Concrete Calculator
Estimate the cubic yards of concrete and number of bags needed for any slab or pour.
Reviewed by the ToolNestr Editorial Team — July 2026
How the concrete calculator works
The concrete calculator estimates the total volume of concrete needed for a slab based on length, width, and depth. It converts the volume from cubic feet to cubic yards — the standard unit for ordering ready-mix concrete. It also calculates the number of bags required based on the bag size you choose.
Concrete volume is straightforward: multiply the length by the width to get the surface area, then multiply by the depth. Since depth is typically measured in inches while length and width are in feet, the calculator converts depth to feet by dividing by 12. The result in cubic feet is then divided by 27 to get cubic yards, since one cubic yard equals 27 cubic feet.
For bagged concrete, the calculator uses standard yields: a 60 lb bag covers approximately 0.45 cubic feet, while an 80 lb bag covers about 0.6 cubic feet. Ready-mix orders are typically placed by the cubic yard and delivered by a concrete truck carrying 8-12 cubic yards per load.
Mix proportioning and yield figures follow ACI 211.1 (Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete).
Measure the area
Measure the length and width of the slab area in feet. Measure the planned depth in inches.
Choose bag type
Select 60 lb bags, 80 lb bags, or ready-mix. The calculator adjusts the bag count accordingly.
Review results
Check cubic yards for ready-mix or bag count for DIY. Add 5-10% extra for waste and spillage.
The formula explained
Volume in cubic feet
V = L × W × (D ÷ 12). A slab 20′ × 10′ with a 4-inch depth gives 20 × 10 × (4÷12) = 66.67 cu ft.
Convert to cubic yards
CY = V ÷ 27. From the example: 66.67 ÷ 27 = 2.47 cu yd. You would order 2.5 cubic yards from the ready-mix company.
Bag count
Bags = V ÷ bag yield. Using 60 lb bags (0.45 cu ft each): 66.67 ÷ 0.45 = 148 bags. With 80 lb bags (0.6 cu ft): 66.67 ÷ 0.6 = 111 bags.
Worked example
A 24′ × 20′ driveway at 5 inches deep, with 10% extra for waste.
Use cases for the concrete calculator
Driveway and patio
Plan your driveway or patio pour with confidence. Know exactly how much concrete to order, whether you are using bagged mix for a small patio or ready-mix for a full driveway. Add rebar or wire mesh reinforcement for strength.
Foundation footings
Calculate concrete for foundation footings and stem walls. Footings require precise volume estimates since they are below grade and harder to adjust during the pour.
Walkways and paths
Garden paths and front walkways typically use 4-inch thick concrete. The calculator helps you compare the cost of bagged vs. ready-mix concrete for these smaller projects.
Pool deck and steps
Pool surrounds, steps, and benches all need concrete. Measure the irregular shape as a series of rectangles and add the volumes together for an accurate total.
Tips for ordering concrete
Always order 10% extra
Subgrades are rarely perfectly level, and some concrete is lost to spillage and the truck chute. Adding 10% to your estimate ensures you do not run short. Any leftover concrete can be used for a small project or disposed of according to local regulations.
Understand ready-mix minimums
Most ready-mix trucks carry a minimum load of 3-4 cubic yards. If your project needs less, you will pay a short-load fee. Compare the cost of bagged concrete versus a short load of ready-mix to find the cheaper option for small pours.
Reinforcement matters for strength
Concrete is strong in compression but weak in tension. For slabs that will bear weight — driveways, garage floors, foundations — add rebar or welded wire mesh. The calculator does not include reinforcement materials, so budget for them separately.
Real-world context
A typical residential driveway replacement is one of the most common places this calculator gets used. A 20 ft by 40 ft driveway poured 5 inches thick needs 20 × 40 × (5/12) = 333 cubic feet, or about 12.3 cubic yards — right at the edge of a single ready-mix truck's typical 8 to 12 cubic yard capacity, which is exactly why contractors size driveway sections to avoid a costly second delivery mid-pour.
Small DIY projects lean the other way. A 3 ft by 4 ft mailbox post pad, 4 inches deep, needs only 4 cubic feet — about nine 60 lb bags — well under a ready-mix truck's minimum load, so bagged concrete is the practical (and usually cheaper) choice for anything under roughly 1 cubic yard.
Foundation footings sit at the higher-stakes end of the scale. A continuous footing 60 ft long, 16 inches wide, 8 inches deep needs 60 × (16/12) × (8/12) = 53.3 cubic feet, about 2 cubic yards — and because footings are poured below grade and hard to patch once backfilled, contractors routinely round this kind of estimate up rather than down.
Common misconceptions
"A cubic yard of concrete and a cubic yard of gravel weigh about the same." Not quite. Normal-weight concrete runs about 150 lb per cubic foot (roughly 4,050 lb per cubic yard), while loose gravel is closer to 100-120 lb per cubic foot. Concrete is denser because cement paste fills the gaps between aggregate almost completely.
"If I mix the bags a little wet, I'll get more volume out of them." This backfires. Adding excess water beyond the bag's specified ratio dilutes the cement paste, weakening the cured concrete's compressive strength — it doesn't meaningfully increase yield, and can turn a driveway-rated pour into one that cracks and spalls years early.
Related calculators
Frequently asked questions
How many cubic feet are in a cubic yard?
There are 27 cubic feet in one cubic yard (3 ft x 3 ft x 3 ft). This is the standard unit for ordering ready-mix concrete.
How many bags of concrete do I need per cubic yard?
A 60 lb bag yields about 0.45 cubic feet, so you need roughly 60 bags per cubic yard. An 80 lb bag yields about 0.6 cubic feet, needing about 45 bags per cubic yard.
Should I order extra concrete?
Yes, order 5-10% extra to account for spillage, uneven subgrade, and slight over-excavation. Ready-mix trucks carry a minimum load, so check with your supplier.
What is the minimum depth for a concrete slab?
A 4-inch thickness is standard for patios, walkways, and garage floors. Driveways need 5-6 inches. Heavy-load areas may require 8 inches or more.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.