# Concrete Calculator

To figure concrete, multiply length × width × thickness in feet to get cubic feet, then divide by 27 for cubic yards. Order 4–10% extra. A 12 × 12 ft slab 4 in thick is 1.78 cubic yards, or about 1.96 yards with 10% extra: 88 bags of 80 lb mix.

## Key takeaways

- Cubic yards = length (ft) × width (ft) × thickness (in ÷ 12) ÷ 27.
- An 80 lb bag of concrete mix yields about 0.6 cu ft, so a cubic yard takes 45 bags. A 60 lb bag yields 0.45 cu ft (60 per yard).
- The National Ready Mixed Concrete Association advises ordering 4–10% more than the plan volume for spillage, form bulge and low spots in the subgrade.
- Always convert inches to feet before you multiply. A 4 in slab is 0.333 ft thick, not 4.

## How do you calculate how much concrete you need?

Work out the volume in cubic feet, divide by 27 to get cubic yards, then add a waste allowance. Ready-mix plants sell by the cubic yard; bagged mix is sold by weight, so divide cubic feet by the bag's yield to get a bag count.

**Formula**

```
Cubic yards = L × W × (T ÷ 12) ÷ 27 × (1 + waste)
```

Where:
- L = length in feet
- W = width in feet (for a footing, width in inches ÷ 12)
- T = thickness or depth in inches
- 27 = cubic feet in one cubic yard (3 × 3 × 3)
- waste = extra to order, 0.04–0.10 per NRMCA guidance

For a round column, pier or post hole, the volume is π × r² × height, where r is the radius in feet (diameter in inches ÷ 24). For bags: bags = cubic feet with waste ÷ bag yield, rounded up.

1. **Measure the form, not the plan.** Measure the inside of the forms. Use the real depth: a 2×4 form board is 3.5 in tall, a 2×6 is 5.5 in.
2. **Convert inches to feet.** Divide thickness, footing width and column diameter by 12 before you multiply.
3. **Multiply and divide by 27.** Length × width × thickness in feet gives cubic feet. Divide by 27 for cubic yards.
4. **Add waste.** Multiply by 1.04–1.10. Use the high end for uneven subgrade, long footings or a pump truck.
5. **Pick bags or ready-mix.** Divide cubic feet by the bag yield for a bag count. Above about a yard, price a ready-mix truck too.

> **Worked example: a patio slab**
>
> A 12 × 12 ft patio, 4 in thick: 12 × 12 × 0.333 = **48 cu ft**.
>
> 48 ÷ 27 = 1.78 cubic yards. With 10% waste: 52.8 cu ft, or **1.96 cubic yards**.
>
> In bags: 52.8 ÷ 0.6 = **88 bags of 80 lb mix**, or 118 bags of 60 lb. That is more than three tons of bags to lift, so get a 2-yard ready-mix price before you decide.

## Footings and columns

A continuous footing is just a long, narrow slab. 96 ft of footing 16 in wide and 8 in deep is 96 × 1.333 × 0.667 = 85.3 cu ft, or 3.16 yards. With 10% waste, order 3.48 yards. Trenched footings often run wider than planned where the bank caves in, so lean toward the high end of the waste range.

For round forms, 6 tube forms 12 in across and 4 ft deep hold π × 0.5² × 4 × 6 = 18.8 cu ft (0.7 yd). With 10% waste that is 35 bags of 80 lb mix, a typical bag job for a deck.

## How many bags of concrete are in a yard?

It takes about 45 bags of 80 lb concrete mix, 60 bags of 60 lb or 90 bags of 40 lb to make one cubic yard, based on the manufacturer's stated yield per bag. Those counts have no waste in them.

**Bag yield and bags per cubic yard**

| Bag size | Yield per bag | Bags per cubic yard | Bags per 10 × 10 ft slab, 4 in thick (no waste) |
| --- | --- | --- | --- |
| 80 lb | 0.6 cu ft | 45 | 56 |
| 60 lb | 0.45 cu ft | 60 | 75 |
| 40 lb | 0.3 cu ft | 90 | 112 |

_Yields from QUIKRETE Concrete Mix product information, checked 2026-10-12. Other brands are close; check the bag._

## How much concrete for common jobs?

**Concrete by job (10% waste included in the last two columns)**

| Job | Size | Net cubic yards | Order (cu yd) | 80 lb bags |
| --- | --- | --- | --- | --- |
| Shed pad | 8 × 10 ft × 4 in | 0.99 | 1.09 | 49 |
| Patio | 12 × 12 ft × 4 in | 1.78 | 1.96 | 88 |
| Sidewalk run | 30 × 4 ft × 4 in | 1.48 | 1.63 | 74 |
| Two-car garage slab | 24 × 24 ft × 4 in | 7.11 | 7.82 | 352 |
| Single-car driveway | 12 × 40 ft × 4 in | 5.93 | 6.52 | 294 |
| Driveway for heavier vehicles | 12 × 40 ft × 6 in | 8.89 | 9.78 | 440 |

_A 4 in slab is the common minimum for a residential driveway carrying passenger cars; heavier vehicles may call for 5–6 in (Concrete Network). Your local code, the soil and the engineer's drawings decide the real thickness._

## Should you use bags or order ready-mix?

Bags make sense for post holes, footings for a deck, small pads and repairs: anything up to about a yard where a truck can't reach or the minimum order would waste most of the load. Above that, ready-mix usually wins on labor alone. A single yard is about 45 bags of 80 lb mix, all mixed and placed by hand before the first batch sets.

- **Ask about short-load fees.** Many plants charge extra for loads under their minimum. Get the price per yard and the fee in the same call.
- **Round up to the plant's increment.** Ready-mix is usually sold in quarter or half yards. Running short on a pour is far more expensive than half a yard of extra.
- **Plan the leftover.** Have a spare form or a spot for extra concrete so overage doesn't end up as a disposal problem.

## Common concrete estimating mistakes

1. **Leaving thickness in inches.** 12 × 12 × 4 = 576 is cubic-foot nonsense. Divide 4 by 12 first: 12 × 12 × 0.333 = 48 cu ft.
2. **Using nominal lumber sizes.** A 2×4 form is 3.5 in, not 4 in. If you screed to the top of the form, that is your slab depth.
3. **Forgetting thickened edges and turned-down footings.** Calculate those as footings and add them to the slab.
4. **Skipping waste.** Subgrade is never perfectly flat. An inch of low spot across a 400 sq ft slab is another 1.2 yards.
5. **Counting bags without yield.** Bag weight is not volume. Check the yield printed on the bag.
6. **Not counting every footing run.** Walk the plan and add up every leg, including interior bearing footings and pads.

Once the yards and bags are known, BRIX turns them into a line-item quote on site and follows up until the customer signs.

- [Quotes and estimates](https://www.brix.so/features/quotes-estimates): Builds accurate quotes on site and follows up on every open estimate.
- [Sales automation](https://www.brix.so/features/sales-automation): Follows up every lead and open estimate automatically by text and email, and asks every customer for a review after the job.

## Frequently asked questions

### How many 80 lb bags of concrete make a yard?

About 45. An 80 lb bag of concrete mix yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45. Add 4–10% for waste.

### How much concrete do I need for a 10 × 10 slab?

A 10 × 10 ft slab 4 in thick is 33.3 cubic feet, or 1.23 cubic yards. With 10% waste, order about 1.36 yards, or 62 bags of 80 lb mix.

### How much extra concrete should I order?

The National Ready Mixed Concrete Association recommends ordering 4–10% more than the volume calculated from plan dimensions. Use the high end for uneven or soft subgrade, long trenched footings, or a pump, and the low end for a well-formed slab on a compacted, level base.

### How thick should a concrete driveway be?

A 4 in slab is a common minimum for a residential driveway that carries passenger cars and SUVs. Driveways that carry RVs, trucks or heavy equipment are often 5–6 in. Local code and soil conditions can require more, so check before you pour.

### How do I calculate concrete for a round post hole?

Use π × radius² × depth, with all measurements in feet. A 12 in hole 4 ft deep is 3.14 × 0.5 × 0.5 × 4 = 3.14 cubic feet, or six 80 lb bags with 10% waste. Subtract the post's volume if you want to be exact.

### Is it cheaper to mix bags or order ready-mix?

For small jobs under about a yard, bags are usually cheaper and simpler. Above that, compare the ready-mix price including any short-load fee against the bags plus the labor to mix them. Labor usually tips it toward the truck.

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## Sources

- [QUIKRETE — Concrete Mix No. 1101 data sheet (yield per bag)](https://www.quikrete.com/PDFs/DATA_SHEET-Concrete%20Mix%201101.pdf) (checked 2026-10-12)
- [NRMCA — CIP 31, Ordering Ready Mixed Concrete](https://www.nrmca.org/wp-content/uploads/2021/01/31pr.pdf) (checked 2026-10-12)
- [NRMCA — CIP 8, Discrepancies in Yield](https://www.nrmca.org/wp-content/uploads/2021/01/08pr.pdf) (checked 2026-10-12)
- [Concrete Network — How Thick Should a Concrete Driveway Be?](https://www.concretenetwork.com/concrete/concrete_driveways/thickness.html) (checked 2026-10-12)

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Written by BRIX Editorial Team. Last updated 2026-10-12.
Canonical: https://www.brix.so/tools/concrete-calculator