Concrete Calculator

How the Concrete Calculator Works

The concrete calculator is one of the most-used construction tools online — searched over 550,000 times per month in the US, peaking in spring and summer when contractors and DIYers pour slabs. It estimates concrete volume for any shape: rectangular slabs, circular pads, and cylindrical footings or columns.

Slab formula: Volume (cubic yards) = Length (ft) × Width (ft) × Thickness (ft) ÷ 27. The ÷ 27 converts cubic feet to cubic yards (1 cubic yard = 27 cubic feet).

Worked example — 20×30 ft driveway, 4 inches thick: Volume = 20 × 30 × (4/12) ÷ 27 = 20 × 30 × 0.333 ÷ 27 = 200 ÷ 27 = 7.41 cubic yards. Always add 10% for waste and uneven subgrade: 7.41 × 1.10 = 8.15 cubic yards ordered.

Column/footing formula: Volume = π × r² × Height ÷ 27. A 12-inch diameter (r = 0.5 ft) footing 3 ft deep: V = 3.14159 × 0.25 × 3 ÷ 27 = 0.0873 cubic yards per footing.

Concrete Slab Calculator: Coverage by Project Type

The concrete slab calculator gets 49,500 searches per month on its own — the single most common concrete calculation. Here's what each project typically needs:

• 10×10 ft patio (4" thick): 10 × 10 × 0.333 ÷ 27 = 1.23 cu yd = ~56 bags of 80-lb mix
• 20×20 ft garage floor (4" thick): 4.94 cu yd = ~222 bags or 1 short-load ready-mix truck
• 100 ft sidewalk, 4 ft wide, 4" thick: 4.94 cu yd = same as garage floor above
• 2-car driveway 20×40 ft (5" thick): 12.35 cu yd = requires ready-mix delivery

How Many Bags of Concrete Do I Need?

This is one of the most common questions on job sites. Bag yield varies by weight:

• 40-lb bag: Yields 0.30 cubic feet. Need 90 bags per cubic yard.
• 60-lb bag: Yields 0.45 cubic feet. Need 60 bags per cubic yard.
• 80-lb bag: Yields 0.60 cubic feet. Need 45 bags per cubic yard.
• 50-lb bag (Quikrete 5000): Yields 0.375 cubic feet. Need 72 bags per cubic yard.

Quick formula: Cubic feet needed = Length × Width × Thickness (all in feet). Then divide by bag yield. Example: 10 × 10 × 0.333 = 33.3 cubic feet. Using 80-lb bags: 33.3 ÷ 0.60 = 56 bags.

Concrete Yardage Calculator: When to Order Ready-Mix

The concrete yardage calculator (14,800 monthly searches) is critical for any pour larger than a few bags. Ready-mix thresholds to know:

• Under 0.5 cu yd (13.5 cu ft): Bags are practical — about 24 bags of 80-lb mix
• 0.5–1.0 cu yd: Gray area; bags become laborious but short-load concrete gets expensive
• Over 1.0 cu yd: Ready-mix typically wins on labor cost and quality consistency
• Under 4 cu yd: Expect a short-load surcharge from ready-mix companies ($50–$150)
• Full truck (8–10 cu yd): Best unit price; typical cost $140–$200 per cubic yard delivered

Concrete must be placed within 90 minutes of batching. Have your subgrade, forms, reinforcement, and crew fully ready before the truck arrives.

Recommended Thickness by Application

• Walkways and patios: 4 inches minimum (3.5" finished after forming)
• Residential driveways: 4–6 inches for passenger vehicles
• Driveways with RV/truck traffic: 6 inches with 3,500+ psi mix
• Garage floors: 4–6 inches; vapor barrier recommended under slab
• Foundation footings: 8–12 inches or per structural engineering specs
• Fence posts: Footing depth = 1/3 of post height above grade, minimum 24" for frost-free areas

Concrete Cost Calculator: Budget Estimate

Concrete cost (4,400 monthly searches) varies widely by region. As a baseline for 2025:

• Ready-mix concrete: $140–$200 per cubic yard (material only)
• 80-lb bag (Quikrete/Sakrete): $7–$9 at home improvement stores
• Labor for pour + finish: $3–$10 per square foot depending on complexity
• Reinforcement (rebar #4): $0.70–$0.90 per linear foot
• Wire mesh (6×6 W1.4): $0.15–$0.25 per square foot

For a 400 sq ft driveway (20×20) at 4 inches thick: ~6 cubic yards of concrete = ~$900–$1,200 material cost. Add $2,400–$4,000 for professional pour, finish, and curing = total $3,300–$5,200 installed.

Frequently Asked Questions

Why add 10% extra to my concrete order?

Subgrade is never perfectly flat, forms bow slightly, and some concrete is inevitably lost to spillage and transitions. A 10% overage is the industry standard for flat work. For complex projects with many corners, curves, or deep footings, use 15%. Running out of concrete mid-pour forces a cold joint (a weakness in the slab), so ordering slightly more is always the better call.

What concrete mix strength do I need?

PSI (pounds per square inch) indicates compressive strength at 28 days. Common mixes: 2,500 psi for residential sidewalks and patios; 3,000 psi for driveways and most slabs; 3,500–4,000 psi for garage floors with heavy vehicles; 4,000+ psi for footings and structural elements. Most bagged concrete is 4,000 psi — the extra strength is worth the minimal cost difference on small projects.

How long does concrete take to cure?

Initial set: 24–48 hours (can walk on it carefully). Usable strength (about 70%): 7 days. Full design strength (28-day cure): 28 days. For driveways, wait 7 days before light vehicles and 28 days before heavy vehicles. Keep concrete moist the first 7 days — rapid drying causes surface cracking. Curing blankets or plastic sheeting help in hot or windy conditions.