How Many Footings Does a Deck Need?

The number of footings comes straight from how far your beam can span between posts. A beam that reaches farther needs fewer posts — and every post needs a footing — so beam size and footing count are directly linked.

Posts across the beam

Look up your beam's maximum post spacing in IRC Table R507.5, then divide your deck's width by it and round up. A 16-ft-wide deck with a beam good for 8 ft between posts needs three posts (two spans), so three footings along the beam, plus whatever the ledger and any stair posts require.

Fewer, bigger footings vs more, smaller ones

• Upsizing the beam (e.g. 2-ply to 3-ply) can delete a post and a footing.
• Each footing still has to be big enough for the load its post carries — fewer posts means each footing carries more.
• On poor soil, you may need wider footings or more posts to keep bearing pressure in check.

The calculator reports the post count, spacing and the footing size each one needs, so you know exactly how many holes to dig and how big.