Deck Calculator
Enter the size of your deck and the boards you plan to use to see how many boards to buy, how many joists fit underneath and how many fasteners you need.
Enter the deck size and board details.
On this page
How to use the deck calculator
- Enter the length along the boards — the direction the decking runs — and the width across them. Boards run perpendicular to the joists.
- Choose the decking and board length. Buying boards as long as the deck (or exactly half of it) avoids waste and messy joints.
- Set the joist spacing your decking allows, and read the number of joists and fasteners.
How to calculate deck boards
Worked example: a 16 × 12 ft deck in 5/4×6 decking, 16 ft boards
- Rows: 144 in ÷ (5.5 + 0.1875) = 25.3 → 26 rows
- Boards: 26 rows × 1 board per row = 26, plus 10% = 29 boards (416 linear feet)
- Joists at 16 in: 192 ÷ 16 + 1 = 13 joists
- Screws: 26 × 13 × 2 = 676 screws. Buy by count rather than by weight — the box lists how many screws it holds.
Maximum joist spacing by decking
The decking, not the joists, often sets the spacing. The International Residential Code (Table R507.7) limits joist spacing for wood decking laid square to the joists; diagonal layouts need closer spacing. Composite and PVC decking follow the manufacturer's rating.
| Decking | Boards square to joists | Boards diagonal |
|---|---|---|
| 5/4 (1¼ in) wood | 16 in | 12 in |
| 2 in (nominal) wood | 24 in | 16 in |
| Composite / PVC | Per manufacturer — most are rated for 16 in square, 12 in diagonal | |
Boards and fasteners per 100 square feet
| Decking | Linear feet per 100 sq ft | Face screws per 100 sq ft (16 in o.c.) |
|---|---|---|
| 5.5 in boards, 3/16 in gap | about 211 ft | about 320–350 |
| 3.5 in boards, 3/16 in gap | about 325 ft | about 490–530 |
Before adding extra for cuts. Screw counts rise at 12 in joist spacing.
Tips for fewer joints and less waste
- Match board length to the deck. A 16 ft deck is perfect for 16 ft boards; a 14 ft deck wastes 2 ft per row from 16 ft boards but may still beat joints.
- Land every butt joint on a joist — ideally a doubled joist or blocking — and stagger joints from row to row.
- Check the gap on the label. Wet pressure-treated boards are often laid tight because they shrink; composites need specific gaps for expansion.
- Picture-frame borders and diagonal patterns look great but add 5–10% more waste and need blocking.
Frequently asked questions
How many deck boards do I need for a 12 × 12 deck?
With 5.5 in boards and a 3/16 in gap: 26 rows. With 12 ft boards that's 26 boards, or 29 with 10% extra.
How many screws per deck board?
Two per joist crossing. A 16 ft board over joists 16 inches apart crosses 13 joists, so it takes 26 screws.
Does this calculate joist and beam sizes?
No. Joist, beam and post sizes depend on span, wood species, grade and load. Use the American Wood Council's DCA 6 prescriptive deck guide or your local code, and get a permit where required.
How much extra decking should I buy?
About 10% for straight layouts; 15% or more for diagonal decking, picture frames or lots of cut-outs around posts and stairs.
Sources
- International Residential Code, Table R507.7 — maximum joist spacing for decking.
- American Wood Council, DCA 6: Prescriptive Residential Wood Deck Construction Guide — framing design.