How to use the deck calculator
- Enter the deck length (the direction the boards run) and width (across the boards).
- Enter the board’s actual width and the gap between boards.
- Enter the board length you will buy and the joist spacing.
- Keep 10% extra for cuts and defects, and add a price per board for the cost.
Deck formulas
Rows of boards = Deck width ÷ (Board width + Gap) → round up Boards per row = Deck length ÷ Board length → round up Boards = Rows × Boards per row × (1 + extra %) Joists = Deck length ÷ Joist spacing + 1 Screws = Rows × Joists × 2
Worked example
A 16 × 12 ft deck with 5.5 in boards, 3/16 in gaps, 16 ft boards and joists at 16 in on centre:
- Rows = 144 ÷ 5.6875 = 25.3 → 26 rows
- Boards = 26 × 1 = 26; +10% = 29 boards
- Joists = 192 ÷ 16 + 1 = 13 joists, each 12 ft
- Screws = 26 × 13 × 2 = 676
Deck boards needed per 100 ft²
| Board (actual width) | Gap | Linear feet per 100 ft² |
|---|---|---|
| 3.5 in (2 × 4) | 3/16 in | 325 |
| 5.5 in (5/4 × 6, 2 × 6) | 3/16 in | 211 |
| 5.5 in | 1/4 in | 209 |
| 140 mm (metric) | 5 mm | 6.9 m per m² |
Wood or composite?
Pressure-treated pine is the cheapest and easiest to cut, but needs sealing and may warp or check. Cedar and redwood look beautiful and resist rot naturally but cost more. Composite and PVC decking cost more up front but need little maintenance, don’t splinter, and often carry 25-year or longer warranties. They usually need closer joist spacing and hidden fasteners or colour-matched screws, so check the maker’s guide before you finalise the frame.
Deck building tips
- Check permits — most decks attached to a house or more than about 30 in above ground need one.
- Plan board joints over joists and stagger them across rows; add a double joist or blocking at joints.
- Use the right fasteners: coated or stainless screws rated for treated lumber, and joist hangers at the ledger.
- Leave the right gap for drainage and expansion, and keep the frame at least a little off the ground for airflow.
- Picture-frame borders and diagonal layouts look great but use 10–15% more boards.
Estimating the whole deck budget
Decking boards are usually only about a third to a half of the total material cost. Remember the framing (joists, beams, rim joists and ledger), posts and footings, concrete, joist hangers and structural screws, flashing tape over the joists, and railings and stairs if the deck is raised. A quick way to plan is to price the boards with this calculator, then add a similar amount again for the frame and hardware, and more for railings. Ordering 10% extra boards covers cutting waste and lets you pick the straightest lengths for the most visible rows.