Deck board calculator: how many boards, joists and screws do I need?
Board rows across the deck, the joists under them and the fasteners at every crossing, plus waste. All sizes are typical values you can change.
Deck board calculator
Enter the deck and your boards. Board size, gap, joist spacing and waste are typical values: enter the ones from your product and its installation instructions.
Typical size: check your product
Typically 16 in center to center for wood boards laid square to the joists, 12 in for diagonal boards; the board's label applies
You need
Enter the length and width of the deck.
Deck boards by deck size
Boards of 16 ft × 5 1/2 in along the length, 1/8 in gap, joists 16 in apart, center to center:
| Deck | Boards (length bought) | Joists | Screws or clips |
|---|---|---|---|
| 10 × 10 ft (100 sq ft) | 22 (352 ft) | 99 ft | 436 / 218 |
| 12 × 10 ft (120 sq ft) | 22 (352 ft) | 110 ft | 484 / 242 |
| 12 × 12 ft (144 sq ft) | 26 (416 ft) | 132 ft | 572 / 286 |
| 16 × 12 ft (192 sq ft) | 29 (464 ft) | 171.6 ft | 744 / 372 |
| 20 × 12 ft (240 sq ft) | 36 (576 ft) | 211.2 ft | 916 / 458 |
| 20 × 16 ft (320 sq ft) | 49 (784 ft) | 281.6 ft | 1232 / 616 |
| 24 × 12 ft (288 sq ft) | 43 (688 ft) | 250.8 ft | 1087 / 544 |
Includes 10 % waste on boards and joists and 10 % spare fasteners. Boards: whole boards, in brackets the length bought.
For 10 × 10 ft the rows decide: each 10 ft row takes a board of its own, because the 6 ft offcut cannot fill another row.
Worked example: a 16 × 12 ft deck
- Area: 16 × 12 ft = 192 sq ft, boards 16 ft × 5 1/2 in, 1/8 in gap, joists 16 in apart, boards along the length.
- Rows across the 12 ft width: (12 ft + 1/8 in) ÷ (5 1/2 in + 1/8 in) = 25.62, so 26 rows of 16 ft, 416 ft of boards.
- Plus 10 % waste: 457.6 ft ÷ 16 ft per board = 28.6, you order 29 boards; 48 ft is left over.
- Check row by row: whole boards per row, offcuts used only where they are long enough for the whole piece, gives 26 boards. You order the larger figure, here 29.
- Joists across the boards: 16 ft ÷ 16 in + 1 = 13 joists of 12 ft, 156 ft, with waste 171.6 ft.
- 26 rows × 13 joists = 338 crossings: with 10 % spare 744 screws or 372 hidden clips.
Slabs instead of boards? The patio slab and pedestal calculator gives the slabs and pedestals.
What to measure before you calculate
Measure the deck where the boards will go, not the yard. The length along the house and the depth out into the yard make the rectangle for the calculator. If one side is angled or the deck wraps around a corner, split it into rectangles and calculate each one.
- Measure length and depth in at least two places: if the figures differ, the area is not square and the longer side counts.
- Check the height: board, joist and support together have to stay below the door threshold, with some room so that water does not run toward the door.
- Find out what is underneath: concrete, pavers, gravel or soil decides what the joists rest on.
- Measure the existing slope with a straightedge and a level across the whole depth.
- Note the cut-outs: steps, posts, planting beds and drains need cuts and often joists of their own.
Joist spacing and fastening rules
In the United States the International Residential Code, section R507 on decks, sets the rules most towns adopt. Wood boards about 1 in thick span at most 16 in when they run square to the joists and 12 in when they run diagonally; boards 2 in thick span 24 in and 16 in. Composite and plastic boards must show their span rating on the label. Every board is fixed to every joist with two screws or two nails, or with the clip system the board maker approves.
Many towns ask for a permit for a deck attached to the house, larger than 200 sq ft or higher than 30 in above the ground; above 30 in a guard is required. Ask your building department before you order.
- Screws: coated or stainless deck screws about 2 1/2 times as long as the board is thick, two per joist, about 3/4 in from the board edge.
- Butt joints sit on a doubled joist or on blocking, so that each board end gets its own screws; stagger the joints of neighboring rows.
- Treated wood rated for ground contact for the frame near the soil, and end-grain preservative on every cut the supplier asks for.
Slope, water and the house wall
A deck slopes slightly away from the house, about 1/8 in per foot, so that water leaves along the boards. The slope comes from the framing, not from boards cut at an angle.
No water may stand under the deck. Keep the ground below sloped away from the house and the drains free. Where the deck is fixed to the house, flashing over the ledger keeps water out of the wall; the house wall and its weather barrier must not disappear behind the deck.
Wood or composite
Pressure-treated pine, cedar, redwood and tropical hardwoods move mostly in width: they swell when wet and shrink when dry, hence the gap. Hardwood is predrilled, or it splits at the screws. Wood turns gray over time unless it is oiled.
Composite boards grow mostly in length when the sun heats them. They usually need closer joists than thick wood boards, a gap at butt joints and ends, and often the maker's clip system. Enter its joist spacing and gap in the calculator: a closer spacing gives more joists and more clips.
Choose the board length to suit the deck
The board length decides the offcuts. On a 12 × 10 ft deck with 16 ft boards, every 12 ft row takes a board of its own and the 4 ft offcut fills no other row: 22 boards, 352 ft bought for 264 ft laid. With 12 ft boards it is 25 boards, 300 ft with 10 % waste.
When a board length matches the deck, or the rows come out in whole boards, there are hardly any offcuts. Run the lengths on offer through the calculator and compare the length you buy, not only the number of boards.
Common mistakes
- Using the joist spacing of thick wood boards for thin or composite boards: the boards bounce and crack at the screws.
- Butt joints on a single joist: both ends share a narrow strip of wood and the screws split out.
- Joists on soil or in a hollow where water stands.
- No slope, or a slope toward the house.
- Plain steel screws in treated or tropical wood: they corrode and leave dark stains.
- Ordering only the length plus waste without checking the rows: with short rows the waste figure is not enough.
- Mixing boards from different deliveries: color and size can differ.
Tools and materials
- Boards, joists, supports (footings, posts and beams, or pedestals and pads) and landscape fabric
- Coated or stainless deck screws or the board maker's clip system, plus spacers for the gap
- Cordless drill driver, for hardwood a countersink drill bit
- Miter saw or circular saw, straightedge, level, string line and tape measure
- End-grain sealer for the cut ends, if the board maker asks for it
Frequently asked questions
- How many deck boards do I need for a 16 × 12 ft deck?
- With boards 5 1/2 in wide and a 1/8 in gap, running along the 16 ft side: 26 rows of 16 ft, 416 linear feet, with 10 % waste 457.6 ft. In 16 ft boards that is 29 boards. With joists 16 in on center you need 13 joists of 12 ft, 156 ft, with waste 171.6 ft, and 744 screws or 372 hidden clips.
- How far apart should deck joists be?
- For wood boards about 1 in thick (sold as 5/4 boards), the International Residential Code allows joists at most 16 in center to center when the boards run square to the joists and 12 in when they run diagonally; boards 2 in thick span up to 24 in. Composite and plastic boards carry their own span rating on the label, often 16 in or 12 in. A closer spacing means more joists and more screws.
- How many screws per deck board?
- Two at every joist the board crosses, or one hidden clip per crossing with a clip system; the International Residential Code asks for two screws or two nails per board at each joist. A 16 ft board on joists 16 in on center crosses 13 joists and takes 26 screws or 13 clips. Your fastener system's instructions apply.
- How big should the gap between deck boards be?
- About 1/8 in for kiln-dried wood and most composite boards, so that water drains and the boards can move; the calculator starts from 1/8 in. Wet pressure-treated boards shrink as they dry and are often laid with a very small gap or none. The board maker's figure applies. The gap counts in the number of rows: the wider it is, the fewer rows.
- Which way should deck boards run?
- Usually parallel to the house, square to the joists that run out from the house, so that water runs off along the boards; the deck slopes slightly away from the house. Diagonal boards look different but need closer joists and more waste. The direction changes the quantities: rows across the shorter side are more but shorter, with fewer butt joints. Try both directions in the calculator and compare.
- What do the joists rest on?
- Never directly on soil. On a raised deck the joists sit on beams carried by posts on concrete footings, and on the house side usually on a ledger bolted to the house frame. On a slab or patio they rest on sleepers or adjustable pedestals with rubber pads. How far beams and footings may be apart depends on the joist size and the wood: the span tables of your local code and the product instructions apply. Every butt joint gets a doubled joist or blocking.
Draw the deck in the planner
Upload your floor plan and the free planner works out the exact packs and waste before you order.
create a layout planCalculation with typical values for boards, gap, joist spacing and fasteners, not products. Rules from the International Residential Code, section R507 (decks); your local building department and the manufacturer's instructions apply.