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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.
When to use
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).
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.
Inputs
| Parameter | Type | Unit | Required | Description |
|---|---|---|---|---|
shape | enum: strip | round_pier | square_pier | default strip | Strip footing (length × width × depth) or piers (count × cross-section × depth). | |
unit | enum: m | ft | 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 | no | Total length of the strip footing (m or ft); not used for piers. Range: > 0, ≤ 10000 |
width | number | m or ft | no | Width of the strip footing or side length of a square pier (m or ft). Range: > 0, ≤ 100 |
depth | number | m or ft | yes | Footing thickness or pier depth (m or ft). Range: > 0, ≤ 100 |
diameter | number | m or ft | no | Diameter of a round pier (m or ft); required for round_pier. Range: > 0, ≤ 100 |
count | integer | default 1 | Number of identical piers (or strip footings). Range: ≥ 1, ≤ 1000 | |
waste_percent | number | % | 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. Range: ≥ 0, ≤ 50 |
bag_size | enum: 80_lb | 60_lb | 50_lb | 25_kg | 20_kg | default 80_lb | Bag size used for the bag count, with its typical yield of placed concrete. |
Outputs
| Output | 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. |
Example
Strip footing 10 m × 0.5 m × 0.3 m, 10 % waste, 25 kg bags: {"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: {"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
Machine access
- API:
GET https://tttkmbb.com/api/v1/calculate/concrete-footing(query parameters) orPOSTwith a JSON body{"inputs": {...}} - Schema: https://tttkmbb.com/api/v1/calculators/concrete-footing · Markdown: https://tttkmbb.com/construction/concrete-footing.md · JSON definition: https://tttkmbb.com/construction/concrete-footing.json
- MCP: server
https://tttkmbb.com/mcp, toolrun_calculator with calculator_id="concrete-footing" - OpenAPI operationId:
estimate_concrete_footing - Freshness:
static. Authentication: none. Rate limit: fair use (see rate limits).
Sources
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 calculators
- Concrete Slab Calculator — Slab volume and bags.
- Cylinder Calculator — Volume of a round pier from radius and height.
- Rebar Calculator — Reinforcement for the footing or slab.