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Concrete Calculator
Last updated: June 19, 2026
A concrete calculator is a concrete volume estimation utility used to estimate the volume and quantity of concrete required for rectangular slabs. It calculates the necessary volume in cubic yards and cubic feet, and projects the total number of pre-mixed concrete bags needed in common weights like forty-pound, sixty-pound, or eighty-pound bags. Contractors, builders, and DIY homeowners use this calculator to estimate material costs, prevent shortage or excess waste, and prepare concrete orders.
Enter your slab's length, width, and thickness. We compute the exact volume in cubic yards and feet, plus the number of pre-mix bags you'll need at the size you choose.
Quick Answer
Estimate concrete for a slab. Enter length, width, and thickness to get cubic yards, cubic feet, and how many 40, 60, or 80 lb bags to buy, including a waste allowance.
Slabs are commonly 4–6 inches.
10% covers spillage and edge overpour.
Bag size
80 lb bags (incl. waste)
62
61.1 exact, rounded up
At 1.23 yd³, ready-mix from a truck is usually cheaper and faster than mixing this many bags by hand.
Examples
10 ft × 10 ft × 4 in slab
≈ 1.23 yd³ · 56 × 80 lb bags
20 ft × 12 ft × 6 in patio
≈ 4.44 yd³ · ready-mix recommended
3 ft × 3 ft × 4 in pad
≈ 0.11 yd³ · 5 × 80 lb bags
How it works
Volume in cubic feet is length × width × thickness, with thickness converted from inches to feet. We divide by 27 to get cubic yards, then divide by each bag's yield to get bag count.
Volume · L × W × (T ÷ 12) ÷ 27 = yd³
Bag yield: 40 lb ≈ 0.30 ft³ · 60 lb ≈ 0.45 ft³ · 80 lb ≈ 0.60 ft³
Bags of concrete for common slab sizes (4-inch slab, 80 lb bags)
Below is a handy lookup table for common concrete slab sizes computed at a standard 4-inch thickness using 80 lb bags (0.60 cubic feet yield per bag, rounded up to the nearest whole bag, assuming no waste):
| Slab Size (ft) | Volume (cu ft) | Volume (cu yd) | 80 lb Bags (no waste) |
|---|---|---|---|
| 8 × 8 ft | 21.33 ft³ | 0.79 yd³ | 36 bags |
| 10 × 10 ft | 33.33 ft³ | 1.23 yd³ | 56 bags |
| 10 × 12 ft | 40.00 ft³ | 1.48 yd³ | 67 bags |
| 12 × 12 ft | 48.00 ft³ | 1.78 yd³ | 80 bags |
| 16 × 20 ft | 106.67 ft³ | 3.95 yd³ | 178 bags |
| 20 × 20 ft | 133.33 ft³ | 4.94 yd³ | 223 bags |
Note: Always add 5–10% for waste to cover uneven subgrades, edge overpour, and spillage. The calculator above includes a customizable waste allowance automatically.
How to calculate concrete in cubic yards
Calculating concrete volume involves finding the three-dimensional space of your slab form and converting it into cubic yards (the standard industrial ordering unit):
Formula
Volume (cu ft) = Length (ft) × Width (ft) × (Thickness (in) ÷ 12)
Volume (cu yd) = Volume (cu ft) ÷ 27
Worked Example
For a standard 10 × 10 ft slab at 4 inches thick:
- Convert thickness: 4 in ÷ 12 = 0.333 ft
- Calculate cubic feet: 10 × 10 × 0.333 = 33.33 ft³
- Convert to cubic yards: 33.33 ÷ 27 = 1.23 yd³
How many bags per cubic yard
If you are using pre-mixed bagged concrete, you must divide one cubic yard (27 cubic feet) by the specific yield of the bag weight you purchase:
- 80 lb bags (yields 0.60 ft³): 27 ÷ 0.60 = 45 bags per cubic yard.
- 60 lb bags (yields 0.45 ft³): 27 ÷ 0.45 = 60 bags per cubic yard.
- 40 lb bags (yields 0.30 ft³): 27 ÷ 0.30 = 90 bags per cubic yard.
Bags vs ready-mix
For small pads, fence posts, deck footings, or minor repair patches, mixing concrete bags by hand is cost-effective and practical. However, at around 1 cubic yard or more (which requires 45 of the heavy 80 lb bags), ordering ready-mix concrete from a delivery truck is highly recommended. Ready-mix saves extensive manual labor, guarantees a uniform cure, and is often priced competitively once you factor in delivery and aggregate consistency.
How thick should a slab be
Slab thickness depends directly on the load it will support:
- 4 inches: The standard minimum thickness for walkways, garden paths, backyard patios, and small shed pads.
- 5 to 6 inches: Required for residential driveways, garage floors, or parking pads designed to hold passenger vehicles and light trucks.
- Local standards: Always check municipal building codes and soil compaction profiles, as local frost lines and sandy soils often dictate thicker edges or rebar reinforcement.
Common mistakes
- Entering thickness in feet: Entering "4" for thickness directly into a cubic-feet formula without dividing by 12. This treats the slab as 4 feet thick instead of 4 inches, multiplying your material estimate by 12.
- Skipping the waste allowance: Ordering only the exact mathematical volume. Uneven subgrades and minor spills always happen; failing to add 5–10% extra can leave you short on a pour.
- Confusing concrete with cement: Cement is simply the gray powder binder (composed of limestone and calcium). Concrete is the final structural aggregate mixture of cement, sand, gravel, and water.
Related home measurement calculators
Sources: Concrete volume yields and bag coverage values conform to manufacturer product specifications, including published technical data sheets from Quikrete and Sakrete. Last reviewed: June 2026.
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Frequently asked questions
A 10×10 foot slab at 4 inches thick requires 56 bags of 80 lb concrete mix before accounting for waste (or 62 bags if including a standard 10% waste margin).
It takes exactly 45 bags of 80 lb concrete mix to equal one cubic yard of concrete, as each 80 lb bag yields 0.6 cubic feet of volume.
It takes exactly 60 bags of 60 lb concrete mix to equal one cubic yard of concrete, as each 60 lb bag yields 0.45 cubic feet of volume.
There are exactly 27 cubic feet in one cubic yard of volume. If you need to convert between these units, use our dedicated <Link href="/cubic-feet-in-a-cubic-yard" className="text-primary hover:underline">cubic feet to cubic yards converter</Link>.
Cement is a binding ingredient (a fine powder made of limestone and clay) used to manufacture concrete, which also includes aggregates like sand, gravel, and water.
A typical concrete slab should be 4 inches thick for patios, walkways, and shed bases, and 5 to 6 inches thick for driveways or heavy load-bearing surfaces.
You should order ready-mix concrete from a delivery truck for projects requiring more than 1 cubic yard (about 45 bags of 80 lb mix) to save labor and ensure structural consistency.
We recommend adding a 10% waste allowance to your calculations to cover spillage, uneven ground, and formwork variations.
Concrete takes about 28 days to reach its full structural design strength, though it is usually dry enough to walk on after 24 to 48 hours.
No, you should pour concrete over a compacted base of gravel or crushed stone rather than bare dirt to prevent cracking from settling and shifting.
This concrete calculator currently models flat rectangular slabs only, but you can calculate footing volume by treating it as a small deep slab.
The standard pre-mixed concrete bag sizes sold in most hardware stores are 40 lb, 60 lb, and 80 lb bags.
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