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.
Concrete needed for common projects
| Project | Cubic yards | 80-lb bags |
|---|---|---|
| Sidewalk, 3 × 20 ft, 4 in | 0.74 | 34 |
| Shed pad, 10 × 12 ft, 4 in | 1.48 | 67 |
| Garage floor, 24 × 24 ft, 6 in | 10.67 | 480 |
| Footing, 40 ft × 16 in × 8 in | 1.32 | 60 |
| Wall, 20 ft × 4 ft × 8 in | 1.98 | 89 |
| Four 12 in round columns, 4 ft | 0.47 | 21 |
These are exact volumes without waste. Add the waste allowance from the calculator before you order.
What a cubic yard of concrete weighs
A cubic yard of normal-weight concrete is about 2 tons, or roughly 4,000 pounds. That is why a ready-mix truck, not a pickup, delivers anything over a fraction of a yard, and why bagged concrete is heavy work: 45 bags of 80 lb make one yard. Use the yards to tons calculator for other materials.
Estimate the cost
Prices depend on your area, the mix and delivery distance, so ask two or three local suppliers for a delivered quote. The concrete cost calculator compares ready-mix with bags using your own prices, including delivery and short-load fees.
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.
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Guides and how-tos
. Estimates only. Always confirm quantities with your supplier or a qualified contractor before ordering, and add extra for waste and site conditions.