← Home & Construction Calculators
Estimate concrete volume, cubic yards, premix bags, weight and project cost for rectangular or circular pours with editable waste and supplier assumptions.
Concrete calculation symbols
Concrete formulas
Every measurement is converted to feet before volume is calculated. Waste is applied after the exact geometric volume.
Visual guide
Measure inside the forms and use average depth where the base varies. For irregular projects, divide the pour into simple shapes and add the results.
Worked slab example
A 20 ft × 10 ft slab at 4 in thick contains:
20 × 10 × 0.3333 = 66.667 ft³
66.667 ÷ 27 = 2.469 yd³
With 10% waste = 2.716 yd³
At 0.6 ft³ per bag, this equals 123 bags. For a project of this size, ready-mix delivery is generally more practical than hand mixing.
Round column example
A 12-inch-diameter, 8-foot column contains about 6.283 ft³ before allowance.
Typical premix bag yields
| Bag size | Approximate yield |
|---|---|
| 40 lb | 0.30 ft³ |
| 50 lb | 0.375 ft³ |
| 60 lb | 0.45 ft³ |
| 80 lb | 0.60 ft³ |
| 90 lb | 0.675 ft³ |
Always use the actual yield printed by the manufacturer. Mix design, aggregate and product type can change coverage.
Ordering and site preparation
- Verify forms, reinforcement, subgrade and access before the truck arrives.
- Add allowance for uneven grade and unavoidable spillage.
- Confirm the supplier minimum load and short-load charge.
- Schedule enough labor to place and finish within working time.
- Do not add uncontrolled water to improve workability.
Measurement checklist
- Measure every section separately.
- Use consistent units.
- Subtract openings only when they will not be filled.
- Confirm design thickness and reinforcement cover.
- Check weather, curing and joint requirements.
Frequently asked questions
How many cubic feet are in a cubic yard?
One cubic yard contains exactly 27 cubic feet.
How many 80-pound bags are in a cubic yard?
At 0.60 cubic foot per bag, one cubic yard requires 45 bags before waste.
How much extra concrete should I order?
Five to ten percent is common, but irregular excavation or difficult placement may require more.
Does the calculator design the concrete mix?
No. It estimates quantity and cost. Strength, slump, exposure class and reinforcement must follow project specifications.
When should I order ready mix?
Ready mix is usually preferable when bag quantity, labor or consistent batching makes hand mixing impractical.
Common mistakes
- Entering depth in feet when the value is inches.
- Using diameter as radius.
- Ignoring uneven subgrade.
- Using a generic bag yield instead of the label.
- Forgetting delivery, short-load and pump charges.