📏 Slab Dimensions
ft
Measure in feet (convert inches: divide by 12)
Enter a valid length greater than 0.
ft
Width of your slab in feet
Enter a valid width greater than 0.
inches
4" = patio/walk · 6" = garage/driveway · 8" = heavy use
Enter thickness (0.5–24 inches).
Industry standard: always order 10% extra
$/yd³
National avg $140–$200/yd³ in 2026 — leave blank to skip cost
Enter a valid price.
Concrete Volume Needed (with waste)
—
Bags Needed (if using bagged concrete)
⚠️ Disclaimer: Volume calculations use the standard ACI formula. Order from your concrete supplier based on the cubic yard total shown. Actual volume may vary slightly due to subgrade unevenness. Never order less than calculated — running out mid-pour creates a cold joint that compromises structural integrity.
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📏 Circular Slab Dimensions
ft
Full diameter (not radius) in feet
Enter a valid diameter greater than 0.
inches
Thickness in inches
Enter thickness (0.5–24 inches).
$/yd³
Leave blank to skip cost estimate
Enter a valid price.
Concrete Volume Needed (with waste)
—
Bags Needed (if using bagged concrete)
⚠️ Formula: Volume = π × (Diameter ÷ 2)² × Thickness (ft) ÷ 27. Always add 10% overage when ordering.
📏 Column / Post Hole Dimensions
inches
Outer diameter in inches (e.g. Sonotube size)
Enter diameter (1–120 inches).
ft
Height of column or depth of post hole in feet
Enter height greater than 0.
columns
Total columns for deck, porch, or fence posts
Enter number of columns (1–500).
Total Concrete Volume (with waste)
—
Bags Per Column & Total (80 lb bags)
⚠️ Formula: Volume per column = π × (Dia ÷ 24)² × Height ÷ 27 (all converted to feet). Quikrete QUIK-TUBE bag recommendation: 2-foot diameter hole, 4 feet deep = approximately 3.5 bags of 80-lb mix per hole.
Sources & Methodology
All formulas sourced from ACI 318 Building Code (rectangular and circular volume), Quikrete and Sakrete manufacturer bag yield specifications (bag counts), and ConcreteNetwork.com (industry-standard overage guidance).
American Concrete Institute — ACI 318 Building Code
Standard volume formula: V = L × W × T (all in feet) ÷ 27 for rectangular slabs; V = π × r² × h ÷ 27 for circular sections. Minimum slab thickness requirements by application type used in thickness guide.
Quikrete — Product Yield Specifications
Official manufacturer yield data: 80-lb bag = 0.60 cubic feet (0.022 cubic yards), 60-lb bag = 0.45 cubic feet (0.017 cubic yards), 40-lb bag = 0.30 cubic feet (0.011 cubic yards). These are the exact bag yield figures used in this calculator’s bag count calculation.
ConcreteNetwork.com — Concrete Calculator & Ordering Guide
Industry standard 10% overage recommendation, ready-mix ordering practices, and weight benchmark (4,050 lbs per cubic yard for standard structural concrete) used in this calculator.
Exact Formulas Used:
Rectangular: Volume (ft³) = Length × Width × (Thickness ÷ 12) Circular: Volume (ft³) = π × (Diameter ÷ 2)² × (Thickness ÷ 12) Column: Volume (ft³) = π × (Diameter_inches ÷ 24)² × Height_ft × Num_Columns Cubic Yards = Volume (ft³) ÷ 27 80-lb Bags = ⌈Volume (ft³) ÷ 0.60⌉ 60-lb Bags = ⌈Volume (ft³) ÷ 0.45⌉ 40-lb Bags = ⌈Volume (ft³) ÷ 0.30⌉ Waste factor applied BEFORE dividing by 27 and calculating bags. Weight: Volume (ft³) × 150 lbs/ft³ (unit weight of standard 3,000 PSI concrete per ACI).
Rectangular: Volume (ft³) = Length × Width × (Thickness ÷ 12) Circular: Volume (ft³) = π × (Diameter ÷ 2)² × (Thickness ÷ 12) Column: Volume (ft³) = π × (Diameter_inches ÷ 24)² × Height_ft × Num_Columns Cubic Yards = Volume (ft³) ÷ 27 80-lb Bags = ⌈Volume (ft³) ÷ 0.60⌉ 60-lb Bags = ⌈Volume (ft³) ÷ 0.45⌉ 40-lb Bags = ⌈Volume (ft³) ÷ 0.30⌉ Waste factor applied BEFORE dividing by 27 and calculating bags. Weight: Volume (ft³) × 150 lbs/ft³ (unit weight of standard 3,000 PSI concrete per ACI).
Last reviewed: April 2026 | Formulas verified against ACI 318 and manufacturer yield specs
Concrete Slab Calculator Guide 2026 — How to Calculate Concrete the Right Way
Whether you are pouring a patio, driveway, garage floor, or footings for a deck, calculating the right amount of concrete is critical. Order too little and you get a cold joint — a structural weak point. Order too much and you waste money. This guide explains every step of the calculation, the exact formulas, and the industry rules that concrete professionals use every day.
💡 Quick shortcut for 4-inch slabs: Divide your total square footage by 81 to get cubic yards. Example: 200 sq ft patio ÷ 81 = 2.47 cubic yards (before waste). Add 10%: order 2.72 cubic yards. This formula is correct because 4" = 1/3 ft, and 27 ÷ (1/3) = 81.
The Concrete Volume Formula — Verified by ACI
The core formula for any concrete pour is volume = length × width × thickness, with all measurements in feet. This gives you cubic feet. Divide by 27 to convert to cubic yards, which is how ready-mix concrete is sold.
The critical step most people miss: convert thickness from inches to feet before multiplying. A 4-inch slab = 4 ÷ 12 = 0.333 feet. A 6-inch slab = 6 ÷ 12 = 0.500 feet. Every calculator error in concrete comes from forgetting this conversion.
Concrete Thickness Reference by Application
| Application | Recommended Thickness | PSI Minimum | Cubic Yards per 100 sq ft |
|---|---|---|---|
| Garden path / walkway | 3–4 inches | 3,000 PSI | 0.93–1.23 yd³ |
| Patio (foot traffic only) | 4 inches | 3,000 PSI | 1.23 yd³ |
| Residential driveway | 5–6 inches | 3,500 PSI | 1.54–1.85 yd³ |
| Garage floor | 4–6 inches | 3,500 PSI | 1.23–1.85 yd³ |
| RV / heavy vehicle pad | 6 inches | 4,000 PSI | 1.85 yd³ |
| Residential footing | 8–12 inches | 3,000 PSI | 2.47–3.70 yd³ |
| Commercial / heavy load | 6–8 inches+ | 4,000+ PSI | 1.85–2.47+ yd³ |
Bags vs. Ready-Mix — When to Use Each
Bagged concrete (Quikrete, Sakrete) is convenient for small projects but expensive per cubic yard and labor-intensive at scale. Official Quikrete yield specs: Each 80-lb bag yields exactly 0.60 cubic feet (0.022 cubic yards). Each 60-lb bag yields 0.45 cubic feet. Each 40-lb bag yields 0.30 cubic feet.
| Project Size | Bags of 80-lb | Ready-Mix Cost | Bag Cost Estimate | Recommendation |
|---|---|---|---|---|
| 0.5 cubic yards | 23 bags | $70–$100 + delivery min | $140–$210 | Bags practical |
| 1 cubic yard | 45 bags | $140–$200 + short-load fee | $270–$405 | Ready-mix often better |
| 2 cubic yards | 90 bags | $280–$400 | $540–$810 | Ready-mix strongly preferred |
| 5+ cubic yards | 225+ bags | $700–$1,000 | $1,350+ | Ready-mix only |
Bag costs based on 2026 Home Depot/Lowe’s pricing ($6–$9 per 80-lb bag). Ready-mix includes $150–$400 short-load fee for orders under minimum (typically 3–4 yd³).
Concrete Coverage Chart — How Much Area Per Cubic Yard
| Thickness | Coverage per Cubic Yard | Formula (sq ft ÷ X = yd³) |
|---|---|---|
| 2 inches | 162 sq ft / yd³ | sq ft ÷ 162 |
| 3 inches | 108 sq ft / yd³ | sq ft ÷ 108 |
| 4 inches | 81 sq ft / yd³ | sq ft ÷ 81 ✔ |
| 5 inches | 65 sq ft / yd³ | sq ft ÷ 65 |
| 6 inches | 54 sq ft / yd³ | sq ft ÷ 54 |
| 8 inches | 40 sq ft / yd³ | sq ft ÷ 40 |
| 10 inches | 32 sq ft / yd³ | sq ft ÷ 32 |
| 12 inches | 27 sq ft / yd³ | sq ft ÷ 27 |
Why You Must Always Order 10% Extra
The concrete industry universally recommends ordering 5 to 10 percent more concrete than your calculated volume. The reasons are practical and unavoidable:
- Uneven subgrade: Even a carefully graded subgrade has high and low spots that consume more concrete than a perfectly flat calculation assumes.
- Form movement: Concrete pressure causes forms to flex slightly outward, increasing volume consumed.
- Spillage: Every pour loses some concrete in the chute, on the ground, and in equipment.
- Cold joint risk: If you run out mid-pour and must stop, the concrete that has already been placed will begin to set. When you resume with fresh concrete, the new pour cannot chemically bond with the hardened concrete, creating a cold joint — a permanent seam of weakness that reduces slab strength and invites water infiltration.
Concrete Weight
Standard structural concrete (3,000–4,000 PSI) weighs approximately 150 lbs per cubic foot or 4,050 lbs (about 2 tons) per cubic yard. This matters for: truck load limits (ready-mix trucks are typically limited to 10 cubic yards), elevated slab structural design, and concrete breaking and disposal (broken concrete weighs roughly 2,000 lbs per cubic yard).
Frequently Asked Questions
Measure length and width in feet. Convert thickness from inches to feet (divide by 12). Multiply: Length × Width × Thickness = cubic feet. Divide by 27 = cubic yards. Add 10% for waste. Example: 10 ft × 10 ft × (4 ÷ 12 ft) = 33.33 cubic feet ÷ 27 = 1.23 cubic yards. With 10% waste: order 1.36 cubic yards.
There are exactly 27 cubic feet in 1 cubic yard (3 ft × 3 ft × 3 ft = 27 cu ft). This is the fundamental concrete conversion. All concrete volume calculations convert cubic feet to cubic yards by dividing by 27. Ready-mix concrete is sold by the cubic yard; bagged concrete yields are listed in cubic feet.
Per cubic yard: 45 bags of 80-lb mix (each yields 0.60 cubic feet per Quikrete specs), 60 bags of 60-lb mix (each yields 0.45 cubic feet), or 90 bags of 40-lb mix (each yields 0.30 cubic feet). For projects over 1 cubic yard, ready-mix delivery is typically more cost-effective.
Standard thicknesses: Walkways / garden paths = 3–4 inches. Patios = 4 inches. Driveways = 5–6 inches. Garage floors = 4–6 inches. RV pads or heavy vehicle use = 6 inches. Footings = 8–12 inches minimum. ACI 318 and most residential codes specify a minimum of 4 inches for any load-bearing residential slab.
1 cubic yard covers: 81 sq ft at 4 inches thick. 54 sq ft at 6 inches. 40 sq ft at 8 inches. 162 sq ft at 2 inches. The contractor rule for 4-inch slabs: divide total square footage by 81 to get cubic yards. This shortcut is mathematically exact: 27 ÷ (4÷12) = 27 ÷ 0.333 = 81.
Use bags for projects under 0.5–1 cubic yard (fence posts, small repairs). Use ready-mix for 1 cubic yard and above. A typical 10×10 ft patio at 4 inches needs 1.23 cubic yards — technically 56 bags of 80-lb mix, but exhausting to mix by hand. At 2 cubic yards and above, ready-mix is faster, cheaper per cubic yard, and produces a better uniform mix.
Ready-mix concrete costs $140 to $200 per cubic yard in 2026 depending on region, PSI, and order quantity. Short-load fees of $150–$400 apply for orders below the supplier minimum (usually 3–4 cubic yards). Bagged concrete (80-lb bags at $6–$9 each) works out to $270–$405 per cubic yard — far more expensive than ready-mix for larger projects.
A cold joint forms when fresh concrete contacts concrete that has already begun to harden. This creates a permanent seam of weakness because new concrete cannot chemically bond with hardened concrete. Cold joints allow water infiltration and reduce structural strength. They occur when a pour runs short of concrete and must be stopped. Always order 10% extra to ensure you can complete the pour without interruption.
Standard structural concrete weighs 150 lbs per cubic foot, or approximately 4,050 lbs (about 2 tons) per cubic yard. This matters for truck load limits (ready-mix trucks typically carry up to 10 cubic yards), elevated slab structural engineering, and concrete removal (broken concrete weighs roughly 2,000 lbs per cubic yard due to void space).
3,000 PSI for standard residential slabs (patios, walkways). 3,500 PSI for driveways and garage floors. 4,000 PSI for cold-climate driveways (freeze-thaw resistance) and heavy loads. 5,000+ PSI for structural members and high-load commercial applications. PSI does not change the volume calculation — only the concrete mix specification you order from your ready-mix supplier or bag product you select.
Use the circular area formula: Volume = π × (Diameter ÷ 2)² × Thickness (in feet) ÷ 27. Example: 8-ft diameter circular patio, 4 inches thick: π × (4)² × (4÷12) = 3.14159 × 16 × 0.333 = 16.76 cubic feet ÷ 27 = 0.62 cubic yards. Add 10% = order 0.68 cubic yards. Use our Circular Slab tab above for instant results.
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