Concrete Slab Calculator
Total Area
Volume
Slab Diagram
3D ViewReady-Mix Truck Estimator
After calculating your cubic yardage, determine how many truckloads you need. Small trucks carry up to 5 cu yd, while standard full-size trucks carry 8–10 cu yd. Short loads (partial trucks) typically incur an extra fee of $15–$50 per cubic yard under the minimum.
Small Truck
Up to 5 cu yd
Ideal for small patios, walkways, and post holes
Large Truck
8–10 cu yd
Standard for driveways, garages, and foundations
Truckloads Needed
2 loads
1 full + 1 short load
Project Type Tabs
Select your project type to see recommended dimensions, thickness, and concrete specifications. Click any tab below to auto-fill suggestions into the calculator above.
Concrete Strength Selector
Concrete strength is measured in PSI (pounds per square inch) — the compressive load the cured slab can withstand after 28 days. Higher PSI means stronger concrete but also higher cost. Select your project type to see the recommended mix strength.
3,500 PSI — Suitable for foundations, footings, and structural slabs. Provides a margin of safety above the standard 3,000 PSI residential mix.
Recommended Strength by Project
Material Breakdown
Standard concrete is a mix of cement, sand (fine aggregate), gravel (coarse aggregate), and water. For every cubic yard of concrete, the typical proportions are shown below. Enter your volume to see the material quantities needed.
Cement
Sand
Gravel
Water
How to Calculate Square Feet for Concrete
Concrete projects always start with the surface area measurement — the square footage of the slab. From there, you factor in the depth (thickness) of the pour to determine the volume of concrete required. Concrete is ordered and delivered in cubic yards, so converting from square feet to cubic yards is the critical second step. Convert volume with the square feet to cubic feet calculator, estimate cost via the price per square foot calculator, plan the job with our project estimation calculator, or use the main square footage calculator.
Step 1: Calculate the Area
Measure the length and width of the slab in feet and multiply them together. For circular slabs (like a round patio or column footing), use the formula: Area = π × (Diameter ÷ 2)². Our calculator handles both shapes automatically.
Concrete Volume Formula
FormulaExample: 20×10 ft slab at 4 in thick = (20 × 10 × 0.333) ÷ 27 = 2.47 cu yd
Step 2: Convert to Cubic Yards
One cubic yard equals 27 cubic feet. Multiply your area (sq ft) by the depth in feet (divide inches by 12), then divide by 27. Always order an additional 10% overage — concrete must be poured without stopping, and running short mid-pour is catastrophic for curing quality.
Step 3: Bags vs. Ready-Mix
For small pours under 1 cubic yard, premixed 80 lb bags are practical. Each 80 lb bag yields approximately 0.60 cubic feet. For larger pours, ready-mix delivered by truck is more economical. The industry breakeven is typically around 1–1.5 cubic yards (27–40 cubic feet).
Recommended Concrete Slab Thickness by Project
| Project Type | Recommended Thickness | Notes |
|---|---|---|
| Walkway / Sidewalk | 3–4 inches | Foot traffic only |
| Residential Patio | 4 inches | Standard residential pour |
| Garage Floor | 4–6 inches | Vehicle weight considered |
| Driveway | 4–6 inches | Passenger cars; 6 in for trucks |
| Pool Deck | 4 inches | Non-load-bearing, decorative |
| Structural Foundation | 6–8+ inches | Engineer specification required |
Reinforcement Estimator
Concrete has excellent compressive strength but is weak in tension. Reinforcement (rebar or wire mesh) provides tensile strength to prevent cracking from shrinkage, temperature changes, and load stress. Use this estimator to calculate your rebar and mesh requirements.
Rebar Calculator
Concrete Curing Guide
Curing is the chemical process where cement hydrates and gains strength over time. Proper curing is critical — concrete that dries too fast cracks and loses up to 50% of its potential strength. Below is the standard curing timeline for a typical 3,500 PSI mix at 70°F.
| Day | Strength | Status | Guidelines |
|---|---|---|---|
| 1 | 20% | ⬤ Very Weak | Remove forms only; no foot traffic |
| 3 | 40% | ⬤ Developing | Light foot traffic OK; keep moist |
| 7 | 70% | ⬤ Moderate | Normal walking; no vehicles yet |
| 28 | 100% | ⬤ Full Strength | Full design load; vehicles OK |
Keep surface moist for at least 7 days using wet burlap, plastic sheeting, or curing compound.
Maintain temperature between 50–85°F. Below 50°F, curing slows dramatically.
Don't rush — concrete continues gaining strength for months. 28-day strength is the design benchmark.
Weather Recommendations
Weather conditions during and after the pour have a massive impact on concrete quality and curing. Plan your pour around optimal conditions when possible.
Hot Weather Pouring
Above 90°F (32°C)
- Pour early morning or late evening
- Use cold water or ice in the mix
- Apply evaporation retarder immediately
- Begin curing within 20 minutes of finishing
- Set time can be 50% faster — plan crew accordingly
Cold Weather Pouring
Below 40°F (4°C)
- Use heated water (not exceeding 140°F)
- Never pour on frozen ground
- Use insulated blankets or heated enclosures
- Consider accelerators in the mix
- Extend curing time to 14+ days
Rain Precautions
Any precipitation
- Never pour during active rainfall
- Cover fresh concrete immediately if rain starts
- Rain on fresh concrete dilutes surface and weakens it
- Wait 24 hours after rain for sub-base to dry
- Slight mist after initial set can actually help curing
Wind Effects
Above 15 mph
- Wind accelerates surface evaporation
- Causes plastic shrinkage cracking
- Use windbreaks or temporary barriers
- Apply evaporation retarder spray
- Begin wet curing immediately after finishing
Ideal Pouring Temperature
50°F – 85°F (10°C – 29°C) — moderate humidity, low wind, overcast skies. This window gives the best conditions for proper hydration, finishing time, and curing strength.
Common Concrete Projects and Square Foot Estimates
Driveway Concrete
A standard two-car driveway is 20 × 20 ft = 400 sq ft. At 6 inches thick, that is (400 × 0.5) ÷ 27 = 7.41 cubic yards. Add 10% overage = 8.15 cu yd. Contact your ready-mix supplier for local pricing, typically ranging from $120–$200 per cubic yard depending on mix strength and region.
Patio Slab
A 15 × 12 ft patio (180 sq ft) at 4 inches thick requires (180 × 0.333) ÷ 27 = 2.22 cubic yards. This is near the ready-mix breakeven. If your patio is below 1.5 cubic yards, 80 lb bags may be more practical and cost-effective than ordering a truck delivery with a minimum order fee.
Column Footings
Circular column footings are measured by diameter. A 24-inch diameter footing at 12 inches deep uses: π × (1)² × 1 ft = 3.14 cubic feet = 0.116 cubic yards per footing. Multiple footings are summed — use the quantity field to calculate all footings in one step.
Fence Post Holes
Concrete fence post holes are cylindrical. For a 10-inch diameter hole at 24 inches deep, volume = π × (0.417)² × 2 = 1.09 cubic feet = 0.04 cubic yards. For 20 fence posts, that equals 0.8 cu yd — well within DIY bag territory using 80 lb Quikrete bags.
Common Project Library
Quick reference estimates for popular concrete projects. All calculations include 10% overage. Costs are estimated at $150 per cubic yard (ready-mix delivered).
20×20 Driveway
Popular10×12 Patio
Garage Slab
Shed Base
Basketball Court
Specialized Square Feet Calculators
Each tool is purpose-built with its own dedicated calculator, visual diagrams, and step-by-step guidance.
Triangle
Find the area of any triangle using base × height or Heron's formula with three side lengths.
Irregular Shape
Break any complex shape into rectangles and triangles. Add sections and sum them automatically.
Inches
Enter measurements in inches and get instant square feet results. Includes a quick-reference conversion table.
From Meters
Convert metric measurements to square feet. Perfect for international real estate and imported materials.
Linear Feet
Convert linear footage to square feet for lumber, carpet, fencing, decking, and rolled materials.
Sq Meters
Convert sq ft to sq m with live dual display. Essential for international real estate and architecture.
Sq Yards
Divide sq ft by 9 to get sq yd. Perfect for carpet ordering, sod purchases, and fabric estimates.
Acres
Convert property area from sq ft to acres. Ideal for land sales, farming, zoning, and lot size comparisons.
Cubic Feet
Add depth to any area to calculate volume. Essential for concrete, mulch, soil, gravel, and pool planning.
House
Add rooms one by one and get your home's total square footage, with a live bar chart breakdown per room.
Room
Measure rectangular or L-shaped rooms for flooring, paint, and renovation budgeting with instant cost estimates.
Wall
Get net wall area for paint, wallpaper, or tile by subtracting doors and windows from the gross surface.
Tile
Enter room size and tile dimensions to calculate tile count, boxes to buy, and total material cost including waste.
Concrete
Get slab area, cubic yards of concrete, bags needed, and pour cost for any project.
Land
Measure any property lot or parcel in sq ft and acres. Supports rectangular, triangular, and irregular shapes.
BTU Calculator
Calculate heating and cooling BTU needs per square foot based on climate zone, ceiling height, and insulation.
Cost
Estimate total project cost from area and price per square foot. Compare scenarios with tax and markup.
Flooring
Find exact flooring material for hardwood, laminate, vinyl, or carpet with waste factor and box count.
Roof
Get true roof surface area using pitch multiplier. Includes roofing squares, shingle bundles, and cost.
Paint Area
Get paintable wall area with door and window deductions. Calculates gallons, coats, and total paint cost.
Estimation
Add multiple areas, apply buffer percentages, and get quick estimates for budgeting any project.
Related Calculators
Explore other free square footage and cost tools:
Frequently Asked Questions
Multiply the length by the width of the slab in feet. A 20 × 10 ft slab = 200 square feet. This is your surface area. Then factor in depth to find the concrete volume needed.
It depends on thickness. At 4 inches thick: (200 × 0.333) ÷ 27 = 2.47 cu yd. At 6 inches thick: (200 × 0.5) ÷ 27 = 3.70 cu yd. Always add 10% overage to your order.
One 80 lb bag yields approximately 0.60 cubic feet of concrete. Since 1 cubic yard = 27 cubic feet, you need 27 ÷ 0.60 = 45 bags per cubic yard. For 60 lb bags (0.45 cu ft each), you need 60 bags per cubic yard.
The standard residential driveway thickness is 4 inches for passenger cars. For heavy vehicles, trucks, or RVs, increase to 5–6 inches. Driveways with cold climates or expansive soils should also consider a thicker pour with proper sub-base preparation.
Installed concrete typically costs $4–$8 per square foot for a plain slab. Decorative finishes (stamped, colored, exposed aggregate) add $3–$10 per sq ft. Material-only (ready-mix delivered) runs $120–$200 per cubic yard depending on your region and mix strength (PSI).
Yes — always order 10% more than your calculated volume. Concrete cannot be paused mid-pour; running short causes cold joints that structurally weaken the slab. The small cost of extra material is insignificant compared to the risk of a failed pour.
Use the formula: Area = π × r² (where r is the radius, half the diameter). Multiply by the depth in feet, then divide by 27. Our calculator handles circular slabs automatically — just select "Circular Slab" and enter the diameter.
Square feet measures the flat surface area (2D). Cubic yards measures volume (3D) — the amount of concrete required to fill that area to a given depth. Concrete is ordered in cubic yards; square footage is just the starting point for that calculation.
For 80 lb bags (yielding 0.60 cu ft each): 27 ÷ 0.60 = 45 bags per cubic yard. For 60 lb bags (0.45 cu ft each): 27 ÷ 0.45 = 60 bags. For 40 lb bags (0.30 cu ft each): 27 ÷ 0.30 = 90 bags. Ready-mix delivery is far more practical for anything over 1 cubic yard.
A standard residential patio slab should be 4 inches thick. This provides adequate strength for foot traffic, patio furniture, and grills. If you plan to place heavy items such as a hot tub or masonry fireplace, increase to 6 inches and consider reinforcing with rebar or wire mesh.
Wet (freshly mixed) concrete weighs approximately 150 lbs per cubic foot, or about 4,050 lbs per cubic yard. Cured concrete weighs slightly less at around 145 lbs per cubic foot (3,915 lbs per cubic yard) due to water evaporation during the curing process.
There are exactly 27 cubic feet in one cubic yard (3 ft × 3 ft × 3 ft = 27 cu ft). This is the standard conversion factor used in all concrete calculations when ordering ready-mix by the yard.
Professional contractors measure all dimensions in feet, calculate total cubic feet (length × width × depth), divide by 27 to get cubic yards, then add 5–10% overage for waste, spillage, and uneven subgrade. They also account for slope, formwork, and any irregular edges that increase volume beyond theoretical calculations.