Deck Footing Calculator — Post Holes & Concrete Volume
Calculate the number of deck footings needed, the volume of concrete per footing, and total concrete bags required. Enter deck size and post spacing to get footing layout, depth below frost line and total concrete volume for your deck foundation.
How the deck footing calculator works
- 1Post grid: number of posts = (deck length ÷ post spacing + 1) × (number of beam lines)
- 2Footing diameter: typically 300–400 mm for residential decks
- 3Footing depth: below frost line (minimum 600 mm, up to 1200 mm in cold climates)
- 4Concrete volume per footing: π × (diameter/2)² × depth (m³)
- 5Total concrete = volume per footing × number of footings
- 6Bags needed = total m³ ÷ volume per 25 kg bag (typically 0.012 m³/bag)
Worked example
Deck: 4 m × 6 m, posts at 1.8 m centres, 9 posts total. 300 mm diameter footings, 800 mm deep.
Volume per footing: π × 0.15² × 0.8 = 0.057 m³. Total: 9 × 0.057 = 0.51 m³. Bags: 0.51 ÷ 0.012 = 43 bags of 25 kg. Consider ready-mix for more than 0.5 m³.
Frequently asked questions
How deep should deck footings be?
Footings must extend below the frost line to prevent heave — typically 600 mm in southern coastal regions, 800–1000 mm in inland areas, and up to 1200 mm in the coldest zones. Always check local building codes.
What diameter should deck footings be?
250–300 mm is standard for most residential decks. Larger decks or decks with long spans may require 350–450 mm footings. Use a post hole digger or auger to achieve round, consistent-diameter holes.
How many footings do I need for a deck?
As a rule, place footings at every post location — typically 1.5–2.5 m apart along each beam line. A 4×6 m deck with 1.8 m post spacing on two beam lines needs approximately 8–10 footings depending on the framing design.
Should I use ready-mix or bagged concrete for deck footings?
For fewer than 10 footings or volumes under 0.5 m³, bagged concrete is easier and more practical. For larger decks, order ready-mix concrete — it is cheaper per m³ and easier to place in multiple holes quickly.
Do deck posts need to be set in concrete?
In Norway and most cold climates, yes — posts are typically set in a concrete footing with the post sitting above ground on a post base connector (not embedded in concrete) to prevent rot. The post base is cast into the footing, not the post itself.