Concrete Calculator
A 10×10 ft slab at 4" thick needs 1.23 cubic yards — about 62 bags (80-lb) or 100 bags (50-lb). Ready-mix: $155-200. Bags: $310-400.
Concrete Calculator
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About This Calculator
Whether you're pouring a patio slab, setting fence post footings, or building a foundation wall, getting the right amount of concrete is critical. Order too little and you'll have a weak cold joint mid-pour; order too much and you're paying for a wasted partial yard. This calculator handles slabs, footings, columns, walls, curbs, and steps — enter your dimensions and get results in cubic yards, cubic feet, and exact bag counts for 50-lb, 60-lb, and 80-lb bags. Cost estimates cover both ready-mix truck delivery and bagged concrete from home improvement stores.
How to Calculate Concrete Volume
The basic formula for rectangular slabs is:
Volume (cubic yards) = Length (ft) x Width (ft) x Thickness (in) / 324
The 324 constant comes from 12 inches per foot times 27 cubic feet per cubic yard. For circular slabs, replace length x width with the circle area formula (pi x radius squared).
Worked Examples
- 10x10 patio, 4" thick: 10 x 10 x 4 / 324 = 1.23 cubic yards (about 62 bags of 80-lb mix)
- 20x24 garage slab, 6" thick: 20 x 24 x 6 / 324 = 8.89 cubic yards (ready-mix delivery recommended)
- Fence post footing, 12" diameter x 36" deep: pi x 0.5^2 x 3 = 2.36 cubic feet per post. For 20 posts: 47.1 cu ft = 1.75 cubic yards
Types of Concrete for Different Projects
Not all concrete is the same. The mix you choose depends on the structural requirements, exposure conditions, and curing time available.
| Mix Type | PSI Rating | Best For | Cost per Bag (80-lb) |
|---|---|---|---|
| Standard Mix | 3,000-3,500 | Walkways, patios, general slabs | $5.00-$5.50 |
| High-Strength | 5,000+ | Driveways, garage floors, footings | $6.50-$7.50 |
| Fast-Setting | 4,000 | Fence posts, small repairs, cold weather | $7.00-$8.00 |
| Crack-Resistant | 4,000 | Slabs, driveways (fiber-reinforced) | $6.00-$7.00 |
| Ready-Mix (truck) | 3,000-5,000 | Any project over 1 cubic yard | $125-$165/yard |
Cost Comparison: Bags vs. Ready-Mix
The crossover point where ready-mix delivery becomes cheaper than bags is approximately 1 cubic yard. Below that, bags are more practical despite the higher per-yard cost. Above 2 cubic yards, ready-mix saves both money and significant labor.
| Project Size | Bags (80-lb) | Bag Cost | Ready-Mix Cost | Winner |
|---|---|---|---|---|
| 0.5 cubic yards | 25 bags | $144 | $200+ (short-load fee) | Bags |
| 1.0 cubic yards | 50 bags | $288 | $165 | Ready-Mix |
| 2.0 cubic yards | 100 bags | $575 | $290 | Ready-Mix |
| 5.0 cubic yards | 250 bags | $1,438 | $725 | Ready-Mix |
Tips for a Successful Concrete Pour
- Always add 10% overage — uneven subgrade, form deflection, and spillage consume more concrete than the math predicts
- Prepare the subbase — compact 4-6 inches of gravel below the slab for drainage and to prevent cracking
- Use rebar or wire mesh for slabs over 4 inches thick or any driveway application
- Pour in mild weather — ideal temperature is 50-80F. Below 40F, concrete sets too slowly; above 90F, it cures too fast and cracks
- Cure properly — keep concrete moist for at least 7 days. Covering with plastic sheeting or applying curing compound prevents surface cracking
Frequently Asked Questions
How many bags of concrete do I need for a 10x10 slab?
A 10×10 ft slab at 4 inches thick requires 1.23 cubic yards, which equals about 62 bags of 80-lb concrete mix or 100 bags of 50-lb mix. At $5-6.50 per 80-lb bag, materials cost $310-400.
How much does a yard of concrete cost?
Ready-mix concrete delivered by truck costs $125-165 per cubic yard in most US markets. There's typically a short-load fee of $50-100 for orders under 5 yards. Bagged concrete runs $4-6.50 per 80-lb bag, making it $200-325 per cubic yard — more expensive but practical for small jobs.
What is the 324 rule for concrete?
Dividing by 324 converts length (ft) × width (ft) × depth (inches) directly to cubic yards. The number 324 equals 12 (inches per foot) × 27 (cubic feet per cubic yard). This shortcut eliminates the need for multiple unit conversions.
How thick should a concrete slab be?
A standard patio or walkway slab is 4 inches thick. Driveways need 5-6 inches for vehicle weight. Garage floors and areas with heavy loads should be 6 inches. Foundation footings are typically 8-12 inches thick and extend below the local frost line.
Should I use bags or ready-mix concrete?
Use bags for jobs under 0.5 cubic yards — fence posts, small pads, repairs. For anything larger, ready-mix truck delivery is cheaper per yard, stronger (consistent mix), and far less labor. The crossover point is roughly 1 cubic yard: above that, bags become impractical and more expensive.
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