# Concrete Calculator

- **URL:** https://codeasystem.com/calculators/construction/concrete-calculator/
- **Category:** construction
- **Description:** Estimate cubic yards of concrete for a slab, plus the number of 80-lb or 60-lb bags to buy with a waste allowance.
- **Primary output:** Concrete volume (10% waste included): 5.43 yd³

## Inputs
- Slab length (name: `length`, type: number, example: 20)
- Slab width (name: `width`, type: number, example: 20)
- Thickness (name: `thickness`, type: number, example: 4)
- Waste allowance (name: `waste`, type: number, example: 10)

## Outputs
- Concrete volume (10% waste included): 5.43 yd³
- Volume without waste: 4.94 yd³ (133.33 ft³)
- Volume with waste: 146.67 ft³
- 80-lb bags (≈0.60 ft³ each): 245 bags
- 60-lb bags (≈0.45 ft³ each): 326 bags

## Formula / methodology
```
ft³ = L(ft) × W(ft) × T(in) / 12     yd³ = ft³ / 27
Bags = ft³(with waste) ÷ yield   (80-lb ≈ 0.60 ft³, 60-lb ≈ 0.45 ft³)
```

A rectangular slab’s volume is its footprint area times thickness; convert inches to feet first so the units cancel. Divide by 27 to express the result in cubic yards, the unit ready-mix trucks price in.

The waste allowance (default 10%) covers uneven subgrade, spillage and over-excavation. Bag counts divide the wasted volume by each bag size’s typical yield and always round up, because a partial bag is not an option.

## Assumptions & limitations
- Rectangular pour on a flat, even subgrade.
- 80-lb bag yields ≈0.60 ft³ and 60-lb bag yields ≈0.45 ft³ of mixed concrete.
- Waste allowance applied multiplicatively before rounding bags up.
- Footings, thickened edges and rebar displacement are not modeled.

## How to use
1. **Measure the pour**; Enter the length and width in feet and the slab thickness in inches; 4 inches is standard for patios, 5–6 for driveways.
2. **Keep some waste**; Leave the 10% waste allowance unless you are ordering ready-mix against a formed, exact-volume pour.
3. **Choose bags or truck**; Compare the bag counts against the cubic-yard figure: anything over about 1 yd³ is usually cheaper delivered by truck.

## Example
A 20 ft × 20 ft patio at 4 inches thick is about 133 ft³ = 4.94 yd³; adding 10% waste gives roughly 5.43 yd³; about 245 of the 80-lb bags or 326 of the 60-lb bags.

Result for these inputs:

```
Concrete volume (10% waste included): 5.43 yd³
```

## About this calculator
### Cubic yards, feet and bags

Ready-mix suppliers sell by the cubic yard, but most DIY pours start life measured in feet and inches. One cubic yard is 27 cubic feet; a slab just 12×12 feet at 4 inches already eats nearly two yards, which is why small projects go the bag route.

### Why order more than the math says

Subgrades are rarely perfectly flat and forms flex. A 10% cushion prevents the classic cold joint that appears when a pour runs short mid-slab. For hand-mixed jobs, buying one extra bag beats a weak spot forever.

### How thick should it be?

Walkway and shed pads get by at 3–4 inches, patios and interior slabs at 4, and vehicle traffic wants 5–6 inches with reinforcement. Doubling thickness doubles cost, so match the spec to the load instead of guessing high everywhere.

## FAQs
### How many 80-lb bags make one cubic yard?

About 45 bags: 27 ft³ ÷ 0.60 ft³ per bag. For 60-lb bags it is roughly 60 bags.

### Should I include waste in my order?

Yes. A 10% allowance is standard for DIY pours; ready-mix orders often round up to the nearest quarter or half yard instead.

### Does rebar or wire mesh change the volume?

Reinforcement occupies negligible volume. The subgrade unevenness covered by the waste allowance matters far more.

### What if my slab is not rectangular?

Split it into rectangles, run each through the calculator, then add the results before applying waste once.

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Last updated: 2026-08-23 · Version: 1.0.0 · [HTML version](https://codeasystem.com/calculators/construction/concrete-calculator/)