Deck details
Type feet and inches like 12'6" or just 12.5. Boards run along the length and joists run across the width. This is a material estimate, not a building plan: sizes must follow your local building code. Everything is saved on this device.
About the deck calculator
This free deck calculator tells you how many deck boards, joists, rim joists, beams, posts, screws or clips and bags of concrete you need for a simple rectangular deck. Enter the length and width, choose your boards, and you get a shopping list you can copy.
Use it to plan a budget and to talk to your supplier. It is not a building plan. Joist, beam and post sizes must follow your local building code, and a professional or your building office should check them.
Features
- Sections — add more rectangles for an L-shaped deck, each attached or freestanding
- Deck boards — set the board width, gap, board length and waste
- Joists — 12, 16 or 24 in spacing (30, 40 or 60 cm), or your own
- Rim joists and ledger — counted for each section, with the length to buy
- Beams and posts — a rule-of-thumb count you can change, marked as an estimate
- Screws or clips — deck screws or hidden fasteners, rounded up to the next 50
- Concrete — bags for the footings, from the hole size and depth
- Cost and list — add your prices, then copy the shopping list
How to use it
- Choose Feet & inches or Metres & cm
- Enter the length (the way the boards run) and the width (the way the joists run)
- Choose Attached to the house or Freestanding. Use Add another section for an L-shaped deck
- Pick the board width, gap, board length, waste and joist spacing
- Choose deck screws or hidden fasteners, and enter the deck height
- Open More options to change the beams, posts, footings and prices
- Read the list, then press Copy the list
How the numbers are worked out
Each section is counted on its own:
| Item | How it is counted |
|---|---|
| Rows of boards | Width ÷ (board + gap), rounded up |
| Deck boards | Rows × boards per row, plus waste |
| Joists | Length ÷ spacing, rounded up, + 1 |
| Rim joists | 1 if attached (+ 1 ledger), 2 if freestanding |
| Screws | Rows × joists × 2, up to the next 50 |
| Hidden clips | Rows × joists, up to the next 50 |
If a row fits in one board, each board gives as many rows as fit. If a row is longer than a board, the boards are joined and the off-cuts are used. Joist and rim lengths are rounded up to the next 2 ft (0.3 m). The joist count includes the two end joists.
Beams, posts and footings are estimates
These three numbers are rules of thumb, not engineering:
- Beams — one for each beam spacing across the width, 10 ft (3 m) by default. A freestanding deck gets one more at the house side.
- Posts — one for each post spacing along a beam, 8 ft (2.4 m) by default, plus one. One footing for each post.
- Footings — a hole 12 in (30 cm) wide and 36 in (90 cm) deep, plus 10% extra concrete.
The safe distances depend on your joist and beam size, wood type and snow load. Footings often must go below the frost line, and that depth changes from place to place. Ask your building office, then enter their numbers under More options.
Worked examples
These numbers come from the calculator itself and were also checked by hand:
| Deck | Boards | Joists | Beams, posts, bags |
|---|---|---|---|
| 16 × 12 ft, attached | 29 of 16 ft | 13 of 12 ft | 2, 6, 26 |
| 6 × 4 m, freestanding | 54 of 3.6 m | 16 of 4.2 m | 3, 12, 70 |
The first deck uses 5½ in boards, a ⅛ in gap, 10% waste and joists at 16 in. That is 26 rows and about 700 screws. The second uses 140 mm boards, a 5 mm gap, 15% waste and joists at 40 cm. That is 28 rows and about 900 screws.
Frequently asked questions
How many deck boards do I need?
Divide the deck width by the board width plus the gap, and round up to get the rows. A 12 ft deck with 5½ in boards and a ⅛ in gap has 26 rows. Multiply by the boards for each row and add waste.
What gap should I leave between deck boards?
Many builders use about ⅛ in (3 mm) for wood, up to 5 mm in some countries. For composite boards, follow the maker's instructions.
What joist spacing should I use?
16 in (40 cm) is the most common. Diagonal boards and many composite boards need closer joists, such as 12 in (30 cm). Check the board maker and your local code before you use 24 in (60 cm).
How many screws do I need for a deck?
The calculator uses 2 screws where each board crosses a joist. A 16 × 12 ft deck with joists at 16 in has 338 crossings, so about 700 screws.
Does this calculator tell me what size joists and beams to use?
No. Sizes and spans come from your local building code. Ask your building office or a professional.
Does it include stairs and railings?
No. Stairs, railings, joist hangers, bolts, flashing and post brackets are not counted. Add them to your list.
Good to know
- Call before you dig. Underground pipes and cables are a real risk.
- Many places need a permit for a deck. Ask before you buy.
- Beams and rim joists over about 20 ft (6 m) are joined from shorter boards.
- The numbers are estimates. Buy a few extra boards and one extra bag.
- Everything stays on your device.
Related tools
- Concrete Calculator — bags of concrete for slabs and holes
- Fence Calculator — posts, rails and pickets for a fence
- Gravel Calculator — gravel under the deck
- Square Footage Calculator — measure your deck area
- Unit Converter — feet, inches and metres
