Four inches is 0.333 ft, not 4 ft. The calculator handles this automatically, but doing the math by hand without converting is the most common source of 12x oversized orders.
Concrete Calculator: Slab Volume, Cubic Yards & Bag Count
Size a rectangular slab pour - patio, shed, walkway, or equipment pad - before you order bags or schedule a ready-mix delivery.
Default settings
- Default thickness is 4 inches, which is a common slab-planning baseline but not a structural recommendation.
- Default overage is 10% to cover spillage, subgrade irregularity, and small finishing losses.
- Bag-yield constants stay fixed at 0.45 cu ft for 60 lb bags and 0.60 cu ft for 80 lb bags.
How this estimate works
- Slab area = length x width
- Raw cubic feet = slab area x thickness converted into feet
- Adjusted cubic feet = raw cubic feet x (1 + overage percent)
- Cubic yards = adjusted cubic feet / 27
- Bag counts = adjusted cubic feet / bag yield, rounded up
Related pages
Use a guide if bag yield or conversions need a closer look.
How these estimates workReview how rounding, unit conversion, and default settings affect the estimate.Concrete bag yield guideTranslate cubic feet and cubic yards into practical bag counts and ready-mix breakpoints.Square feet to cubic yards guideUse the same area-times-depth pattern to reason about other bulk material calculations.Gravel calculatorCompare slab-pour math with bulk aggregate ordering by cubic yards and tons.
FAQ
Quick answers if you need them.
Why does the calculator show both 60 lb and 80 lb bag counts?
Bag yield changes by size. Many users compare labor and haul effort between both bag types.
When should I stop thinking in bags and switch to ready-mix?
Once the pour exceeds about 1 cubic yard, ready-mix usually becomes more practical.
How the concrete estimate is calculated
The tool follows the same five-step pattern that hand calculations use.
- Slab area = length x width (in feet or meters).
- Raw volume = slab area x thickness, with thickness converted into the same unit. Four inches becomes 0.333 ft; ten centimeters becomes 0.10 m.
- Adjusted volume = raw volume x (1 + overage %). The default 10% covers spillage, uneven subgrade, and small finishing losses.
- Cubic yards = adjusted cubic feet / 27. Suppliers price ready-mix per cubic yard, so this is the number you order against.
- Bag count = adjusted cubic feet / bag yield, rounded up. Yields are 0.45 cu ft for 60 lb bags and 0.60 cu ft for 80 lb bags.
Bag yields above are planning constants. Actual bag yield can vary by a few percent with mix consistency and curing conditions, but for ordering purposes the constants are stable enough to use directly.
Bag size reference table
How much concrete each bag size yields and which projects each size fits best.
| Bag size | Yield (cu ft) | Bags per cubic yard | Best for |
|---|---|---|---|
| 40 lb bag | 0.30 cu ft | 90 bags | Tiny repair pours, post holes |
| 60 lb bag | 0.45 cu ft | 60 bags | Walkways, small slabs, patches |
| 80 lb bag | 0.60 cu ft | 45 bags | Patios, pads, sheds |
Worked examples by project type
Four common rectangular pours, with cubic yards and bag counts side by side.
| Project | Dimensions | Raw volume | Adjusted | 60 lb bags | 80 lb bags |
|---|---|---|---|---|---|
| Patio slab | 12 ft x 10 ft, 4 in thick, 10% overage | 40 cu ft | 44 cu ft / 1.63 cu yd | 98 bags | 74 bags |
| Equipment pad | 3 ft x 3 ft, 6 in thick, 10% overage | 4.5 cu ft | 4.95 cu ft / 0.18 cu yd | 11 bags | 9 bags |
| Walkway section | 24 ft x 3 ft, 4 in thick, 10% overage | 24 cu ft | 26.4 cu ft / 0.98 cu yd | 59 bags | 44 bags |
| Shed foundation | 8 ft x 10 ft, 4 in thick, 10% overage | 26.67 cu ft | 29.33 cu ft / 1.09 cu yd | 66 bags | 49 bags |
Patio slab. Right at the boundary. Compare ready-mix delivery cost against 6 to 7 store runs of 60 lb bags.
Equipment pad. Easy bag pour. Mix in a wheelbarrow or small mixer; ready-mix not needed.
Walkway section. Bags work but expect 2 to 3 hours mixing. Consider pouring in two sessions with a cold joint.
Shed foundation. Ready-mix usually wins here on time and finish quality.
Bags vs ready-mix: when to switch
The deciding factor is volume, not just budget.
Under 0.5 cubic yards (roughly 30 bags of 80 lb mix or fewer), bags are usually the right call. You can mix in batches, finish small areas at a time, and skip delivery fees.
0.5 to 1 cubic yard is the gray zone. Bags still work but mixing time grows fast, and finishing the slab in one continuous pour becomes harder. This is the range where many DIYers switch to a rented towable mixer or call for a short-load delivery.
Above 1 cubic yard, ready-mix is almost always the better choice. The pour finishes uniformly, labor is dramatically lower, and the cost per cubic yard is competitive once you factor in store runs, mixing labor, and the risk of cold joints between batches. Ask the supplier about short-load fees - most plants charge them below 3 to 4 cubic yards.
Common mistakes that ruin a concrete estimate
The five errors that show up most often in DIY slab planning.
Soil that has not been compacted absorbs visibly more concrete than a hard-packed base. On loose or sloped ground bump overage to 15 percent.
60 lb yields 0.45 cu ft; 80 lb yields 0.60 cu ft. Using the wrong constant under-orders the smaller bag by about 33 percent.
Most ready-mix plants charge a short-load fee below 3 to 4 cubic yards. A 1.5 yd pour at short-load pricing can cost more per yard than 4 yd at standard rates. Always ask.
If you cannot pour a slab in one continuous session, plan the cold joint at a control joint or transition - not in the middle of a span - so the seam does not become a future crack.
Frequently asked questions
Answers to the questions that come up most often when planning a small slab.
How many bags of concrete do I need for a 10 x 12 slab at 4 inches thick?
A 10 x 12 foot slab poured at 4 inches thick is 40 cubic feet of concrete before overage. With a 10 percent overage buffer for spillage and uneven subgrade, the adjusted volume is about 44 cubic feet. Using fixed planning yields, that becomes roughly 98 bags of 60 lb concrete (0.45 cu ft per bag) or 74 bags of 80 lb concrete (0.60 cu ft per bag). At that count, hand-mixing is operationally large, and ready-mix delivery is usually the more practical option.
How do I convert cubic feet of concrete to cubic yards?
Divide cubic feet by 27. One cubic yard equals 3 ft x 3 ft x 3 ft = 27 cubic feet. The calculator does this conversion automatically and shows both units so you can match it against ready-mix supplier quotes, which are almost always priced per cubic yard.
What is the standard thickness for a concrete slab?
For non-structural residential slabs - patios, sheds, walkways, and basic equipment pads - 4 inches is the common planning baseline. Driveways supporting vehicles typically use 4 to 6 inches. Garage floors and slabs that will see heavier loads often start at 5 to 6 inches. Anything structural, including footings and slabs tied into a foundation, should follow a stamped drawing or local building code rather than a general baseline.
How much concrete does one 80 lb bag cover?
A standard 80 lb bag of pre-mixed concrete yields about 0.60 cubic feet of cured concrete. A 60 lb bag yields about 0.45 cubic feet, and a 40 lb bag yields about 0.30 cubic feet. These planning constants assume the mix is hydrated correctly. Slumpy or wet mixes can yield slightly less because excess water reduces final volume after curing.
When should I switch from bags to ready-mix concrete?
Once total volume reaches about one cubic yard - roughly 60 bags of 80 lb mix - the bag workflow gets heavy and time-sensitive. Above 1 to 1.5 cubic yards most pros recommend ready-mix delivery. Ready-mix arrives uniformly mixed, lets you finish the slab as a single pour, and usually costs less per cubic yard than the equivalent in bags once you include labor and store runs.
Why do I need an overage on concrete orders?
Overage covers real-world losses that the raw math does not: spillage during pouring, uneven subgrade absorbing slightly more material than the calculated depth, edge spread before forms catch the mix, and small finishing waste. A 5 to 10 percent overage is typical for slabs. Pours with rough subgrade, sloped sites, or complex formwork often go to 10 to 15 percent.
Can I use this calculator for footings, foundations, or structural slabs?
This calculator is built for rectangular planning estimates - patios, sheds, walkways, small pads, and similar small pours. It is not engineering software. Footings, foundation walls, structural slabs, and any pour tied into a load-bearing system should follow a stamped drawing, an engineer's specification, or the local code authority. Use the result here as a first-pass planning number only.
Does the calculator account for rebar or wire mesh?
No. Reinforcement volume is a rounding error at the planning stage - rebar and wire mesh displace less than one percent of typical slab volume. The overage buffer already covers this. Reinforcement still matters for slab durability; this calculator just does not subtract its volume from the concrete order.
Related calculators and guides
Pages that pair with the concrete estimate.
Concrete bag yield guide Why 0.45 and 0.60 cu ft are the planning constants and how to check them against your bag label. Square feet to cubic yards guide The area-times-depth pattern behind every bulk material estimate. Gravel calculator Estimate the gravel base under a slab pour in cubic yards and tons. How these estimates work Rounding, overage, and unit handling across every calculator on the site.