Use this free deck board calculator for a straight rectangular deck surface. Enter the run direction, actual board width, required side gap and stock length to estimate rows and boards. It does not count screws or design the frame.
How it works
For N rows there are N – 1 side gaps. Rows = round up ((deck width in inches + gap) / (actual board width + gap)). Each row runs along the deck length. Boards per row = round up (run length / stock board length). Multiply rows by boards per row, add the chosen extra percentage, then round up. End cuts and joint gaps need a separate layout.
Worked example
A 16 x 12 foot deck with 5.5 inch boards and a 1/8 inch gap: (144 + 0.125) / (5.5 + 0.125) = 25.6222, so 26 rows. The length-only estimate assigns one 16-foot stock board per row; confirm trimming and end gaps before assuming a nominal 16-foot board will span the finished run. 26 + 10% = 28.6, so buy 29 boards.
What this does not cover
- Joists, hardware, railings and stairs.
- Picture-frame borders or diagonal layouts, which use more boards.
- Staggered end joints, which depend on your joist layout.
All calculators | Gravel | Concrete | Fence post concrete
What you’ll need
A plain checklist. Needs vary by project, so follow the instructions on your materials.
- Joist hangers and structural screws or nails rated for the use – check the product label.
- Level and square – to keep the frame true.
- Drill and driver – for hundreds of fasteners.
- Circular or miter saw – for cutting boards to length.
- Tape measure and chalk line – for layout.
Popular tools: Concrete | Gravel | Mulch | Paint | Tile | Flooring | Topsoil | Sod | Roofing | Drywall | All calculators
Share this calculator
Linking to this page is welcome. Copy the snippet below to add a link to your site, forum post or newsletter.
Estimates only. Confirm quantities and product details with your supplier before you order.
Guide: How many deck boards do I need?
Decking calculator: board direction changes the order
The length input is the direction the boards run. Width is the distance across the rows. Swapping them keeps the square footage the same but can change the stock-board count. This is why area divided by one board’s area is only a rough first estimate. The calculator budgets stock pieces for each row and does not assume an offcut from one row can be reused in another.
Use actual board width and the right gap
A nominal board name is not its installed face width. Trex’s 2024 guide lists several decking profiles with 5.5-inch actual width. Its side-to-side gap requirement is at least 3/16 inch, with different guidance for docks and heavily wooded areas. End-to-end and abutting gaps have their own rules. Those are Trex instructions, not a universal setting for every wood or composite deck. Check the current guide for the product you buy.1
Our default 1/8-inch gap is an arithmetic example, not a Trex installation recommendation. To calculate with a 3/16-inch side gap, enter 0.1875. For 1/4 inch enter 0.25. Width presets are examples; use a product whose actual width matches the preset. If none matches, do not force the result to fit a different board.
Free deck board count chart
Straight rows with 5.5-inch faces, 3/16-inch side gaps and an example 10 percent extra. Stock boards are assigned row by row with no reuse. Nominal stock length is used before end trimming; these are our arithmetic estimates, not manufacturer orders.
| Board run x row span | Stock length | Rows | Pieces per row | Boards with 10% extra |
|---|---|---|---|---|
| 12 x 10 ft | 12 ft | 22 | 1 | 25 |
| 16 x 12 ft | 16 ft | 26 | 1 | 29 |
| 20 x 12 ft | 20 ft | 26 | 1 | 29 |
| 16 x 16 ft | 16 ft | 34 | 1 | 38 |
| 24 x 12 ft | 12 ft | 26 | 2 | 58 |
| 24 x 16 ft | 16 ft | 34 | 2 | 75 |
Two more checked examples
20 x 12-foot deck, 20-foot stock, 3/16-inch side gap: (144 + 0.1875) / (5.5 + 0.1875) = 25.3516, rounded to 26 rows. One nominal 20-foot piece per row gives 26 pieces. Multiply by 1.10 = 28.6, rounded to 29 boards. The last row may need ripping to fit the available width.
24 x 16-foot deck, 16-foot stock, same width and gap: (192 + 0.1875) / 5.6875 = 33.7912, rounded to 34 rows. Each 24-foot run needs two stock pieces in this conservative method, so 34 x 2 = 68. With 10 percent extra: 74.8, rounded to 75 boards. That can overbuy compared with a detailed cutting plan: an 8-foot offcut might be reusable, but butt joints must land on the planned supports and follow fastening rules.
Why a stock length equal to the deck run needs a check
Trex’s 2024 guide requires clean cutting at least 3/16 inch from each board end. A stock board exactly 16 feet long is therefore not automatically a usable 16-foot finished piece. Actual delivered length, trim allowance, overhang and required end or abutting gaps determine the final cut. Our estimate has no end-trim input. For a run near a stock-length boundary, make a cutting plan before using the board count as a purchase list.1
Fasteners, joists and borders are not counted
This calculator counts surface boards only. The Trex guide specifies two screws per joist where screws attach decking, but a useful screw count also needs the joist layout, board ends and approved fastening system. Hidden fasteners use a different takeoff. We do not invent a screw quantity from square footage alone. Read the installation guide, and use our deck-board quantity guide for more layout examples.1
Turn the board estimate into a purchase list
Our planning checklist: compare the same decking product and layout before comparing prices. The calculator counts surface boards for straight rows, not the framing, fastening system or a complete installed deck.
- Record the board direction. Give the supplier the run length, distance across the rows and separate border or stair areas. Keep diagonal layouts and picture-frame borders on their own takeoff.
- Name the exact product. List the collection, profile, color, actual face width and available stock lengths. Confirm the gap and trimming requirements from that product’s installation guide rather than treating the calculator defaults as instructions.
- Check the cut list. Ask how end trimming, supported joints and reusable offcuts affect the order. A nominal stock board as long as the deck run may not provide that finished length after required trimming.
- Separate the other materials and work. Ask which framing, fasteners, fascia, railings, stairs, removal, disposal and installation work are included. Do not assume a surface-board price includes those items.
- Check the order terms. Ask about stock, delivery and unloading, tax, returns and matching extra boards later. Record how product substitutions or added work will be approved and priced.
Use our project planning and quote checklist to track included work, exclusions and changes. These are our comparison prompts, not a deck design or promises about a supplier’s terms.
Frequently asked questions
Can I use this for diagonal boards or picture-frame borders?
No, not as a complete takeoff. Diagonal cuts change the runs, usable lengths and waste. A border removes some field area and adds separate board runs. Plan those sections separately, with the supporting frame and manufacturer instructions.
Is 10 percent extra always enough?
No. It is an example input you can change. Straight full-length rows, complicated cuts, rejected boards and planned touch-up stock have different allowances. The tool does not optimize cut reuse. Compare its result with a row-by-row cutting list.
Why does the formula add one gap to the width?
N boards have N – 1 gaps between them, so covered width is N x board width + (N – 1) x gap. Solving for N gives (target width + gap) / (board width + gap). Adding one gap prevents counting a nonexistent gap after the final row. Perimeter clearances are separate and not included.
Sources
- Trex: 2024 Decking Installation Guide (manufacturer dimensions, side and end gaps, end trimming and fastening guidance; read October 6, 2026)