Deck lumber calculator: what is counted
This deck lumber calculator lists every part of a rectangular, ledger-attached deck. It assumes the decking runs parallel to the house and the joists run out from it:
- Decking: rows across the depth, boards per row from your stock length, and screws at two per board per joist, the IRC fastening for decking[1].
- Framing: joists, ledger, rim joist, a doubled beam and posts at your beam span. Size the joists with the joist span calculator and lay out the frame in the deck framing calculator.
- Railing: posts at your spacing and enough balusters to keep each gap under the limit.
Deck rules that change the count
| Item | Rule | Source |
|---|---|---|
| Live load | 40 psf, or the ground snow load if greater | R301.5, R507.1[1] |
| 5/4 (1-1/4 in) decking | joists at most 16 in apart, 12 in when laid diagonally (3+ joists) | Table R507.7 |
| 2x decking | joists at most 24 in apart | Table R507.7 |
| Decking fasteners | 2 threaded nails or 2 No. 8 screws at each joist | R507.7 |
| Ledger | at least 2x8; never on stone or masonry veneer | R507.9.1.1 |
| Posts | 6x6 minimum in the prescriptive guide | DCA 6[2] |
Deck railing calculator: posts and balusters
A guard is required where the deck is more than 30 in above the ground within 36 in of the edge; it must be at least 36 in high, and no opening may pass a 4 in sphere[3]. The calculator spaces balusters evenly in each section between rail posts and keeps the gap under the maximum you set (3-7/8 in by default, leaving a margin under 4 in). On stair sides the limit is a 4-3/8 in sphere, and guards there may be 34 in high[3].
Deck stain calculator: how much stain
Stain is bought by coverage: divide the area to coat by the coverage per gallon (or litre) printed on the can, and do it once per coat. The area is the deck surface (width × depth) plus the railings and stairs. Rough-sawn or weathered wood drinks more than smooth new boards, so use the low end of the can's range. Keep the deck area from the results above as your starting number.
Free deck calculator tips
- Order deck boards in lengths that span the full width or break over joists; DCA 6 asks each board to bear on at least three joists[2].
- For decking alone, wood or composite, the deck board calculator adds clips, border and fascia.
- Steps down to the yard: the deck stair calculator.
Common deck mistakes
These are the deck errors that fail inspections or waste material:
- Joists too far apart for the decking. 5/4 boards that cross three or more joists need joists at 16 in, 12 in when diagonal; boards resting on only two joists need 12 in and 8 in (IRC Table R507.7, 2021 and 2024)[1].
- Ledger on veneer. A ledger may not be fastened to stone or masonry veneer[1][2].
- Too much cantilever. Joists may overhang the beam by no more than a quarter of the back span[2].
- No guard over 30 in. Decks more than 30 in above grade need a 36 in guard[3].
- Baluster gaps of 4 in or more. The opening must stop a 4 in sphere[3].
Frequently asked questions
How many deck boards do I need for a 12 × 16 deck?
With 5-1/2 in boards and 3/16 in gaps running the 16 ft way, 26 rows of 16 ft boards, plus about 10% for waste.
How many joists for a 16 ft deck?
At 16 in centres, 192 ÷ 16 = 12, plus 1: 13 joists. At 12 in centres, 17.
How far apart should deck joists be?
16 in for 5/4 deck boards laid square to the joists and 12 in when they run diagonally, for boards that cross three or more joists; up to 24 in for 2x decking (IRC Table R507.7). Composites follow the maker.
How much stain do I need for a deck?
Deck area (plus railings and stairs) ÷ the coverage per gallon on the can, times the number of coats.