Deck Calculator
Enter your deck size, board width, gap and joist spacing to find how many deck boards, joists, beams, posts and screws or hidden fasteners you need. Add prices for a full cost estimate and see a framing plan.
Material prices (optional, for a cost estimate)
Framing plan (top view)
Materials & cost breakdown
Estimates only. Joist, beam and post sizes and spans depend on lumber species, grade, load and your local building code, so check span tables or ask your building department before you build.
How to use the Deck Calculator
- Choose units and whether the deck is attached to the house with a ledger board or freestanding.
- Enter the deck size: the length along the house (boards run this way) and the width out from the house.
- Describe the boards: actual board width, gap between boards and the board length you will buy.
- Set the framing: joist spacing, max joist span and max post spacing from your span tables, plus fasteners and waste.
- Add prices (optional) and press Calculate to get the materials list, total cost and framing plan.
Deck material formulas
| Quantity | Formula |
|---|---|
| Board rows | rows = ⌈(deck width + gap) ÷ (board width + gap)⌉ |
| Boards (board ≥ deck length) | boards = ⌈rows ÷ ⌊board length ÷ deck length⌋⌉ |
| Boards (board < deck length) | boards = ⌈rows × deck length ÷ board length⌉ |
| Boards with waste | ⌈boards × (1 + waste %)⌉ |
| Joists | ⌈deck length ÷ joist spacing⌉ + 1 |
| Beam lines | ⌈deck width ÷ max joist span⌉, plus 1 if freestanding |
| Posts | beam lines × (⌈deck length ÷ max post spacing⌉ + 1) |
| Face screws | rows × joists × 2 |
| Hidden clips | (rows + 1) × joists |
Worked example: 16 × 12 ft attached deck
- Rows: width 12 ft = 144 in. (144 + 0.125) ÷ (5.5 + 0.125) = 25.6, rounded up to 26 rows.
- Boards: 16 ft boards cover one 16 ft row each, so 26 boards. With 10% waste: 26 × 1.1 = 28.6, rounded up to 29 boards.
- Joists at 16 in on centre: 192 ÷ 16 = 12 spaces, so 13 joists, each 12 ft long.
- Beam and posts: a 12 ft span needs one beam at the outer edge. With posts no more than 6 ft apart: ⌈16 ÷ 6⌉ + 1 = 4 posts.
- Screws: 26 rows × 13 joists × 2 = 676 screws, or two boxes of 350.
Typical deck board and joist sizes
| Board | Actual width | Common joist spacing |
|---|---|---|
| 5/4 × 6 wood (radius edge) | 5.5 in (140 mm) | 16 in (400–450 mm) |
| 2 × 6 wood | 5.5 in (140 mm) | 16 to 24 in |
| 2 × 4 wood | 3.5 in (89 mm) | 16 in |
| Composite board | 5.5 in (140 mm) | 12 or 16 in (check the manufacturer) |
| Hardwood (ipe) 1 × 6 | 5.5 in (140 mm) | 16 in |
Allowable joist and beam spans depend on lumber species, grade, size, spacing and design load. Always check a span table or your local building code before building.
Frequently asked questions
How many deck boards do I need?
Divide the deck width plus one gap by the board width plus gap to get the number of rows, then multiply by the boards needed per row and add waste. A 16 × 12 ft deck with 5.5 in boards and 1/8 in gaps needs 26 rows. With 16 ft boards that is 26 boards, or 29 with 10% waste.
How many joists do I need for a deck?
Divide the deck length by the joist spacing, round up and add one for the end joist. A 16 ft (192 in) deck with joists at 16 in on centre needs 192 ÷ 16 = 12 spaces, so 13 joists.
How many screws do I need for decking?
Use two screws where each board crosses each joist. Multiply the number of board rows by the number of joists, then by 2. A 16 × 12 ft deck with 26 rows and 13 joists needs 26 × 13 × 2 = 676 screws.
What joist spacing should I use?
16 inches on centre is the most common spacing for wood decking laid at 90° to the joists. Many composite boards require 12 or 16 inches, and diagonal decking needs closer spacing. Always follow the board manufacturer's instructions.
How much waste should I allow?
10% is typical for a simple rectangular deck with boards laid straight. Use 15% for diagonal patterns, picture-frame borders or many cut-outs, and 5% if your board length exactly matches the deck length.
What gap should I leave between deck boards?
Kiln-dried wood and composite boards usually need about 1/8 to 3/16 inch (3–5 mm). Wet pressure-treated lumber is often laid tight because it shrinks as it dries. Hidden fastener clips set the gap for you.
Does this calculator size my joists and beams?
No. It counts members based on the spans you enter. The lumber size needed for a given span depends on species, grade, spacing and load, so use a span table or your local building code to choose joist, beam and post sizes.