Concrete Calculator
Cubic yards, bag count, and cost for slabs from length, width, and thickness.
What it does
Concrete sells by volume: cubic yards for ready mix trucks, bags for hand mixing. The core math is a box: length times width times thickness. Waste allowance goes on top because subgrade is never perfectly level and some mix sticks to every tool.
The tool above takes slab dimensions in feet with thickness in inches or feet. It returns cubic yards including waste, the count of 80 lb bags for the same job, and the cost at your per yard price.
Four inches suits patios, walks, and light shed slabs. Driveways and garage floors usually want five to six inches with reinforcement. Thickness beats strength grade for residential cracking: a thin slab fails no matter the mix.
How to use
- Switch the unit system first: imperial for feet and inches, metric for meters and centimeters.
- Measure length and width in feet at the longest points.
- Enter thickness. Most patios use 4 inches, driveways 5 to 6.
- Keep 10% waste for normal ground, more for rough or sloped bases.
- Under about half a yard, bags beat the truck short load fee. Above two yards, the truck wins.
Formula
- L x W x T
- Slab box in feet. Thickness divided by 12 when inches.
- 27
- Cubic feet per cubic yard.
- 0.6
- Cubic feet per 80 lb bag after mixing.
- waste
- Spillage and uneven base share as a rate.
One 80 lb bag of premix fills 0.6 cubic feet after mixing, and a 60 lb bag fills 0.45. Ready mix is ordered in quarter or half yard steps in many areas, so round the result up to the supplier step.
Worked example
Example: 10 x 10 ft patio, 4 inches thick, 10% waste
Length 10, width 10, thickness 4 inches, waste 10%, price $150 per yard.
- Thickness in feet: 4 / 12 = 0.3333.
- Raw volume: 10 x 10 x 0.3333 = 33.33 ft3.
- With waste: 33.33 x 1.10 = 36.67 ft3.
- Cubic yards: 36.67 / 27 = 1.36 yd3. Order 1.5.
- Bags: 36.67 / 0.6 = 61.1, so 62 bags of 80 lb.
- Cost: 1.36 x $150 = $203.70 before delivery fees.
Reference table
| Slab | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|
| 8 x 8 ft | 21.33 | 0.79 | 36 |
| 10 x 10 ft | 33.33 | 1.23 | 56 |
| 12 x 12 ft | 48.00 | 1.78 | 80 |
| 20 x 10 ft drive strip | 66.67 | 2.47 | 112 |
Limits
- Flat slabs assumed. Footings and curves need separate shapes.
- Subgrade quality moves waste a lot.
- Bag yields vary by brand and mix.
- Delivery fees and short load charges excluded.
Common mistakes
- Skipping waste. Real subgrade eats 5 to 10% on most home jobs.
- Pouring thinner than 4 inches to save a bag, then watching it crack in a year.
- Ordering exact yards. Trucks batch in steps, so round up and use the rest in a walk.
- Mixing bag counts across brands. Yields differ, so check the bag, not memory.
- Forgetting the base. Compacted gravel matters more than extra cement.
Try it yourself
Quick checks. Answers sit tucked under each question.
8 x 8 ft slab, 4 in thick, no waste?
64 x 0.3333 = 21.33 ft3. That is 0.79 yd3 or 36 bags of 80 lb.
Bags of 80 lb per full yard?
27 / 0.6 = 45 bags.
Questions
How many 80 lb bags make a yard?
45 bags. A yard holds 27 cubic feet and each 80 lb bag fills 0.6, so 27 / 0.6 = 45.
How thick should a patio be?
4 inches with a compacted base for foot traffic. Go 5 to 6 inches for anything a car drives on.
Bags or ready mix?
Bags under roughly half a yard. Ready mix above that, since hand mixing 60 bags by wheelbarrow is a long day.
Does rebar change the volume?
No, steel displaces almost nothing. Order the same concrete and add the mesh or bar separately.
Can I pour in cold weather?
Above freezing with care: warm water, covers, and no frozen ground. Below that, wait for a better day.