Plan your entire concrete pour in one place. Calculate concrete yards or bags for multiple rectangular or circular slabs at once, estimate subbase gravel and rebar requirements, verify vehicle hauling capacity, and compare ready-mix delivery costs.
Reviewed and updated June 30, 2026
Use it when you want to know
Multi-Slab Pour PlanningCombine multiple slabs of different shapes (like a patio and a matching sidewalk) in a single estimate.
Subbase & Rebar EstimationsCalculate tons of crushed gravel base and the total linear feet or pieces of reinforcing rebar needed.
Bagged vs. Ready-Mix Truck CostCompare local retail bag pricing against ready-mix yard deliveries, accounting for short-load fees.
Calculator
Concrete slab estimating tool
Create a detailed list of all the slabs you are pouring. You can add multiple areas, adjust thickness, and estimate auxiliary materials like gravel subbase and reinforcement.
Estimated concrete volume
0.00 cu yd
Includes 10% waste buffer (0.00 cu yd raw volume).
Cubic Yards0.00
Cubic Feet0.0
Required Bags0
Job materials details
Hauling & Transport Weight0 lbs (0.00 tons)Check cargo payload limits before hauling concrete bags.
Estimated Mixing Water0.0 quarts (0.0 gallons)Approximate water requirements for proper mixing consistency.
Please enter slab dimensions to see recommendations.
Estimated Materials Cost Comparison
Bagged Concrete Cost$0.00Based on bags + subbase/rebar if enabled.
Ready-Mix Truck Cost$0.00Includes short-load fee if applicable.
Guide
How to calculate concrete for a slab project
Using a professional-grade **concrete slab calculator** before starting your pour ensures you purchase the exact quantity of material required for outdoor patios, walkways, shed pads, and driveways. A concrete shortfall mid-pour results in a weak seam called a "cold joint," while overestimating results in wasted materials and money.
How to Estimate Concrete Volume
Concrete volume is measured in cubic yards (commonly just called "yards") or cubic feet. To compute the volume:
Multiply the slab length by the width to get the square footage.
Convert the slab thickness from inches to feet (divide the inches by 12).
Multiply the square footage by the decimal thickness in feet to get total cubic feet.
Divide the cubic feet by 27 to convert the result to cubic yards (since 1 cubic yard = 3 ft × 3 ft × 3 ft = 27 cubic feet).
Use for round patios, column footings, or tree ring borders.
Worked Example: 10x12 Patio at 4 Inches Thick
Let's calculate concrete requirements for a typical rectangular patio measuring 10 ft by 12 ft with a thickness of 4 inches, using standard yields from manufacturers like Quikrete or Sakrete:
Area: 10 ft × 12 ft = 120 sq ft
Thickness in feet: 4 inches ÷ 12 = 0.333 ft
Net Cubic Volume: 120 sq ft × 0.333 ft = 40.0 cu ft
Convert to Cubic Yards: 40.0 cu ft ÷ 27 = 1.48 cu yds
With 10% Waste Factor: 40.0 cu ft × 1.10 = 44.0 cu ft (1.63 cu yds)
Bags of 80 lb needed: 44.0 cu ft ÷ 0.60 cu ft yield = 73.3 → 74 bags
Bags of 60 lb needed: 44.0 cu ft ÷ 0.45 cu ft yield = 97.7 → 98 bags
Why a gravel base is crucial
Directly pouring concrete onto raw soil leads to cracks. Clay soil expands and shrinks with water, while cold winters cause frost heaves. A compacted base of subbase gravel (ideally 3/4-inch crushed road-base stone with fines) spreads the weight of the slab evenly and allows water to drain away.
Rebar reinforcement guidelines
While concrete stands up to high compression loads, it has low tensile strength. Steel rebar grid adds strength and holds the slab together if cracking occurs. Lay rebar on chairs spaced 16 or 18 inches apart, elevated to sit in the middle of the slab's depth, and overlap joints by 12 to 18 inches.
Common DIY pitfalls
Underestimating Thickness: Uneven ground can easily make a 4-inch pour average 4.5 inches, requiring 12% more concrete. Always add waste. Vehicle Overload: SUV payload capacity is usually < 900 lbs (only eleven 80 lb bags!). Hauling 74 bags (nearly 6,000 lbs) requires truck delivery. Excess Water: Soupy concrete is easy to pour but dilutes cement paste, reducing PSI strength by up to 50%.
FAQ
Frequently asked questions
How thick should a concrete slab be?
For residential walkways, patios, and shed pads, a thickness of 4 inches (10 cm) is typical. For driveways, heavy equipment pads, or parking areas, 5 to 6 inches is recommended to handle the load without cracking.
How many 80 lb bags of concrete make a cubic yard?
An 80 lb bag of concrete yields 0.60 cubic feet of mixed material. Since a cubic yard is 27 cubic feet, it takes exactly 45 bags of 80 lb concrete to make one cubic yard. If using 60 lb bags, you need 60 bags per cubic yard.
Do I need a gravel base (subbase) for a concrete slab?
Yes, in most climates. A 4-inch base of compacted crushed gravel provides crucial drainage and a stable foundation, preventing soil movement and frost heaves from cracking the concrete slab.
How do you calculate rebar for a slab?
Rebar is calculated by layout runs. For an 18-inch grid pattern, you determine the runs required along both the length and width of the slab, multiply by the slab dimensions to get total linear feet, and then add 10% to 15% for lap joints (where rebar overlaps).
When is it better to order a ready-mix truck instead of mixing bags?
As a general rule: if your project volume exceeds 1 cubic yard (roughly 45 bags of 80 lb concrete), mixing by hand gets extremely exhausting. For projects over 2 cubic yards (90 bags), ordering a ready-mix truck is highly recommended. However, be aware of "short-load fees" charged by ready-mix suppliers for deliveries under 3 to 5 cubic yards.
What is the standard waste factor for concrete slabs?
It is standard practice to add a 10% waste buffer. This covers over-excavation (uneven ground can easily make a 4-inch slab average 4.25 inches thick, requiring 6% more concrete), spillage, and errors.