Calculate deck materials, posts, and hardware
A deck calculator estimates lumber, hardware, and materials for building a deck. Accurate planning ensures you have everything needed before starting construction.
The deck estimate works from three rules. Decking rows are the deck width divided by the board width, rounded up, and each row runs the full deck length with 10 percent added for waste. Joists run across the deck width and number the deck length divided by the spacing, plus one for the end. Beams are two runs of the deck length, with a post every 6 feet along each beam plus one more. Gaps between boards are not modelled, so check your product's spacing if the deck width is close to a whole number of boards.
Decking Boards = (Deck Area / Board Width) × 1.1 waste factor
| Calculation | Expression | Result |
|---|---|---|
| Decking rows for a 16 × 12 ft deck | 12 ft ÷ (5.5 in ÷ 12), rounded up | 27 rows |
| Decking length with 10% waste | 27 rows × 16 ft × 1.1, rounded up | 476 linear ft |
| Joists at 16 in spacing | (16 ft × 12 ÷ 16) + 1 | 13 joists of 12 ft |
| Posts for two beams | (16 ft ÷ 6, rounded up, + 1) × 2 | 8 posts |
16" on center: Standard for residential decks, supports most decking. 12" on center: Required for diagonal decking and some composites. 24" on center: Only for 2× decking on low-traffic areas.
Calculate deck area, divide by board coverage, add 10-15% waste. For diagonal patterns, add 20%. Order extra boards to match weathering for future repairs.
Roughly 350 screws per 100 sq ft of decking (2 per joist intersection). Hidden fastener systems: approximately 90 clips per 100 sq ft. Buy extra for mistakes.
Posts: 4×4 for low decks, 6×6 for higher decks. Beams: Typically doubled 2×8, 2×10, or 2×12 depending on span. Check local code requirements and span tables.
Typical footing: 18" diameter × 42" deep (below frost line). Uses about 3 cubic feet of concrete. Pre-mix bags: Two 80 lb bags per footing. Adjust for local frost depth requirements.
Pressure-treated wood: Affordable, requires annual maintenance, 15-20 year lifespan. Composite: Higher cost, minimal maintenance, 25-30+ year lifespan, won't splinter.
Thirteen, each 12 feet long: the 16-foot length is 192 inches, 192 ÷ 16 = 12 spaces, and one more joist closes the end. At 12-inch spacing the same deck needs 17 and at 24-inch spacing it needs 9.