Concrete Calculator: Cubic Yards & Bags
Pick your shape — slab, footing, wall, round column or steps — enter the dimensions, and get the concrete you need in cubic yards, premix bags and estimated cost.
A 10 × 10 ft slab that is 4 inches thick needs 1.23 cubic yards of concrete, or about 56 bags of 80-lb mix (one cubic yard is about 45 bags). Formula: length × width × thickness (inches ÷ 12) ÷ 27 = cubic yards. Add 5–10% extra for waste.
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How to calculate how much concrete you need
Concrete is sold by volume — cubic yards from a ready-mix truck, or bags of dry premix measured by their yield in cubic feet. Every calculation follows the same three steps: find the volume of the shape in cubic feet, add a waste allowance, then convert to yards or bags.
80-lb bags = cubic feet ÷ 0.60 | 60-lb bags = ÷ 0.45 | 40-lb bags = ÷ 0.30
Formulas for each shape
| Shape | Volume (cubic feet) |
|---|---|
| Slab, patio, driveway, walkway | length (ft) × width (ft) × thickness (in ÷ 12) |
| Footing or trench | length (ft) × width (in ÷ 12) × depth (in ÷ 12) |
| Wall | length (ft) × height (ft) × thickness (in ÷ 12) |
| Round column, pier, post hole | π × (diameter in ÷ 24)² × height (ft) × quantity |
| Solid steps | width (ft) × riser (ft) × tread (ft) × n(n+1) ÷ 2 |
| Circular slab | π × (diameter ÷ 2)² × thickness (in ÷ 12) |
| Hollow tube or ring | π × (outer radius² − inner radius²) × length |
| Curb and gutter | (curb depth × curb height + gutter width × flag thickness) ÷ 144 × length |
For an L-shaped or irregular pour, split it into rectangles, calculate each, and add them. For several different shapes — say a patio plus a footing — run the calculator once per shape and add the yardage.
Worked example: a garage apron
You want a 12 × 20 ft apron, 5 inches thick. Volume = 12 × 20 × 5 ÷ 12 = 100 cubic feet = 100 ÷ 27 = 3.70 cubic yards. With 10% waste you need 4.07 cubic yards; most suppliers round up to the next quarter or half yard, so ask for about 4¼. At $165 per yard that's roughly $670 delivered, whereas 183 bags of 80-lb mix would be impractical — a good illustration of why trucks win above one yard.
How far does a cubic yard of concrete go?
| Thickness | 1 cubic yard covers |
|---|---|
| 3 in | 108 sq ft |
| 4 in | 81 sq ft |
| 5 in | 65 sq ft |
| 6 in | 54 sq ft |
| 8 in | 41 sq ft |
| 10 in | 32 sq ft |
Concrete slab size chart (cubic yards)
Quick reference for common slab sizes, before waste. Add 5–10% for a real order. The last column shows how many 80-lb bags a 4-inch slab needs.
| Slab size | 4 in thick | 5 in thick | 6 in thick | 80-lb bags (4 in) |
|---|---|---|---|---|
| 8 × 8 ft (64 sq ft) | 0.79 | 0.99 | 1.19 | 36 |
| 10 × 10 ft (100 sq ft) | 1.23 | 1.54 | 1.85 | 56 |
| 10 × 12 ft (120 sq ft) | 1.48 | 1.85 | 2.22 | 67 |
| 12 × 12 ft (144 sq ft) | 1.78 | 2.22 | 2.67 | 80 |
| 12 × 16 ft (192 sq ft) | 2.37 | 2.96 | 3.56 | 107 |
| 12 × 20 ft (240 sq ft) | 2.96 | 3.70 | 4.44 | 134 |
| 16 × 16 ft (256 sq ft) | 3.16 | 3.95 | 4.74 | 143 |
| 20 × 20 ft (400 sq ft) | 4.94 | 6.17 | 7.41 | 223 |
| 24 × 24 ft (576 sq ft) | 7.11 | 8.89 | 10.67 | 320 |
| 24 × 30 ft (720 sq ft) | 8.89 | 11.11 | 13.33 | 400 |
Bags or ready-mix: which is cheaper?
A cubic yard is 27 cubic feet, so it takes 45 bags of 80-lb mix (60 bags of 60-lb, or 90 bags of 40-lb) to make one yard. Bags cost more per yard than a truck, but they need no minimum order and no pump or chute. As a rule:
- Under about 1 cubic yard (a small pad, a few footings, post holes): bagged premix.
- 1 to 2 yards: compare both; short-load fees on a truck can make bags competitive.
- More than 2 yards: ready-mix almost always wins on cost, speed and finish quality.
Slab thickness and strength
| Use | Typical thickness | Typical strength |
|---|---|---|
| Walkways, patios, shed floors | 4 in | 3,000 psi |
| Driveways, garage floors | 5–6 in | 3,500–4,000 psi |
| Heavy-duty pads and slabs with vehicles over 10,000 lb | 6–8 in or engineered | 4,000+ psi |
In freeze–thaw climates ask your supplier for air-entrained concrete. Slabs are normally poured over 4 inches of compacted gravel (use the gravel calculator for the base) with wire mesh or rebar for crack control.
How much extra should you order?
Ten percent is a safe default. Subgrades are never perfectly level, forms bulge slightly, and some concrete stays in the truck or wheelbarrow. Running short in the middle of a pour creates a cold joint, which is far more expensive to fix than a few extra bags are to buy. Use 5% for a precisely dug, small footing and up to 15% for uneven ground.
Preparing for a ready-mix pour
- Finish the base first. Excavate, compact 4 inches of gravel, set forms and rebar or mesh before the truck is scheduled.
- Give the driver access. A truck weighs around 30 tons when full; plan where it will stand, and have wheelbarrows or a pump ready if it cannot reach the forms.
- Plan the finishing crew. Screed, float and edge while the surface is still workable. Hot, windy weather shortens that window.
- Cure it. Keep the slab moist or covered for several days; it reaches most of its strength in about a week and design strength in 28 days.
Common mistakes to avoid
- Mixing units. Thickness is usually in inches; length and width are in feet. Convert inches to feet (÷ 12) before multiplying.
- Forgetting the thickness varies. A "4-inch slab" that settles to 5 inches in places uses 25% more concrete.
- Not subtracting rebar and post volume. Usually negligible, but post holes with large posts can reduce volume noticeably.
- Ordering late. Ready-mix is perishable. Have forms, base and finishing help ready before the truck arrives.
Frequently asked questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. That is about 45 bags of 80-lb mix (27 ÷ 0.60), 60 bags of 60-lb mix, or 90 bags of 40-lb mix.
How many bags of concrete do I need for a 10×10 slab?
A 10 × 10 ft slab at 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. With 10% waste you need about 62 bags of 80-lb mix — at that size a ready-mix truck is usually the better choice.
How much does a cubic yard of concrete weigh?
Standard concrete weighs roughly 4,000–4,050 pounds (about 2 tons) per cubic yard, depending on aggregate and moisture.
How much does a yard of concrete cost?
Ready-mix typically costs about $130–$200 per cubic yard delivered in most US markets, plus short-load, fuel or pump fees. Bagged concrete costs around $5–$8 per 80-lb bag.
How do I calculate concrete for a footing?
Multiply the footing length (ft) by its width (in ÷ 12) and depth (in ÷ 12) to get cubic feet. Choose "Footing or trench" above and the calculator does the conversion.
How thick should a concrete slab be?
Four inches for patios, walkways and sheds; five to six inches for driveways and garage floors; thicker or engineered for heavy loads.
How much extra concrete should I order?
Order 5–10% more than the calculated volume. Use the higher end for uneven ground, small forms or hand-mixed batches.
Should I use bags or ready-mix?
Bags make sense for jobs under about one cubic yard. Above that, mixing dozens of bags by hand is slow and the finish is less consistent, so ready-mix is usually the better value.
Sources and further reading
- Concrete driveways and slabs — American Cement Association
- National Ready Mixed Concrete Association — NRMCA: ready-mix industry resources
- American Concrete Institute — ACI: concrete codes and guides
- Silica, crystalline (cutting concrete, block, brick and stone) — U.S. Occupational Safety and Health Administration
- NIST Special Publication 811: conversion factors — National Institute of Standards and Technology
These references support the general guidance on this page. The calculator applies the standard formulas shown above to the numbers you enter.
Related calculators
. Estimates only. Always confirm quantities with your supplier or a qualified contractor before ordering, and add extra for waste and site conditions.