# 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.

**Quick answer:** 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.

_Last updated: October 4, 2026. Estimates only; confirm quantities with your supplier._

## 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.

```
cubic yards = cubic feet ÷ 27
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](../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](../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](../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](https://www.cement.org/cement-concrete/paving/buildings-structures/concrete-homes/products/driveways) — American Cement Association
- [National Ready Mixed Concrete Association](https://www.nrmca.org/) — NRMCA: ready-mix industry resources
- [American Concrete Institute](https://www.concrete.org/) — ACI: concrete codes and guides
- [Silica, crystalline (cutting concrete, block, brick and stone)](https://www.osha.gov/silica-crystalline) — U.S. Occupational Safety and Health Administration
- [NIST Special Publication 811: conversion factors](https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors) — National Institute of Standards and Technology