Concrete Calculator
Calculate required concrete volume in cubic yards, cubic feet, and pre-mixed bag quantities (80lb, 60lb, 40lb) for concrete slabs, footings, and piers with customizable waste margins.
Calculate Concrete Volume
Enter slab or footing dimensions to compute required ready-mix or bagged concrete.
Calculation Results
Calculated using standard volume formula: Volume (cu yds) = (Length × Width × (Thickness / 12)) / 27 × (1 + Waste %)
Quick Summary
The Concrete Calculator estimates the precise volume of concrete needed for construction projects including house foundations, patio slabs, driveways, footings, and retaining wall posts.
- Cubic Yards (yd³): Standard measurement for ordering truck-delivered ready-mix concrete (1 yd³ = 27 cu ft).
- Pre-Mix Concrete Bags: Quantities required for smaller DIY projects (80lb bags yield 0.60 cu ft; 60lb bags yield 0.45 cu ft).
- Waste Margin Inclusion: Adds 10% extra volume to account for subgrade settlement, form deflection, and site spillage.
How to Use the Concrete Calculator
- Enter the Slab Length in feet (e.g. 20 ft).
- Enter the Slab Width in feet (e.g. 20 ft).
- Enter the Thickness / Depth in inches (e.g. 4 inches for residential patios or walkways).
- Adjust the Waste Margin (%) (10% is industry standard).
- Click Calculate to instantly view cubic yards, cubic feet, pre-mix bag counts, and estimated cost.
Concrete Formula & Method
Concrete volume is calculated by multiplying length by width by thickness, converting all measurements into feet, and dividing by 27 to yield cubic yards:
Cubic Feet (ft³) = Length (ft) × Width (ft) × Thickness (ft)
Cubic Yards (yd³) = (Cubic Feet / 27) × (1 + Waste Margin % / 100)
Pre-mixed bag conversions:
- 80 lb Bags Needed: Cubic Feet / 0.60 cu ft per bag
- 60 lb Bags Needed: Cubic Feet / 0.45 cu ft per bag
- 40 lb Bags Needed: Cubic Feet / 0.30 cu ft per bag
Worked Example
Example: 20ft × 20ft Patio Slab at 4 Inches Thick with 10% Waste
- Surface Area: 20 ft × 20 ft = 400 sq ft
- Thickness in Feet: 4 inches / 12 = 0.333 ft
- Base Volume: 400 sq ft × 0.333 ft = 133.33 cu ft
- Base Cubic Yards: 133.33 / 27 = 4.94 cu yds
- With 10% Waste: 4.94 × 1.10 = 5.43 Cubic Yards
Applying pre-mix bag conversions yields:
146.67 cu ft / 0.60 = 245 Bags of 80lb Concrete
What This Calculator Includes vs. Does Not Include
What This Calculator Includes
- Rectangular & Square Slab Calculations: Accurate 3D volumetric calculations.
- Bag Volume Conversions: Exact counts for 80lb, 60lb, and 40lb bags.
- Safety Waste Factor: Adjustable percentage for uneven ground or spillage.
What This Calculator Does Not Include
- Rebar & Wire Mesh Reinforcement: Steel rebar grid requirements must be calculated separately.
- Concrete Truck Delivery Fees: Short-load fees for orders under 5 cubic yards.
Tips & Best Practices
- Order Ready-Mix for Projects Over 1 Cubic Yard: Mixing more than 40 to 50 bags of concrete by hand in a wheelbarrow is physically exhausting; ready-mix trucks save significant labor.
- Always Account for Subgrade Unevenness: Excavated soil is rarely perfectly flat. Adding 10% waste prevents running out of concrete mid-pour.
- Use Control Joints Every 8 to 10 Feet: Control joints control random cracking by creating planned weak points for thermal expansion.
Common Mistakes to Avoid
- Forgetting to Convert Inches to Feet: Multiplying length in feet by thickness in inches without dividing by 12 results in a massive 12x volume calculation error.
- Pouring Concrete Too Thin for Driveways: Residential driveways carrying heavy vehicles require at least 5 to 6 inches of concrete with rebar.
Frequently Asked Questions (FAQ)
How many cubic yards of concrete are in a 20x20 slab at 4 inches thick?
A 20x20 foot slab at 4 inches thick requires approximately 4.94 cubic yards of concrete (or 5.43 cubic yards with a standard 10% waste margin).
How many 80lb bags of concrete make one cubic yard?
It takes 45 bags of 80lb pre-mixed concrete to equal one cubic yard (27 cubic feet).
How many 60lb bags of concrete equal one cubic yard?
It takes 60 bags of 60lb pre-mixed concrete to equal one cubic yard.
How thick should a residential concrete patio slab be?
A standard residential patio or walkway slab should be 4 inches thick. Driveways supporting vehicles should be 5 to 6 inches thick.
Why should I add a waste percentage to my concrete order?
Adding a 5% to 10% waste margin accounts for formwork deflection, uneven ground excavation, and concrete left inside mixing trucks or pumps.
What is short-load fee for ready-mix concrete?
Ready-mix batch plants charge a short-load fee when ordering less than a full truckload (usually under 5 or 6 cubic yards).
How many square feet does 1 cubic yard of concrete cover at 4 inches?
One cubic yard of concrete covers exactly 81 square feet at a depth of 4 inches (or 108 sq ft at 3 inches).
How long does concrete take to cure before walking or driving on it?
Concrete takes 24 to 48 hours to cure enough for foot traffic, and 7 days for heavy vehicle traffic. Full structural curing occurs at 28 days.
Is this concrete calculator free to use?
Yes, TibCal's Concrete Calculator is 100% free with no registration required.