The Most Expensive Concrete Mistake Is Ordering the Wrong Amount
Picture it: the forms are set, the rebar is tied, your help showed up at 7 a.m., and halfway through the pour the truck runs dry. The plant cannot send another load for two hours, and by then the first half has started to set. The seam where the two loads meet is called a cold joint, and it will be the weakest line in your slab for the rest of its life.
Almost every version of this disaster traces back to bad volume math. Concrete is sold by the cubic yard, 27 cubic feet, whether you buy bagged mix or order a ready-mix truck. The gap between a smooth single pour and an expensive callback is usually a few bags nobody bothered to count.
How Do You Calculate Cubic Yards of Concrete?
You are finding the volume of a box, a trench, or a cylinder, then converting to yards:
Cubic yards = (length ft × width ft × thickness in ÷ 12) ÷ 27
- Slab or footing: length × width × (thickness in inches ÷ 12) = cubic feet
- Round column: 3.14 × (diameter in inches ÷ 24)² × height in feet = cubic feet
- Convert: divide cubic feet by 27 to get cubic yards
- Bags: divide cubic feet by 0.6 for 80-lb bags or 0.45 for 60-lb bags, then round up
Watch your units. If the tape measure reads inches but the sketch is in feet, run everything through our unit converter first. And if the plans call for a 4-1/2 inch slab, our fraction to decimal calculator turns that into 4.5 in one step.
Worked Example: How Much Concrete Do I Need for a 10x10 Slab?
| Step | Math | Result |
| Volume | 10 × 10 × (4 ÷ 12) | 33.33 cu ft |
| Cubic yards | 33.33 ÷ 27 | 1.23 cu yd |
| With 10% waste | 33.33 × 1.10 ÷ 27 | 1.36 cu yd |
| 80-lb bags | 1.36 × 45, rounded up | 62 bags |
| 60-lb bags | 1.36 × 60, rounded up | 82 bags |
Memorize one number: a cubic yard is exactly 45 eighty-lb bags (27 ÷ 0.6).
Bags Per Cubic Yard at a Glance
| Bag size | Yield per bag | Bags per cubic yard |
| 80-lb | 0.60 cu ft | 45 |
| 60-lb | 0.45 cu ft | 60 |
| 50-lb | 0.375 cu ft | 72 |
| 40-lb | 0.30 cu ft | 90 |
What Concrete Costs Per Yard in 2026
| What | Typical 2026 price |
| Ready-mix, delivered | $145–195 per cubic yard |
| 80-lb bagged mix | $5–9 per bag |
| Short-load fee (orders under 3–5 yards) | $50–150 |
Cement is the pricey ingredient. A yard of mix carries roughly six to seven sacks of Portland cement, which is why delivered prices climb when energy and freight costs do. Bags win up to about 1 to 2 cubic yards. Past that, a ready-mix truck is cheaper, faster, and far more consistent than hand-mixing 90 bags in a wheelbarrow. Always ask for the delivered price, not just the per-yard price, because short-load and fuel surcharges can dwarf the headline number.
Your Pour-Day Plan
- Order 10% extra. Spillage, an uneven sub-base, and bowing forms all eat concrete. Running short mid-pour costs far more than one extra bag.
- Confirm delivery details: short-load fees, chute reach, and whether you need a wheelbarrow crew on standby.
- Cut control joints every 8 to 10 feet so shrinkage cracks land where you planned.
- Cure it properly: keep it moist for 7 days, wait 24 to 48 hours before foot traffic, 7 days before driving, and know full strength takes about 28 days.
Plug your dimensions into the tool above and you will walk into the store or the ready-mix office knowing exactly what to ask for. Measure twice, pour once.
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
A 10x10 slab at 4 inches thick needs 33.33 cubic feet, or 1.23 cubic yards. Add 10% for waste and you need 1.36 yards, about 62 eighty-lb bags.
How many bags of concrete are in a cubic yard?
45 eighty-lb bags, 60 sixty-lb bags, 72 fifty-lb bags, or 90 forty-lb bags. An 80-lb bag yields about 0.6 cubic feet, and 27 ÷ 0.6 = 45.
How do you calculate cubic yards of concrete?
Multiply length × width × depth with all three in feet, then divide by 27. If the depth is in inches, divide it by 12 first. For a round column, use 3.14 × radius² × height, then divide by 27.
Should I add extra concrete for waste?
Yes, order 10% extra. Spillage, an uneven sub-base, over-excavation, and forms that bow all consume more than the perfect mathematical volume. This calculator adds 10% by default, and you can toggle it off.
Last updated: 2026 | Bag yields are industry averages; check your product label. Confirm footing dimensions against local building codes.