and you will not miss either.
The steps
- Measure length and width in feet. Round up to the nearest foot to stay safe.
- Set your thickness in feet. Most slabs are 4 inches, which is 0.333 feet. A 5 inch slab is 0.417 feet, and 6 inches is 0.5 feet.
- Multiply for cubic feet: length x width x thickness.
- Divide by 27 to get cubic yards.
- Add 5 to 10 percent for waste and slab edge, then round up to a quarter or half yard for ordering.
Worked example: a 20 by 24 slab at 4 inches
- Area: 20 x 24 = 480 square feet
- Thickness: 4 inches = 0.333 feet
- Volume: 480 x 0.333 = 159.8 cubic feet
- Cubic yards: 159.8 / 27 = 5.92 yards
- Add 8 percent waste: 5.92 x 1.08 = 6.39 yards
- Order: 6.5 yards
That job is well over a cubic yard, so a ready-mix truck is the right call. To check the price, run the same slab through the concrete cost estimator, which adds delivery and any short-load fee for you.
Estimating slab bags for a small pour
For a small slab under about a cubic yard, you might hand-mix bags instead. Bag yields are printed on the sack, but as a guide an 80 lb bag makes about 0.6 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.3. Divide your cubic feet by the bag yield to get the count. A 4 by 4 pad at 4 inches is 5.3 cubic feet, which is about 9 of the 80 lb bags. The concrete slab calculator gives you the bag count for all three sizes at once.
Common slab-estimating mistakes
- Forgetting the waste factor. Running short mid-pour means a cold joint or a second delivery, both bad.
- Mixing up inches and feet. Thickness is in inches; convert to feet before you multiply.
- Ignoring the subgrade. A dished-out or uneven base holds more concrete than the flat number suggests. Add waste on the high side.
- Not rounding up. Order to the next quarter or half yard. A little extra is cheap insurance.
Once you have the volume, price it in the concrete cost estimator or read how much does concrete cost for current rates. For the full step-by-step method across any project, start at the concrete estimate guide.
Sources
Frequently asked questions
How many yards of concrete do I need for a slab?
Multiply length by width by thickness in feet, divide by 27, and add 5 to 10 percent for waste. A 20 by 24 slab at 4 inches works out to about 6.5 yards after waste.
How do I estimate bags of concrete for a slab?
Find the volume in cubic feet, then divide by the bag yield. An 80 lb bag makes about 0.6 cubic feet, so a 5.3 cubic foot pad needs about 9 bags.
How thick should a concrete slab be?
Four inches is standard for patios, walkways, and shed floors. Go to 5 or 6 inches for driveways or anything carrying heavy loads. Thickness changes your volume, so re-run the estimate if you change it.
Should I add extra concrete when estimating a slab?
Yes. Add 5 percent on flat, well-formed slabs and up to 10 percent on rough or uneven subgrade, then round up to the next quarter or half yard.