Free concrete slab calculator: enter the slab length, width, thickness and waste percentage to get the cubic yards and 80 lb bags of concrete needed now.
Results are estimates for planning purposes and are not financial, medical, or professional advice.
How to Use the Concrete Slab Calculator
Enter your slab dimensions and a waste allowance to find out exactly how much concrete to order, in cubic yards for ready-mix or in 80 pound bags for mixing by hand.
- Enter the slab length and width, choosing feet or meters.
- Enter the thickness in inches — 4 inches is typical for patios and walkways, 5 to 6 for driveways.
- Set a waste percent. Five to 10 percent is standard; uneven subgrade deserves the high end.
- Press calculate to see cubic yards and the bag count, both with and without waste.
Concrete Volume Formula
Volume in cubic feet equals length times width times thickness in feet, so divide the inch thickness by 12 first. Divide cubic feet by 27 to get cubic yards, the unit ready-mix suppliers sell in. An 80 pound bag of concrete mix yields about 0.6 cubic feet, so bags needed equals cubic feet divided by 0.6, rounded up, with the waste allowance added. One cubic yard works out to 45 eighty-pound bags — a handy cross-check: 27 cubic feet divided by 0.6 per bag.
Volume (cu ft) = Length × Width × (Thickness in inches ÷ 12)
Cubic yards = Cubic feet ÷ 27
80-lb bags = Cubic feet ÷ 0.6 × (1 + waste %), rounded up
Example
A 10 by 12 foot slab at 4 inches thick gives 10 times 12 times 4 twelfths, which is 40 cubic feet or 1.48 cubic yards. At 0.6 cubic feet per 80 pound bag, that is 66.67 bags, and with 10 percent waste you should buy 74 bags — the classic awkward middle ground where bags are laborious but ready-mix carries a minimum charge.
Common Slabs at a Glance
| Slab | Cubic feet | Cubic yards | 80-lb bags (10% waste) |
|---|---|---|---|
| 10 × 10 ft, 4 in | 33.33 | 1.23 | 62 |
| 12 × 12 ft, 4 in | 48.00 | 1.78 | 88 |
| 20 × 20 ft, 4 in | 133.33 | 4.94 | 245 |
| 24 × 24 ft, 6 in | 288.00 | 10.67 | 528 |
Slab Thickness Guide
| Project | Recommended thickness |
|---|---|
| Garden walkway, light patio | 4 in |
| Patio with heavy furniture or a hot tub | 5 in |
| Residential driveway | 5 to 6 in |
| Garage floor with vehicle loads | 6 in |
| Shed or light outbuilding floor | 4 in, with thickened edges |
Ordering Tips
Measure twice and order once. Confirm your subgrade is compacted and level before calculating, since soft spots swallow extra concrete. For bag mixes, buy a few extra beyond the calculator result and keep the receipt, since unopened bags are returnable at most suppliers. Schedule delivery early in the day so finishing is not rushed, and have help ready: concrete waits for no one. In hot weather, dampen the subgrade first to stop it sucking water from the mix.
Bags or ready-mix? Below about 1 cubic yard, bags win on cost and logistics. Between 1 and 2 yards, compare: ready-mix suppliers carry minimum load charges (often 1 yard) plus short-load fees under about 4 yards, while a 1.5-yard pour is 68 bags to mix by hand. Above 2 yards, a truck is almost always cheaper, faster, and more consistent.
Remember the weight. A cubic yard of concrete weighs roughly 4,000 pounds — two tons. Plan how it gets from the truck to the forms, because a wheelbarrow trip over soft ground gets old fast.
Planning steel too? Estimate rebar separately with our Rebar Calculator — this concrete total covers only the concrete itself.
Frequently Asked Questions
How thick should a concrete slab be?
4 inches suits patios, walkways, and light sheds. Driveways and garage floors usually need 5 to 6 inches.
How many 80 lb bags make a cubic yard?
About 45 bags, since a cubic yard is 27 cubic feet and each bag yields 0.6 cubic feet.
Should I order ready-mix or bags?
Bags suit jobs under about 1 cubic yard. Larger pours are cheaper and easier as ready-mix delivery. Ready-mix suppliers usually have a minimum load charge, often around 1 cubic yard, plus short-load fees below about 4 yards, so compare the total against bag costs before deciding.
Why add a waste percent?
Forms leak, subgrades vary, and spillage happens. A 5 to 10 percent buffer prevents running short mid-pour.
Does this include reinforcement?
No. Rebar or wire mesh is calculated separately based on spacing and slab area.
How many bags of concrete do I need for a 10 by 10 slab?
A 10 by 10 foot slab at 4 inches thick is 33.33 cubic feet, or 1.23 cubic yards. At 0.6 cubic feet per 80 pound bag with 10 percent waste, the calculator gives 62 bags. Below about 1 cubic yard bags are usually the practical choice; above that, compare ready-mix prices.
How much does a cubic yard of concrete weigh?
About 4,000 pounds, or 2 tons, once mixed. That is why large pours arrive by truck: even one cubic yard is 45 heavy bags or several dozen wheelbarrow loads.
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Try our Square Footage Calculator and Flooring Calculator for more construction tools.