About This Tool
Getting the right amount of concrete is critical for any project. Pouring a patio, setting fence posts, or laying a foundation all require precise material estimates. Order too little and you'll face a mid-pour crisis. Concrete waits for no one, and a "cold joint" from a delayed pour creates a weak seam that cracks over time. Order too much and you're paying for waste that hardens in the bag unused. This calculator eliminates the guesswork by converting your project dimensions into cubic yards and showing exactly how many 60lb or 80lb bags you need. It handles the tricky math of converting feet and inches into the cubic yards that suppliers quote, and automatically adds the 10% buffer that experienced contractors always include for spillage, uneven ground, and form bulging. The tool supports three common shapes: rectangles for slabs, patios, and driveways; circles for round column footings and fire pit bases; and triangles for odd-shaped corner pours. Select your project type (slab, driveway, sidewalk, footing, or post holes) and the calculator automatically sets the recommended depth. Choose your concrete strength rating from 2500 to 4000 PSI based on the structural demands of your project, and enter your local cost per cubic yard to get an instant budget estimate. Accurate concrete estimation saves money, prevents structural failures, and keeps your project on schedule.
The Concrete Volume Formula
Concrete volume is calculated using a simple formula:
Volume = Length × Width × Depth
The tricky part is unit conversion. Most measurements are in feet and inches, but concrete is sold by the cubic yard. Here's the math:
- Convert depth from inches to feet (divide by 12)
- Multiply Length × Width × Depth to get cubic feet
- Divide by 27 to convert to cubic yards
For example: A 10ft × 10ft patio at 4 inches deep = 10 × 10 × 0.333 = 33.3 cubic feet = 1.23 cubic yards.
Bags vs. Ready-Mix: Which Should You Choose?
The break-even point is around 1 cubic yard:
- Under 1 yard: Buy bags (60lb or 80lb). More control, no delivery fees, work at your own pace.
- Over 1 yard: Order ready-mix delivery. Faster pour, better consistency, less physical labor.
A 60lb bag yields about 0.45 cubic feet. An 80lb bag yields about 0.6 cubic feet. For most DIY projects, 80lb bags are more cost-effective despite the extra weight.
Why You Need 10% Extra
Never order exactly what the calculator shows. Here's why concrete gets "lost":
- Uneven subgrade: Ground is never perfectly flat, and low spots eat concrete
- Form bulging: Wood forms bow slightly under pressure
- Spillage: Some concrete inevitably misses the forms
- Over-excavation: Digging slightly too deep is common
Running short mid-pour creates a "cold joint," a weak seam that cracks. The 10% buffer costs a few dollars but prevents expensive failures.
Cost Estimation and Budget Planning
Concrete costs vary widely depending on your location, project size, and delivery method. Here's what to budget for:
- Bagged concrete (DIY): An 80lb bag costs $5-7 and yields about 0.6 cubic feet. For small projects under 1 cubic yard, bags are economical. For a 1 cubic yard project, expect to buy 45 bags at roughly $250-315 total.
- Ready-mix delivery: Most suppliers charge $125-150 per cubic yard with a minimum order of 1-2 yards. Delivery fees range from $0 (included in price) to $100+ for small loads or long distances. Short-load fees of $30-50 per yard apply when ordering less than the truck minimum (usually 10 yards).
- Pumping fees: If the truck cannot back up directly to your pour site, a concrete pump costs $150-250 per hour. Backyard patios, second-story pours, and sites with limited access typically require pumping.
Request quotes from at least two local suppliers. Prices fluctuate seasonally, with spring and summer being peak demand periods. Saturday pours often carry surcharges. Order early in the week for best scheduling flexibility.
Common Mistakes to Avoid
Concrete is unforgiving once poured. These common errors lead to cracked slabs, weak foundations, and costly do-overs:
- Skipping the subgrade: Pouring directly on topsoil or uncompacted fill guarantees settling cracks. Remove organic material and compact a 4-6 inch gravel base before pouring.
- Underestimating quantity: Running short during a pour creates cold joints, which are weak seams that crack under stress. The calculator's 10% buffer exists for good reason.
- Adding too much water: Extra water makes concrete easier to work but weakens the final product. A slump test of 4-5 inches is ideal for most residential work. Water-to-cement ratio directly determines strength.
- Neglecting control joints: Cut or tooled control joints every 8-12 feet prevent random cracking by providing weak points where the concrete can crack in a controlled, straight line.
- Improper curing: Concrete that dries too fast loses strength. Keep the surface moist for 7 days by misting with water, covering with plastic sheeting, or applying liquid curing compound. This is especially critical in hot, dry, or windy conditions.