Deck Materials Calculator
Length runs along the house; boards run the same direction.
Carpentry
Deck Calculator
Quick answer: board rows = deck width (in) ÷ (board width + gap), rounded up; joists = deck length (in) ÷ spacing + 1. A 16 × 12 ft deck with 5.5 in boards and 1/8 in gaps needs 26 rows (416 linear ft), 13 joists at 16 in on center, and about 676 screws.
Plan a rectangular deck: decking boards in pieces and linear feet, joist count, deck screws or hidden clips, a rough footing count, and an optional material budget per square foot.
Deck framing plan
How to calculate deck boards, joists, and screws
A deck takeoff has three parts that depend on each other. The deck width and the board-plus-gap width set how many rows of decking you lay. The deck length and the joist spacing set how many joists you need. Rows multiplied by joists gives the number of places each board gets fastened.
Rows = W (in) ÷ (board + gap) · Joists = L (in) ÷ spacing + 1
Screws ≈ rows × joists × 2
- Decide the board direction. This calculator assumes boards run parallel to the house (along the deck length) across joists that run out from the ledger.
- Count the rows. Convert deck width to inches, divide by the board width plus one gap, and round up. The last row is usually ripped to fit.
- Count boards per row. If one stock board covers the deck length, that is one board per row. A longer deck needs two or more boards per row, with butt joints that land on a joist (or doubled joist).
- Add waste. 5–10% covers crooked boards and end trimming; use 15% for diagonal or picture-frame layouts.
- Count joists. Divide the deck length in inches by the spacing, round up, and add one for the starting end.
- Count fasteners. Two face screws per board per joist; hidden clip systems use roughly one clip per board per joist, but follow the clip maker's coverage chart.
Worked example
A 20 ft × 14 ft deck uses 5.5 in boards with a 3/16 in gap, 20 ft board stock, joists 16 in on center, one beam with posts 8 ft apart, and 10% waste.
- Rows: 168 in ÷ 5.6875 in = 29.5 → 30 rows; net decking 30 × 20 ft = 600 linear ft
- Boards: 30 rows × 1 board × 1.10 = 33 boards of 20 ft (660 linear ft)
- Joists: 240 in ÷ 16 in = 15, + 1 = 16 joists, each about 14 ft
- Screws: 30 × 16 × 2 = 960 screws; footings: 20 ÷ 8 = 2.5 → 3, + 1 = 4 footings
Using 12 ft stock on the same deck would take 2 boards per row and 66 boards (792 linear ft) — buying board lengths that match the deck saves material and joints.
Decking and screws per square foot
Linear feet of decking per square foot of deck and approximate screws per 100 ft² (2 screws per board per joist, end joists ignored). Add waste on top.
| Board width + gap | Rows per ft of width | Linear ft per ft² | Screws / 100 ft² @ 16 in | Screws / 100 ft² @ 12 in |
|---|---|---|---|---|
| 5.5 in + 1/8 in | 2.13 | 2.13 | 320 | 427 |
| 5.5 in + 3/16 in | 2.11 | 2.11 | 316 | 422 |
| 3.5 in + 1/8 in | 3.31 | 3.31 | 497 | 662 |
| 3.5 in + 3/16 in | 3.25 | 3.25 | 488 | 651 |
Deck planning tips and common mistakes
- Spans come from tables, not from this tool. Allowed joist spans depend on species, grade, size, and spacing — see IRC Table R507.6. Beam spans and post spacing are in IRC Table R507.5. Many jurisdictions publish a deck guide based on these; follow yours.
- Check the decking's own spacing limit. The IRC limits joist spacing by decking type (Section R507.7), and composite and PVC boards often require 12 in on center for diagonal runs or stairs. Read the product's installation guide before buying joists.
- Gap for the material you bought. Kiln-dried wood and most composites are gapped about 1/8–3/16 in; wet pressure-treated boards are often laid tight because they shrink as they dry. Composite end gaps vary with temperature — use the manufacturer's chart.
- Stagger butt joints. When a row needs two boards, joints must land on a joist; avoid lining up joints on the same joist in adjacent rows. Many builders add a doubled joist or a breaker board instead.
- The ledger is the critical connection. Ledger fastening, flashing, and lateral-load connectors are governed by IRC R507.9 and local amendments. Never attach a ledger through brick veneer or siding alone.
- Footing size and depth are local. Footings must reach below the frost line and be sized for soil bearing. Use the post hole concrete calculator or the footing calculator once you know the size.
Deck calculator FAQ
How many deck boards do I need?
Divide the deck width in inches by the board width plus the gap to get the number of rows, then multiply by the boards needed per row. A 12 ft wide deck with 5.5 in boards and 1/8 in gaps needs 144 ÷ 5.625 = 25.6, so 26 rows. With 16 ft boards on a 16 ft deck that is 26 boards, or 29 with 10% waste.
How many linear feet of decking per square foot?
With 5.5 in wide boards (2×6 or 5/4×6) and a 1/8 in gap, you need about 2.13 linear feet of decking per square foot of deck. With 3.5 in boards (2×4) it rises to about 3.31 linear feet per square foot. Add 5 to 15 percent for cutoffs and rejects.
How many screws do I need for a deck?
Plan on 2 screws per board at every joist it crosses. A 16 × 12 ft deck with 26 rows of boards over 13 joists needs 26 × 13 × 2 = 676 screws. As a rule of thumb, 5.5 in boards on joists 16 in on center take roughly 320 screws per 100 square feet, before extras.
How far apart should deck joists be?
Most residential decks use joists 16 inches on center; 12 inches is common for composite decking, diagonal boards, or a stiffer feel. Allowed spacing depends on the decking product and its direction, and allowed joist spans depend on lumber species, grade, and size, so check IRC Table R507.6, the decking manufacturer, and your local building department.
How many footings does a deck need?
Count the beams and the posts along each beam: posts per beam equals the beam length divided by the post spacing, rounded up, plus one. A 16 ft ledger-attached deck with one beam and posts 8 ft apart needs 3 footings. Allowed post spacing depends on beam size and is set by IRC Table R507.5 and local code.
How we built this calculator: results use the standard volume, area, and coverage formulas shown on this page, with typical U.S. material densities, bag yields, and waste factors that you can override. Written and checked by the YardsCalculator editorial team. Last reviewed: October 2026.