Deck Footing & Beam Calculator
Enter your beam length, joist span, post spacing and soil to size the number of footings, footing diameter, concrete and the beam you need.
Estimates only. Beam and footing sizing follows common span-table values; elevated, hillside and canyon decks in San Diego often need a stamped engineer's plan. We size and permit yours correctly.
Get a Free Structural QuoteHow to Size Deck Beams and Footings
Enter your total beam length, the joist span (how far the joists run out from the house), and the spacing between your beam posts. The calculator works out how many posts and footings you need, the tributary load each footing carries, the footing diameter for your soil, the concrete per footing, and a recommended built-up beam size from standard span tables.
How the Numbers Are Calculated
Posts equal the beam length divided by post spacing, plus one. Each footing carries a tributary area — the post spacing times half the joist span, because the beam supports half the deck and the ledger carries the rest. That area times 50 psf (40 lb live + 10 lb dead load) gives the load in pounds. Divide by your soil bearing capacity and you get the footing area, which sets the diameter. Poor clay soil needs a wider footing than firm gravel for the same load.
The beam size comes from the joist span and the post spacing: the farther apart the posts and the deeper the deck, the larger the beam. Southern Pine spans a little farther than Doug Fir for the same size.
San Diego Footings, Soil and Permits
San Diego County has expansive clay in many inland areas and sandy or rocky soil near the coast and canyons, so footing sizes vary a lot across the region. Hillside and elevated decks in La Jolla, Poway and the East County backcountry often need deeper piers and a stamped structural plan. We size the framing, pull the permit and build to code on every deck installation.
Frequently Asked Questions
How many footings does a deck need?
Divide the beam length by your post spacing and add one. A 20-foot beam with posts every 6 feet needs four footings. Wider post spacing means fewer, but larger, footings and a bigger beam.
What size footing do I need for a deck post?
It depends on the load and soil. Most residential deck footings run 12 to 24 inches in diameter. This calculator sizes each footing from the tributary load and your soil bearing capacity.
How far apart should deck beam posts be?
Commonly 6 to 8 feet, but it depends on the beam size and joist span. Closer posts allow a smaller beam; wider spacing needs a deeper, built-up beam.
Do I need an engineer for deck footings in San Diego?
Ground-level decks usually follow prescriptive span tables. Elevated, hillside and canyon decks often need a licensed engineer's stamped plan, which we handle as part of the permit.