Last summer a guy I know ordered concrete for his patio. Did the math wrong. Ended up with 2.3 cubic yards when he only needed 1.7. That's an extra $187 sitting there going to waste while he scrambled to find somewhere to dump it before it set up. Truck driver wasn't happy either - those guys charge waiting fees after the first 10 minutes, and he burned through 23 minutes trying to figure out what to do with leftover mud.
Concrete math isn't complicated, but mess it up and you're either short (which means an ugly cold joint and a second delivery fee) or you're overpaying for material you'll never use. This calculator does the volume math correctly and shows you exactly how many bags or yards you need, with a waste buffer built in because real-world pours never match the textbook.
Understanding Concrete Measurements
Concrete gets ordered in cubic yards because that's how the industry standardized things decades ago when ready-mix trucks became the norm. One cubic yard is 3 feet by 3 feet by 3 feet. Simple cube. That equals 27 cubic feet total.
Here's where people trip up: they measure a slab that's 10 feet by 12 feet by 4 inches thick, punch those numbers straight into a calculator, and get confused when it says they need 40 cubic feet. But thickness is in inches while length and width are in feet. You have to convert. Take those 4 inches, divide by 12 to get 0.333 feet, then multiply: 10 × 12 × 0.333 = 39.96 cubic feet. Divide by 27 to get 1.48 cubic yards. Round up to 1.5 yards to be safe.
Bagged concrete works differently. An 80-pound bag yields about 0.6 cubic feet when mixed. That means you need roughly 45 bags per cubic yard. For small jobs - fence posts, mailbox footings, small repairs - bags make sense. You mix what you need, no minimum order, no truck scheduling. For anything over 1 yard (that's 45 bags), call the ready-mix plant. Your back will thank you.
The waste factor exists because forms aren't perfect, ground settles unevenly, and you always spill some. Budget 10% extra as standard practice. If you're new to concrete work or dealing with complicated shapes, go 15%. Better to have a wheelbarrow of leftover mix than to come up short halfway through a pour.
Concrete Types and When to Use Each
Not all concrete is the same, and suppliers will absolutely try to upsell you on high-strength mixes you don't need.
| Type | Strength (PSI) | Best For |
|---|---|---|
| Standard | 2500-3000 | Sidewalks, patios, shed slabs - most residential work |
| High-Strength | 4000-5000 | Driveways, garage floors, commercial applications |
| Fast-Setting | 3000-4000 | Post holes, quick repairs - sets in 20-40 minutes |
| Fiber-Reinforced | 3000+ | Crack resistance for slabs without rebar |
| Countertop Mix | 5000-6000 | Smooth finish work, decorative applications |
For most homeowner projects, 3000 PSI is plenty. That's your basic residential mix. Driveways should be 4000 PSI if you're parking trucks or heavy SUVs on it regularly. Don't let a supplier push 5000 PSI for a garden shed slab - you're wasting money.
The bagged-versus-truck decision comes down to volume and convenience. Here's the breakpoint: bagged concrete costs about $4.50 per 80lb bag at most big-box stores. You need 45 bags per yard, so that's $202.50 per cubic yard just in materials, plus your time and labor mixing. Ready-mix in my area runs $135-$145 per yard delivered (your region will vary). Once you cross 1.5 yards, the truck becomes cheaper even before you account for the hours you'll spend mixing bags.
Weather plays into this too. Concrete below 50°F sets too slowly and can freeze before curing properly. Above 90°F it sets too fast and you'll be fighting it the whole pour. Spring and fall are ideal. Summer mornings work if you start early. Winter pours need blankets and sometimes additives, which costs extra.
What Most DIYers Get Wrong
Not having enough help ready. You can't pause a concrete pour to take a break. Once that truck starts flowing, you're committed until it's all placed and screeded. I watched a neighbor try to do a 3-yard driveway solo. Disaster. Concrete was setting up in the back while he was still raking the front. Ended up with visible seams and uneven finish.
Underestimating thickness requirements. Four inches works for foot traffic. Period. You park cars on 4-inch concrete and you'll see cracks within two years, guaranteed. Driveways need 5 inches minimum, 6 if you've got trucks or RVs. Trying to save money by going thinner is penny-wise, pound-foolish.
Forms that aren't strong enough. Wet concrete weighs about 150 pounds per cubic foot and pushes outward with serious force. 2x4 stakes every 3 feet, not every 5. Use stakes on both sides of the form boards. Guy down the street used flimsy forms for a walkway - the concrete bulged out the sides mid-pour and he had to scramble to brace it with whatever he could find. Looked terrible.
Forgetting about base prep. Concrete needs 4-6 inches of compacted gravel underneath for drainage and stability. Pour directly on clay or soft soil and it'll crack when the ground shifts. Also needs to be level - don't try to use concrete to fill low spots. You'll use way more material than calculated.
Missing the truck window. Ready-mix drivers give you about 10 minutes of free time to unload. After that, waiting fees kick in - usually $2-$3 per minute. Seen people get hit with $180 in waiting charges because they weren't organized. Have your forms ready, base compacted, tools staged, and crew standing by before the truck shows up.
Ordering on gut feel instead of math. "Looks like about 2 yards" is how you end up in the situation I described at the start. Do the actual calculation. Measure twice, order once.
Project-Specific Guidance
Slabs & Patios
Standard thickness is 4 inches for anything that's just foot traffic - patios, walkways, shed floors. Calculate length times width times 0.333 (that's 4 inches converted to feet). A 12x16 patio needs 12 × 16 × 0.333 = 63.94 cubic feet = 2.37 yards. Round up to 2.5 yards with waste factor. Base needs to be graded for drainage - minimum 1/8 inch per foot slope away from buildings. Compact that gravel base hard. Rent a plate compactor if the area is bigger than a parking space.
Driveways
Don't cheap out here. Five inches minimum, six if you're parking anything heavier than a sedan. A standard two-car driveway, say 20 feet by 20 feet at 5 inches thick: 20 × 20 × 0.417 = 166.8 cubic feet = 6.2 yards. With 10% waste that's 6.8 yards. This is where rebar matters - #4 rebar in an 18-inch grid pattern, elevated on chairs so it sits mid-slab. Wire mesh doesn't cut it for driveways despite what the big-box stores tell you. Control joints every 10 feet to manage cracking.
Footings
Width depends on what you're supporting, depth depends on frost line. In northern climates frost goes 42-48 inches down - your footings need to go below that or frost heave will shift your structure. Standard deck footing might be 12 inches diameter, 48 inches deep. Calculate as a cylinder: radius squared times pi times depth. For 12-inch diameter that's 0.5² × 3.14 × 4 feet = 3.14 cubic feet per hole. Ten footings = 31.4 cubic feet total = 1.16 yards. This is prime bagged-concrete territory - easier to mix per hole than dealing with a truck.
Fence Posts
Everyone underestimates concrete for fence posts. Standard 4x4 post in an 8-inch diameter hole, 30 inches deep. That's 0.33² × 3.14 × 2.5 feet = 0.86 cubic feet per post. Fifty feet of fence with posts every 8 feet means 7 posts - right around 6 cubic feet total, which is ten 80lb bags. Fast-setting mix is your friend here because you can set posts and move on within an hour instead of waiting overnight.
Resources and Next Steps
Call your local ready-mix plant and verify their pricing before you order. Prices fluctuate based on fuel costs and regional demand. Some plants charge extra for small loads (under 3 yards), others have Saturday delivery fees. Ask about their free time window and waiting charges upfront.
Check your local building department for permit requirements. Most municipalities require permits for slabs over 200 square feet or any footing deeper than 24 inches. Inspectors want to see rebar placement and compacted base before you pour. Getting caught without a permit means tearing it out and starting over.
Have a backup plan for excess concrete. Small amounts can go into quick projects - stepping stones, garden borders, anchor pads. Larger overages need to get spread somewhere fast. You can't just dump it - it'll set into a giant useless lump.
Useful references: The American Concrete Institute publishes residential concrete guidelines. Your local university extension office often has free construction fact sheets. YouTube has decent tutorials but watch ones from actual contractors, not DIY bloggers who've done one project.