# Concrete Slab Calculator

> Computes the concrete volume of a rectangular slab, footing or pad from length, width and thickness, adds a waste allowance and converts it to cubic yards, cubic feet and the number of 80 lb, 60 lb and 25 kg bags of premixed concrete.

- Calculator id: `concrete-slab` · Category: Construction & Home (`construction`) · Tool name: `estimate_concrete_slab`
- Canonical page: https://tttkmbb.com/construction/concrete-slab · This document: https://tttkmbb.com/construction/concrete-slab.md · JSON definition: https://tttkmbb.com/construction/concrete-slab.json

## Purpose

Computes the concrete volume of a rectangular slab, footing or pad from length, width and thickness, adds a waste allowance and converts it to cubic yards, cubic feet and the number of 80 lb, 60 lb and 25 kg bags of premixed concrete.

**Use when:** You need the amount of ready-mix concrete to order or the number of premix bags for a slab, patio, shed base, footing or pad of uniform thickness.

**Do not use when:** The pour is a cylinder such as a post hole or pier (use cylinder for the volume), or you need mortar for brickwork (use brick).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `unit_system` | enum: metric \| imperial |  | optional, default "metric" | Unit system of the length inputs (metric: m / cm / mm; imperial: ft / in). Outputs are reported in both systems. |
| `length` | number | m or ft | required | Slab length in metres or feet. (> 0, max 1000) |
| `width` | number | m or ft | required | Slab width in metres or feet. (> 0, max 1000) |
| `thickness` | number | cm or in | required | Slab thickness in centimetres (metric) or inches (imperial). Typical: 10 cm / 4 in for paths and patios, 15 cm / 6 in for driveways. (> 0, max 500) |
| `waste_percent` | number | % | optional, default 10 | Extra material for cuts, breakage and offcuts, applied as a multiplier (1 + waste_percent/100). Covers uneven sub-base, spillage and over-excavation; 5–10 % is usual. (min 0, max 50) |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `volume_m3` | number | m³ | length × width × thickness, before waste. |
| `volume_cubic_feet` | number | ft³ | Same volume in cubic feet. |
| `volume_cubic_yards` | number | yd³ | Same volume in cubic yards (1 yd³ = 27 ft³ = 0.7646 m³). |
| `volume_with_waste_m3` | number | m³ | Volume × (1 + waste_percent/100). |
| `volume_with_waste_cubic_yards` | number | yd³ | Volume to order in cubic yards. |
| `bags_80lb` | integer |  | ceil(volume_with_waste_ft³ / 0.60 ft³ per bag). |
| `bags_60lb` | integer |  | ceil(volume_with_waste_ft³ / 0.45 ft³ per bag). |
| `bags_25kg` | integer |  | ceil(volume_with_waste_m³ × 96 bags per m³), assuming about 2400 kg of dry premix per m³ of placed concrete. |

## Formula

`volume = length × width × thickness (thickness/100 in m or thickness/12 in ft); with_waste = volume × (1 + waste_percent/100); yd³ = ft³ / 27; m³ = ft³ × 0.028316847; bags_80lb = ceil(ft³ / 0.60); bags_60lb = ceil(ft³ / 0.45); bags_25kg = ceil(m³ × 96).`

Bag yields follow the QUIKRETE data sheet (80 lb → 0.60 ft³, 60 lb → 0.45 ft³). The 25 kg count assumes ≈ 2400 kg of dry mix per m³ (96 bags), a conservative figure: manufacturer yields of 0.010–0.0125 m³ per 25 kg bag correspond to 80–100 bags/m³, so check the bag.

## Data Sources

- QUIKRETE Concrete Mix No. 1101 product data sheet (yield: 80 lb bag 0.60 ft³, 60 lb bag 0.45 ft³) — https://www.quikrete.com/pdfs/data_sheet-concrete%20mix%201101.pdf (reference, retrieved 2026-09-23)
- QUIKRETE – Concrete, mortar and post-setting calculators — https://www.quikrete.com/calculator/main.asp (reference, retrieved 2026-09-23)
- Wikipedia – Concrete (density of normal-weight concrete ≈ 2400 kg/m³) — https://en.wikipedia.org/wiki/Concrete (reference, retrieved 2026-09-23)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/concrete-slab?length=…&width=…&thickness=…`
- `POST https://tttkmbb.com/api/v1/calculate/concrete-slab` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/concrete-slab · OpenAPI operationId `estimate_concrete_slab` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "concrete-slab", "inputs": {…}}`

## Example

- 10 ft × 20 ft slab, 4 in thick, 10 % waste: inputs `{"unit_system":"imperial","length":10,"width":20,"thickness":4,"waste_percent":10}` → `{"volume_cubic_feet":66.67,"volume_cubic_yards":2.469,"volume_m3":1.888,"volume_with_waste_cubic_yards":2.716,"volume_with_waste_m3":2.077,"bags_80lb":123,"bags_60lb":163,"bags_25kg":200}`
- 4 m × 3 m slab, 10 cm thick, 10 % waste: inputs `{"unit_system":"metric","length":4,"width":3,"thickness":10,"waste_percent":10}` → `{"volume_m3":1.2,"volume_cubic_yards":1.57,"volume_with_waste_m3":1.32,"volume_with_waste_cubic_yards":1.727,"bags_25kg":127,"bags_80lb":78,"bags_60lb":104}`

```
GET https://tttkmbb.com/api/v1/calculate/concrete-slab?unit_system=imperial&length=10&width=20&thickness=4&waste_percent=10
```

## Limitations

The pour is a cylinder such as a post hole or pier (use cylinder for the volume), or you need mortar for brickwork (use brick). Bag yields follow the QUIKRETE data sheet (80 lb → 0.60 ft³, 60 lb → 0.45 ft³). The 25 kg count assumes ≈ 2400 kg of dry mix per m³ (96 bags), a conservative figure: manufacturer yields of 0.010–0.0125 m³ per 25 kg bag correspond to 80–100 bags/m³, so check the bag. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Should I order ready-mix or use bags?**

Bags are practical up to roughly 0.5 m³ (about 30 × 80 lb bags); above that, ready-mix delivered by truck is cheaper and gives a consistent mix. Ready-mix is ordered by the m³ or yd³ using the with-waste figure.

**Why do bag counts differ between brands?**

Yield per bag depends on the mix design and how much water is added; the figures here are QUIKRETE's for US bags and a conservative 2400 kg/m³ assumption for 25 kg bags. Use the yield printed on the bag when it differs.

## Related

- [Gravel Calculator](https://tttkmbb.com/construction/gravel.md) — Sub-base aggregate under the slab.
- [Brick Calculator](https://tttkmbb.com/construction/brick.md) — Bricks and mortar for walls on the slab.
- [Cylinder Calculator](https://tttkmbb.com/geometry/cylinder.md) — Volume of round post holes or piers.