Deck calculator

Parts of a deck

1. Ledger. The board fastened to the house. The joists hang from it.
2. Joists. The frame the decking sits on, every 12, 16 or 24 in.
3. Beam. Two boards fastened together, carrying the outer ends of the joists.
4. Post. Holds up the beam. Usually a 6x6.
5. Footing. Concrete in the ground under each post, below the frost line.
6. Decking. The boards you walk on.
7. Railing. Required where the deck is over 30 in above the ground. At least 36 in high.
8. Balusters. The upright pieces in the railing, less than 4 in apart.
9. Stringer. The notched board the stair treads sit on.
10. Handrail. Required on stairs with four or more risers, 34 to 38 in high.

How the deck calculator works

It frames the deck the way most builders do: joists run out from the house, deck boards run along it, and a beam on posts carries the outer end of the joists. The rules come from the deck section of the International Residential Code (IRC), which most US towns use.

• Deck boards. The width is split into rows of 5 1/2 in boards with 3/16 in gaps. Every row needs a full run of boards along the length, and 10% is added for cuts and bad boards.
• Joists. One every 12, 16 or 24 in along the length, plus one at the end. The size is the smallest that spans the width under IRC Table R507.6 for southern pine. If even a 2x12 can't, a second beam goes in the middle.
• Rim joist and ledger. A rim joist along the outer edge, and a ledger board fastened to the house. A freestanding deck gets a second rim instead of the ledger.
• Beam and posts. A doubled 2x10 beam (2x12 for joist spans over 10 ft) on 6x6 posts no more than 8 ft apart. Depending on the span, your beam may need to be bigger or the posts closer; IRC Table R507.5 sets the limits.
• Footings. One under each post, sized by the deck area the post holds up, at 50 lb per sq ft on average soil (IRC Table R507.3.1), then rounded up to a hole you can dig, 36 in deep. In the 16 × 12 ft example that's two 14 in footings of 6 bags each and one 18 in footing of 9 bags. The footing calculator shows how.
• Hardware. A hanger for each joist at the ledger, a hurricane tie where each joist sits on a beam, two ledger fasteners every 16 in, and two deck screws per joist crossing, or one hidden fastener for composite boards.
• Stairs. Risers up to 7 1/2 in and treads of two deck boards (11 1/4 in), with stringers every 16 in, or 12 in for composite. See the deck stair calculator.
• Railing. Along the open sides, minus the stair opening, plus a handrail on both sides of stairs with four or more risers. The deck railing calculator counts the posts and balusters.

Materials for common deck sizes

Length along the house × width out from it. Attached, pressure-treated, 36 in high, joists 16 in apart, 16 ft boards, railing and 4 ft stairs. Concrete in 80 lb bags. Sample prices.

What the estimate includes

The total uses sample 2026 prices: material costs with a 20% markup, as a contractor would charge, and labor at $16 per sq ft for framing and decking, $18 per foot of railing and $75 per step. It doesn't include permits, removing an old deck, delivery, sales tax or design. Prices vary a lot by region and supplier, so treat it as a starting point.

Before you build

• Permit. Under the IRC, a deck needs a permit unless it is 200 sq ft or less, no more than 30 in above the ground, freestanding, and not serving the house's required exit door (Section R105.2). Many towns are stricter.
• Utilities. Call 811 a few days before digging footings.
• Frost depth. Footings go below your local frost line; your building department can tell you how deep that is.
• Ledger. The ledger is where attached decks most often fail. Flash it, and fasten it with bolts, lag screws or approved structural screws, never nails.

Questions