How to use the deck footing calculator
- Beam span is the distance between post centers. Get it from the deck beam span calculator.
- Joist span is how far the joists run from the ledger to the beam. This is the area each post carries.
- Species of the post sets how tall a 6×6 can be. Footing size is the same for every species.
- Corner posts carry less load, and DCA 6 lets their footing size be multiplied by 0.9. This calculator rounds the result up to the next inch.
Reading the result
Pick either the round footing (for a hole or tube form with a spread footing at the bottom) or the square pad. Both carry the same load. The thickness is the minimum depth of concrete in the footing itself, not the depth of the hole. Coordinate it with the post base or anchor you're using.
Example: posts 8' apart carrying 10' joists need a 20" round or 18" × 18" square footing, 8" thick. If the posts are 10' apart and the joists span 18', it goes up to 31" round or 27" square, 13" thick. In SPF the 6×6 post is then limited to just 2' tall.
Depth and soil
- Depth: the bottom of the footing must be at least 12" below undisturbed ground and below the frost line. Your building department sets the frost depth.
- Soil: the table assumes soil that can carry 1,500 psf, the lowest value the IRC allows without testing. Soft clay, fill or wet ground can need bigger footings or an engineer.
- Near the house: footings within 5' of a basement or foundation wall must bear at the same depth as the house footing.
- Call 811 before you dig, and never build footings over septic fields or utility lines.
To figure bags for the whole job, use the concrete calculator with the footing size and your hole depth.
FAQ
How big should deck footings be?
It depends on how much deck area each post carries. Under DCA 6, posts 8' apart with 10' joists need a 20" round or 18" square footing, 8" thick. Longer beams and joists need bigger footings, up to 42" round for 18' beam and joist spans.
How deep should deck footings be?
At least 12" below undisturbed soil and below your local frost line, whichever is deeper. Ask your building department for the frost depth in your area.
How tall can a 6x6 deck post be?
Up to 14' for light loads, per DCA 6 Table 4. With longer beam and joist spans the limit drops. In SPF with an 18' joist span and 14' beam span, a 6×6 is not permitted at all. 8×8 posts can go up to 14' anywhere in the table.
Can I use a 12 inch tube for deck footings?
Only if it sits on a spread footing sized from the table, or your building department approves it. A 12" tube on its own has much less bearing area than the 18" to 42" footings DCA 6 calls for.