How do you calculate deck post spacing?

Measure the total run, choose the maximum allowed spacing, divide the run by that spacing, and round up to the next whole bay. Add one post for posts at both ends. The exact spacing is the total run divided by the number of bays.

How far apart should deck posts be?

Many deck layouts use 6 foot or 8 foot maximum post spacing, but the correct spacing depends on beam size, joist span, loads, footing design, local code, and the hardware being used. Confirm the layout with local building rules or a qualified professional before building.

Does this calculator work for railing posts?

Yes. Enter the railing run length and the maximum allowed post spacing for your railing system. The calculator will show the post count, number of openings, and exact equal spacing between post centers.

Deck calculator

Deck Post Spacing Calculator

Plan evenly spaced deck posts for a beam, railing, or stair rail run. Enter the run length and max spacing to get post count, exact center-to-center spacing, footing positions, and concrete estimates.

Posts needed 4 posts Exact spacing: 6 ft 0 in across an 18 ft run.

Post run

Total run length (feet) Measure from the first post center to the last post center. Maximum spacing (feet) Common targets are 6 ft or 8 ft, depending on the design. End posts

Footing estimate

Footing diameter (inches) Footing depth (inches) Concrete bag size (lb)

This concrete estimate assumes round cylindrical footings. Real projects may need bell-shaped bases, piers, tubes, frost-depth adjustments, or inspection requirements.

Budget check

Price per post ($) Price per concrete bag ($) Extra materials allowance (%)

How to use this deck post spacing calculator

Enter the total run where posts will sit, then enter the maximum spacing you want to allow between post centers. The calculator rounds up to enough bays so no gap is larger than your maximum, then shows the exact equal spacing for a clean layout.

Use the footing fields when you also want a rough concrete order. The result is useful for early planning, shopping lists, and comparing 6-foot versus 8-foot spacing before you finalize the deck design.

Common deck post spacing examples

18 ft run with 6 ft max spacing: 4 posts create 3 equal bays at 6 ft apart.
20 ft run with 8 ft max spacing: 4 posts create 3 equal bays at about 6 ft 8 in apart.
32 ft beam with 8 ft max spacing: 5 posts create 4 equal bays at 8 ft apart.

FAQ

Is 8 feet too far between deck posts?

Eight feet is a common planning number for some deck beam layouts, but it is not automatically right for every deck. Beam size, joist span, height, loads, soil, footing size, connectors, and local code all matter.

Should I measure post spacing edge-to-edge or center-to-center?

Most layout math is easiest center-to-center. Mark the first and last post center, then divide the distance between those marks by the number of bays.

Can I use this for railing post spacing?

Yes. Enter the railing run and the maximum spacing required by your railing system or local rules. The result shows equal openings, but you should still confirm baluster spacing and guardrail requirements separately.