# Concrete Slab Calculator (Yards & Bags)

Feet and inches straight to cubic yards and bag counts.

- Page: https://jianzhujisuan.com/en/concrete-slab-yards
- Category: Concrete (https://jianzhujisuan.com/en/c/concrete)
- Chinese version: https://jianzhujisuan.com/zh/concrete-slab-yards.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Length 10 ft, Width 10 ft, Thickness 4 in, Waste 10%, Bag size 80 lb bag (0.60 ft³ each)): **Concrete required 1.36 yd³**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Length | number | 10 | ft |  |
| Width | number | 10 | ft |  |
| Thickness | number | 4 | in |  |
| Waste | number | 10 | % |  |
| Bag size | select | 80 lb bag (0.60 ft³ each) | — |  |

## Formula

- **Volume (ft³) = L × W × T(in)/12**: convert inches to feet first
- **Cubic yards = ft³ ÷ 27**: 1 yd³ = 27 ft³
- **Bags = volume with waste ÷ bag yield**: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³
- **Weight = volume × 145 lb/ft³**: typical concrete unit weight

## Worked example

A 10 ft x 10 ft driveway slab, 4 in thick, 10% waste: 10 x 10 x (4/12) = 33.3 ft³, 36.7 ft³ with waste, which is 1.36 cubic yards, about 3.7 tons, or 62 bags of 80 lb. That is a half day of mixing for one person and still too slow for two - order 1.5 yards.

## Questions

### How much concrete do I need per square foot at 4 inches?

At 4 in thick a square foot holds 0.333 ft³, so one cubic yard covers about 81 ft². Handy numbers: 3 in covers about 108 ft² per yard, 4 in about 81, 5 in about 65, 6 in about 54. Six inches is the practical minimum for light vehicles - 4 in is fine for walkways and patios, but driveways want 5 to 6 in.

### Should I mix my own or order a truck?

Under about 1 cubic yard (roughly 45 bags of 80 lb) mixing yourself makes sense. From 1 to 3 yards it depends on crew - two people with a mixer can do it. Above 3 yards order a truck. The reason is time, not money: concrete has to be placed and finished before initial set, hand mixing cannot keep up, and inconsistent water per batch leaves patchy strength and a dusty surface. Small orders usually carry a short-load fee, so ask before you book.

### How much waste should I allow?

5% on a flat slab with tight forms, 10% where forms gap, the subgrade is uneven or you are sloping to a drain, and 15-20% for footings, trenches and irregular shapes. Better slightly over than short - a top-up load costs a full delivery and a day. Pour the surplus into a spare form as precast blocks; it beats returning it.

## Related tools

- [Concrete Volume Calculator](https://jianzhujisuan.com/en/concrete-volume): Enter length, width and thickness to get the concrete volume you need and how many truck loads that is.
- [Formwork Area Calculator](https://jianzhujisuan.com/en/formwork-area): Pick a member type, enter section and length, and get the formwork contact area plus sheet and batten estimates.
- [Concrete Mix Design Calculator](https://jianzhujisuan.com/en/concrete-mix-ratio): Work out cement, sand, gravel and water per cubic metre from the target grade and water-cement ratio.
- [Cement Quantity Calculator](https://jianzhujisuan.com/en/cement-quantity): Get total cement, bag count and the cement/fly-ash split for a given volume and grade.
- [Concrete Pour Sequencing Calculator](https://jianzhujisuan.com/en/concrete-pour-plan): Divide a slab into strips and get the volume, truck count and duration for each pour.
- [Concrete Strength Converter](https://jianzhujisuan.com/en/concrete-strength-convert): Convert between grade, MPa, psi and kgf/cm², with cube-to-cylinder correction.

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