Concrete Footing Calculator
Yards and bags for deck, fence, and foundation footings
What is concrete footing calculator?
A concrete footing calculator sizes the volume and bag count for footings — the load-spreading concrete element that transfers post or wall loads into the soil. Footings differ from slabs in three ways: they sit below the frost line, they’re wider than the element they support, and they often include rebar for tensile capacity.
Frost line is the controlling depth. A footing that doesn’t extend below the local frost line will heave every winter as ice forms in the soil under it — lifting the deck or fence and tearing it apart over a few seasons. Frost depths vary by latitude: 12 in in central Florida, 36 in in Pennsylvania, 48 in in Minnesota and Maine. Your local building department publishes the official depth; don’t reuse the depth from a job in another climate.
Footing dimensions follow code, not intuition. The IRC’s Table R403.1 sizes footings by load and soil type. For most residential decks, 16 × 16 × 8 in is the minimum. Heavy snow regions, two-story decks, or weak soils push that to 24 × 24 × 12 in. Round (sonotube) footings should be at least 10-12 in diameter for posts; pier footings under foundation walls run 16-24 in wide.
Why footings are wider than posts. Soil bearing capacity is measured in PSI. A 1,000 lb point load on a 4 × 4 post (12.25 in² of contact) creates 82 PSI — exceeds most soils. Spreading that same 1,000 lb across a 16 × 16 in footing (256 in²) drops the bearing stress to 4 PSI — well within capacity. The wider the footing, the lower the soil stress, the more load you can carry.
Pour logistics. Most footing pours are small (under 1 cu yd total), making bagged premixed concrete the right product. Fast-Setting mix lets you backfill and continue building the same day; standard mix means a 24-48 hour wait before loading. For long foundation footing runs over 1 cu yd, switch to ready-mix.
When to use a concrete footing calculator
- Deck footing — Code requires deck footings below the frost line — typically 36-48 in deep in cold climates. Footing dimensions follow IRC Table R403.1: 16 × 16 × 8 in for most residential decks.
- Fence post footing — Round or square footings 8-12 in wide, 30-36 in deep. The Footing tab handles rectangular; for round, use the Column tab with the diameter.
- Foundation footing run — A continuous strip footing under a foundation wall. Length × width × depth gives volume; multiply by your perimeter feet for the total.
How to use the Concrete Footing Calculator
- Check your local frost line — Footings must extend below the frost line to prevent heaving. Most cold US climates require 36-48 in; Florida and southern Texas allow 12 in. Check your local building department before digging.
- Size the footing per IRC code — Residential deck footings: 16 × 16 × 8 in is the IRC R403.1 minimum for most loads. Heavier roof or deck loads require larger footings — check the code table for your span and load.
- Enter the footing dimensions — The Footing tab handles rectangular shapes. For round (sonotube) footings, switch to the Column tab and use the tube diameter.
- Multiply by footing count — A typical deck has 6-12 footings. Compute one footing's volume, then multiply by your count. The calculator output is per-footing.
Worked examples
Deck footing: 16 × 16 × 8 in
Input: Footing tab: 1.33 ft × 16 in × 8 in
Output: 0.04 cu yd / 2 of 80-lb bagsFence post: 10 in × 36 in deep
Input: Column tab: 10 in dia × 3 ft
Output: 0.06 cu yd / 3 of 80-lb bagsFoundation strip: 40 ft × 16 × 8 in
Input: Footing tab: 40 ft × 16 in × 8 in
Output: 1.32 cu yd / 60 of 80-lb bags — ready-mix