Deck Calculator
Free deck board calculator: enter the deck size, the actual board width and gap, and the board length to see how many decking boards to buy.
- Free, no signup
- Formula shown
- Rounds up to whole units
- Updated October 2026
Boards to buy · 10% allowance
29boards
Rows of boards
26
1 board per row
Linear feet
416 ft
457.6 ft with allowance
- If no offcuts are reused
- 26 boards
- Last row width
- 1-13/16 in
- Deck area
- 192 ft²
The last row would be a narrow strip. Many builders rip the first and last boards to similar widths instead.
Decking boards only. Framing, fasteners, stairs, and railings are not included.
AnswerA 16 × 12 ft deck with 5-1/2 in boards and 3/16 in gaps needs 26 rows (416 linear ft), so buy 29 boards 16 ft long including a 10% allowance.
How to use this deck calculator
- Enter the deck's length and width, and choose which direction the boards run.
- Enter the actual board width, measured with a tape. Nominal sizes are larger than real ones: the softwood lumber standard lists a nominal 6 in width at 5-1/2 in dressed.
- Enter the gap between boards from the board manufacturer's installation instructions.
- Enter the length of the boards you plan to buy.
- Set a waste allowance for end trims, cuts, and boards you reject.
How to calculate deck boards: the formula, shown
Decking is laid in rows. Each row is one board wide plus one gap, except the last row, which has no gap after it. The number of rows is the smallest whole number n where n × board width + (n − 1) × gap covers the deck. Each row is as long as the deck dimension the boards run along. The total length of all rows, plus an allowance, divided by the board length gives the boards to buy.
Default example. A 16 ft × 12 ft deck with boards running the 16 ft length, 5-1/2 in boards, 3/16 in gaps, 16 ft boards, and a 10% allowance. Trex lists its square-edge boards at 5.5 in wide with a minimum 3/16 in side-to-side gap. Your boards and gap may differ. Change the inputs above and the result panel recalculates; this worked example always shows the default.
Across = 12 ft × 12 = 144 in Rows = (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.3516 → 26 rows Last row = 144 − 25 × 5.6875 = 1.8125 in wide (ripped) Linear feet = 26 rows × 16 ft = 416 ft Allowance = 416 × (1 + 10/100) = 457.6 ft Boards = 457.6 ft ÷ 16 ft = 28.6 → 29 boards
In this example the last row is only 1.81 in wide. A small change to the gap or to the overhang at each edge, or ripping the first and last boards to similar widths, gives a neater finish.
Deck boards for common deck sizes
Decking for common deck sizes, computed with the same formula as the calculator. Boards run along the first dimension, 5-1/2 in actual board width, 3/16 in gaps, 16 ft boards, and a 10% allowance. Different boards or gaps change the counts.
| Deck | Rows | Linear feet | 16 ft boards | No offcut reuse |
|---|---|---|---|---|
| 10 × 10 ft | 22 | 220 | 16 | 22 |
| 12 × 12 ft | 26 | 312 | 22 | 26 |
| 16 × 12 ft | 26 | 416 | 29 | 26 |
| 16 × 16 ft | 34 | 544 | 38 | 34 |
| 20 × 12 ft | 26 | 520 | 36 | 52 |
| 20 × 16 ft | 34 | 680 | 47 | 68 |
| 24 × 16 ft | 34 | 816 | 57 | 68 |
How many deck boards do I need to order?
For the default deck, buy 29 boards of 16 ft. With no offcut reuse at all, the layout itself needs 26.
- Cutting assumptions: the board count is based on total linear feet plus your allowance. That assumes offcuts from one row can be used in another. If the deck is longer than your boards, each row needs a butt joint, and the “no offcuts reused” figure shows the count if every row uses fresh boards.
- Choosing board length: boards that match the run (or divide evenly into it) waste the least. Joints must land on framing, so plan them with your joist layout.
- Not included: joists, beams, posts, footings, fasteners, fascia, stairs, and railings. Framing sizes and spans depend on loads and local code, which need proper design inputs. This calculator doesn't estimate them.
- Picture-frame borders and angled layouts use more material. Raise the allowance, or calculate border boards separately.
Tips for planning decking
- Measure several boards. Composite and wood boards can differ slightly from their listed width.
- Check the manufacturer's gap rules for both side-to-side and end-to-end gaps. Some depend on temperature at installation.
- Footings for a ground-level deck often use bagged concrete. Size them with the concrete calculator.
- For an L-shaped deck, split it into rectangles that share the same board direction and add the board counts.
Deck calculator FAQ
How many deck boards do I need?
Work out how many rows of boards cover the deck: divide the deck width plus one gap by the board width plus one gap, and round up. Multiply the rows by the deck length to get linear feet, add an allowance, and divide by the board length. Round up to whole boards.
How many deck boards for a 12 x 16 deck?
With boards running the 16 ft length, 5.5 in boards, 3/16 in gaps, and 16 ft boards, the deck takes 26 rows and 416 linear feet. With a 10% allowance that is 29 boards. Different board widths or gaps change the count.
Should I use the nominal or the actual board width?
Always the actual width. Nominal sizes are larger than the finished board. The American Softwood Lumber Standard lists a nominal 6 in width at 5-1/2 in dressed, and composite boards state their own actual dimensions.
How big should the gap between deck boards be?
Follow the board manufacturer's instructions. For example, Trex states a minimum side-to-side gap of 3/16 in for its decking, and recommends more for docks and heavily wooded areas. Other products differ.
How much extra decking should I buy?
It depends on the layout, the board lengths, and how many cuts you make. The calculator defaults to a 10% allowance as an adjustable example. Use more for diagonal layouts, borders, or when board length doesn't divide evenly into the deck.
Why are there two board counts?
The main count assumes offcuts can be used in other rows, so it is based on total linear feet plus the allowance. The 'no offcuts reused' figure is rows × boards per row. It is the most you would need if every row started with fresh boards.
Does the calculator include joists, beams, or footings?
No. Framing depends on spans, loads, wood species or product, and local code. Those need design inputs this calculator doesn't collect. Use your plans, a span table from an authoritative source, or a qualified professional.
Can I use metric measurements?
Yes. Enter the deck and board length in meters, and the board width and gap in millimeters. The calculator converts everything to the same unit before working out rows.
Related calculators and guides
Calculators
- Concrete Calculator — Slab volume in cubic feet, cubic yards, and cubic meters, with ready-mix order size and bag counts for documented products.
- Paint Calculator — Gallons or liters of paint for your walls and number of coats, using the coverage printed on your paint's label.
- Roof Pitch Calculator — Pitch as x/12, roof angle in degrees, slope percentage, and the sloped length for a given horizontal run.
Guides
- How to Measure an Irregular Area — Split odd shapes into rectangles, triangles, circles, and trapezoids, then average width and depth.
- How to Plan a Bulk Material Delivery — Rounding to supplier increments, questions to ask, and a checklist before bulk material arrives.
Sources
- Trex Decking Installation Guide (2024) — Trex Company. Used for: Board width 5.5 in; minimum side-to-side gap 3/16 in. Checked 2026-10-03.
- Voluntary Product Standard PS 20-25, American Softwood Lumber Standard — National Institute of Standards and Technology. Used for: Nominal 6 in width = 5-1/2 in minimum dressed (dry). Checked 2026-10-03.
- NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity — National Institute of Standards and Technology. Used for: 1 ft = 0.3048 m (exact); 1 in = 2.54 cm (exact). Checked 2026-10-03.