# Concrete Footing Calculator

> Computes the concrete volume of a strip footing (length × width × depth) or of round or square piers (count × cross-section × depth), adds waste and converts it to cubic yards, cubic feet and bags of premixed concrete using manufacturer yields (80 lb 0.60 ft³, 60 lb 0.45 ft³, 50 lb 0.375 ft³, 25 kg 0.0125 m³, 20 kg 0.010 m³). Planning estimate only.

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

## Purpose

Computes the concrete volume of a strip footing (length × width × depth) or of round or square piers (count × cross-section × depth), adds waste and converts it to cubic yards, cubic feet and bags of premixed concrete using manufacturer yields (80 lb 0.60 ft³, 60 lb 0.45 ft³, 50 lb 0.375 ft³, 25 kg 0.0125 m³, 20 kg 0.010 m³). Planning estimate only.

**Use when:** You need the concrete for wall footings, deck or fence post piers, sonotube piers or pad footings, as a ready-mix volume or as a number of bags.

**Do not use when:** The pour is a flat slab (use concrete-slab), you need cement, sand and aggregate for site-mixed concrete (use concrete-mix-ratio), or the footing size itself must be designed for the soil bearing capacity (engineering, not covered).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `shape` | enum: strip \| round_pier \| square_pier |  | optional, default "strip" | Strip footing (length × width × depth) or piers (count × cross-section × depth). |
| `unit` | enum: m \| ft |  | optional, default "m" | Unit of the length inputs (m or ft). Use decimal feet for imperial input (18 in = 1.5 ft). Lengths are reported in both units. |
| `length` | number | m or ft | optional | Total length of the strip footing (m or ft); not used for piers. (> 0, max 10000) |
| `width` | number | m or ft | optional | Width of the strip footing or side length of a square pier (m or ft). (> 0, max 100) |
| `depth` | number | m or ft | required | Footing thickness or pier depth (m or ft). (> 0, max 100) |
| `diameter` | number | m or ft | optional | Diameter of a round pier (m or ft); required for round_pier. (> 0, max 100) |
| `count` | integer |  | optional, default 1 | Number of identical piers (or strip footings). (min 1, max 1000) |
| `waste_percent` | number | % | optional, default 10 | Extra material for cuts, breakage and offcuts, applied as a multiplier (1 + waste_percent/100). Covers over-excavation, uneven trench bottoms and spillage; 10 % is usual for footings dug by hand. (min 0, max 50) |
| `bag_size` | enum: 80_lb \| 60_lb \| 50_lb \| 25_kg \| 20_kg |  | optional, default "80_lb" | Bag size used for the bag count, with its typical yield of placed concrete. |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `volume_m3` | number | m³ | Concrete volume before waste. |
| `volume_ft3` | number | ft³ | Same volume in cubic feet. |
| `volume_yd3` | number | yd³ | Same volume in cubic yards (1 yd³ = 0.7646 m³). |
| `volume_with_waste_m3` | number | m³ | volume × (1 + waste_percent/100). |
| `volume_with_waste_yd3` | number | yd³ | Volume to order in cubic yards. |
| `bags` | integer |  | ceil(volume_with_waste / yield per bag). |
| `bag_yield_used` | string |  | Bag size and yield assumed for the count. |
| `ready_mix_note` | string |  | Whether the volume is practical for bagged mix (below about 0.5 m³ / 0.65 yd³) or should be ordered as ready-mix. |

## Formula

`strip: volume = length × width × depth × count; round_pier: volume = count × π/4 × diameter² × depth; square_pier: volume = count × width² × depth; with_waste = volume × (1 + waste_percent/100); bags = ceil(with_waste / yield_per_bag), yields 0.60 / 0.45 / 0.375 ft³ for 80 / 60 / 50 lb bags and 0.0125 / 0.010 m³ for 25 / 20 kg bags.`

US bag yields follow the QUIKRETE Concrete Mix data sheet and the 25 kg / 20 kg yields are typical manufacturer figures (check the bag: 0.010–0.0125 m³ per 25 kg is common); rebar, formwork, sonotube forms and a gravel bed under the footing are not included. Planning estimate only; check quantities against the supplier's coverage figures and local code before ordering.

## 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 2026-09-24)
- QUIKRETE – Concrete, mortar and post-setting calculators — https://www.quikrete.com/calculator/main.asp (reference, retrieved 2026-09-24)
- Wikipedia – Foundation (engineering) — https://en.wikipedia.org/wiki/Foundation_(engineering) (reference, retrieved 2026-09-24)

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-footing?depth=…`
- `POST https://tttkmbb.com/api/v1/calculate/concrete-footing` 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-footing · OpenAPI operationId `estimate_concrete_footing` 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-footing", "inputs": {…}}`

## Example

- Strip footing 10 m × 0.5 m × 0.3 m, 10 % waste, 25 kg bags: inputs `{"shape":"strip","unit":"m","length":10,"width":0.5,"depth":0.3,"waste_percent":10,"bag_size":"25_kg"}` → `{"volume_m3":1.5,"volume_ft3":52.97,"volume_yd3":1.962,"volume_with_waste_m3":1.65,"volume_with_waste_yd3":2.158,"bags":132,"bag_yield_used":"25 kg bag, 0.0125 m³"}`
- 4 round piers 0.3 m diameter × 1 m deep, 10 % waste, 80 lb bags: inputs `{"shape":"round_pier","unit":"m","diameter":0.3,"depth":1,"count":4,"waste_percent":10,"bag_size":"80_lb"}` → `{"volume_m3":0.283,"volume_yd3":0.37,"volume_with_waste_m3":0.311,"bags":19,"bag_yield_used":"80 lb bag, 0.60 ft³"}`

```
GET https://tttkmbb.com/api/v1/calculate/concrete-footing?shape=strip&unit=m&length=10&width=0.5&depth=0.3&waste_percent=10&bag_size=25_kg
```

## Limitations

The pour is a flat slab (use concrete-slab), you need cement, sand and aggregate for site-mixed concrete (use concrete-mix-ratio), or the footing size itself must be designed for the soil bearing capacity (engineering, not covered). US bag yields follow the QUIKRETE Concrete Mix data sheet and the 25 kg / 20 kg yields are typical manufacturer figures (check the bag: 0.010–0.0125 m³ per 25 kg is common); rebar, formwork, sonotube forms and a gravel bed under the footing are not included. Planning estimate only; check quantities against the supplier's coverage figures and local code before ordering. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**How deep should a footing be?**

Below the local frost line and on undisturbed soil; in the IRC, footings are at least 12 in (300 mm) below grade and 6 in (150 mm) thick, with width set by the soil bearing capacity. Check the local code.

**How many bags for a fence post?**

A 10 in (250 mm) hole 2 ft (0.6 m) deep with a 4 × 4 post takes roughly 0.9 ft³ (0.025 m³) of concrete, i.e. two 80 lb bags; enter diameter 0.25 m, depth 0.6 m and the post count.

**When is ready-mix better than bags?**

Above about 0.5 m³ (30 × 80 lb bags) ready-mix is cheaper and gives a consistent strength; most plants have a minimum order (often 1 m³ or 1 yd³) and charge for short loads.

## Related

- [Concrete Slab Calculator](https://tttkmbb.com/construction/concrete-slab.md) — Slab volume and bags.
- [Cylinder Calculator](https://tttkmbb.com/geometry/cylinder.md) — Volume of a round pier from radius and height.
- [Rebar Calculator](https://tttkmbb.com/construction/rebar.md) — Reinforcement for the footing or slab.