Concrete Calculator — Free Online Concrete Estimator

How to Use the Concrete Calculator

1. Enter your slab dimensions: Type the length and width of your concrete project in feet. If your project area is irregular, divide it into rectangular sections and calculate each one separately. Measure carefully because even a few inches of error can result in a significant difference in the amount of concrete needed for larger projects.
2. Enter the depth: Input the slab depth in inches. Standard residential patios and walkways use 4 inches, driveways need 4 to 6 inches, and foundation slabs require 4 to 6 inches with thickened edges. Always verify depth requirements against your local building codes before pouring.
3. Adjust the price per cubic yard: Enter the current ready-mix concrete price in your area. The default is $150 per cubic yard, which represents a national average for standard concrete. Contact your local supplier for an accurate quote, as prices vary significantly by region and mix type.
4. Set the waste factor: Adjust the waste percentage to account for spillage, overdigging, and uneven subgrade. The default is 10%, which works well for flat, well-prepared slabs. Increase to 15% or 20% for irregular shapes, sloped terrain, or if your subgrade preparation is rough.

Results update instantly as you change any value. The calculator displays cubic yards for ordering ready-mix, the number of 60-lb and 80-lb bags for small projects, total weight for structural planning, and the estimated material cost.

Concrete Volume Formula

Variables Explained

• Length and Width: The horizontal dimensions of the slab in feet. These are multiplied together to get the slab area in square feet. For rectangular slabs, measure the longest and widest dimensions. For L-shaped or irregular areas, break them into rectangles, calculate each volume separately, and add them together.
• Depth: The vertical thickness of the slab. This value is entered in inches and converted to feet by dividing by 12. A 4-inch depth equals 0.333 feet. Depth is the most critical factor affecting volume; even a half-inch increase adds substantial material over a large area.
• Waste Factor: A multiplier to account for real-world material losses. A factor of 1.1 means 10% extra material. Industry standard is 1.1 for flat slabs, 1.15 for irregular areas, and up to 1.2 for complex pours. This prevents costly short orders.
• Cubic Yard Conversion: Since there are 27 cubic feet in one cubic yard, the total cubic footage is divided by 27 to get the quantity in the unit used by ready-mix concrete suppliers for ordering and pricing.

Step-by-Step Example

Suppose you are pouring a 20 x 10 foot patio slab at a standard 4-inch depth with a 10% waste factor:

1. Convert depth to feet: 4 / 12 = 0.333 feet
2. Calculate volume: 20 x 10 x 0.333 x 1.1 = 73.33 cubic feet
3. Convert to cubic yards: 73.33 / 27 = 2.72 cubic yards
4. Calculate 80-lb bags: 73.33 / 0.6 = 123 bags
5. Estimate weight: 73.33 x 150 = 11,000 lbs
6. Estimate cost: 2.72 x $150 = $407.41

This patio requires about 2.72 cubic yards of ready-mix concrete, or 123 bags of 80-lb premix for a DIY approach. At $150 per cubic yard, the material cost is approximately $407.

Practical Examples

Example 1: Kevin's Garage Floor

Kevin is building a detached garage and needs a concrete floor measuring 24 x 24 feet at 5 inches thick. He uses a 10% waste factor:

• Depth in feet: 5 / 12 = 0.417 ft
• Volume: 24 x 24 x 0.417 x 1.1 = 264.24 ft³
• Cubic yards: 264.24 / 27 = 9.79 yd³
• Weight: 264.24 x 150 = 39,636 lbs (about 19.8 tons)
• Cost at $150/yd³: 9.79 x $150 = $1,468.50

Kevin orders 10 cubic yards from his local ready-mix supplier, rounding up to ensure he has enough. The extra half-yard provides a comfortable margin for any subgrade irregularities discovered during the pour.

Example 2: Lisa's Backyard Patio

Lisa wants a 12 x 15 foot patio slab at the standard 4-inch depth. She uses a 10% waste factor and her local concrete price is $165 per cubic yard:

• Volume: 12 x 15 x 0.333 x 1.1 = 65.93 ft³
• Cubic yards: 65.93 / 27 = 2.44 yd³
• 80-lb bags alternative: 65.93 / 0.6 = 110 bags
• Cost (ready-mix): 2.44 x $165 = $402.60

Since Lisa only needs about 2.4 cubic yards, she considers using bagged concrete to avoid the short-load delivery surcharge that her local supplier charges for orders under 3 cubic yards. However, at 110 bags weighing 80 pounds each, the labor of mixing bags individually is considerable.

Example 3: David's Driveway Extension

David is extending his driveway by 30 x 12 feet at 6 inches thick to support his pickup truck and boat trailer. He uses a 15% waste factor due to a slight slope in his yard:

• Volume: 30 x 12 x 0.5 x 1.15 = 207 ft³
• Cubic yards: 207 / 27 = 7.67 yd³
• Weight: 207 x 150 = 31,050 lbs (about 15.5 tons)
• Cost at $150/yd³: 7.67 x $150 = $1,150.50

David orders 8 cubic yards to account for the sloped subgrade. The 6-inch thickness ensures the slab can handle the concentrated wheel loads from his heavy trailer without cracking.

Concrete Reference Table

Table assumes no waste factor. Add 10-15% to these values for real-world projects. Cost calculated at $150/yd³.

Tips and Complete Guide

Choosing Between Ready-Mix and Bagged Concrete

For projects requiring more than 2 cubic yards, ready-mix concrete is almost always the better choice. Ready-mix arrives pre-blended with precise water-to-cement ratios, provides consistent quality, and dramatically reduces labor compared to mixing dozens of bags by hand. A single concrete truck can deliver 8 to 10 cubic yards in one load. For small projects under 1 cubic yard, such as setting fence posts, pouring small footings, or repairing sections of a walkway, bagged premix concrete is convenient and eliminates the delivery fee and potential short-load surcharge. The crossover point where ready-mix becomes more economical than bags is typically around 1.5 to 2 cubic yards.

Preparing the Subgrade for Concrete

A well-prepared subgrade is essential for a durable concrete slab. Start by removing all topsoil, vegetation, and organic material from the area. Compact the exposed soil with a plate compactor or hand tamper to create a firm, uniform base. Add a 4-inch layer of compacted gravel or crushed stone as a drainage layer, then top with a polyethylene vapor barrier if the slab will be enclosed or finished. Install forms that are level and securely staked, using string lines and a spirit level to verify grade. For slabs exposed to freeze-thaw cycles, consider a 6-inch gravel base to prevent frost heave. Proper subgrade preparation prevents cracking, settling, and the need for costly repairs.

Reinforcement Options for Concrete Slabs

Reinforcement helps control cracking and increases the load-bearing capacity of concrete slabs. Welded wire mesh (6x6 W1.4xW1.4) is the most common reinforcement for residential slabs and patios. Number 3 or Number 4 rebar placed on 12 to 18-inch centers provides stronger reinforcement for driveways and garage floors. Fiber-reinforced concrete adds synthetic or steel fibers directly to the mix, reducing surface cracking without the labor of placing mesh or rebar. For any slab that will bear heavy loads, consult a structural engineer to determine the appropriate reinforcement type and spacing. Control joints should be cut at intervals no greater than two to three times the slab thickness in feet to manage inevitable shrinkage cracking.

Common Mistakes to Avoid

• Adding too much water to the mix: Excess water makes concrete easier to pour but dramatically reduces its strength and durability. Follow the manufacturer's recommended water-to-cement ratio and use a concrete vibrator or tapping to consolidate the mix rather than adding more water.
• Skipping the waste factor: Ordering the exact calculated volume without any buffer leads to running short mid-pour, creating cold joints that weaken the slab. Always add at least 10% for standard pours and 15% for complex or uneven areas.
• Neglecting proper curing: Allowing concrete to dry too quickly causes surface cracking and reduced strength. Keep freshly poured concrete moist for at least 7 days using curing compounds, wet burlap, or plastic sheeting.
• Pouring on unprepared ground: Placing concrete directly on topsoil, vegetation, or soft ground leads to settling, cracking, and premature slab failure. Always compact the subgrade and add a gravel drainage base.
• Ignoring weather conditions: Pouring concrete in temperatures below 40 degrees Fahrenheit or above 90 degrees Fahrenheit without proper precautions leads to poor curing and compromised strength. Plan your pour during moderate weather whenever possible.

Frequently Asked Questions