Quick answer
Rows of decking = ⌈deck width × 12 ÷ (board width + gap)⌉, and boards = ⌈rows × deck length × (1 + waste) ÷ board length⌉. A 16 × 12 ft deck with 5.5 inch boards, 1/8 inch gaps, 12 ft boards and 10% waste needs 26 rows and 39 boards.
Enter the deck size, board width, gap, board length and waste. The calculator returns rows, boards and linear feet.
Related tools: Deck Stain Calculator: Gallons of stain for a deck surface. Roof Pitch Calculator: Roof pitch as rise per 12 inches, angle, slope percent and rafter length factor.
How to use the Deck Boards Calculator
- Enter deck length (board direction) (ft) (above 0).
- Enter deck width (ft) (above 0).
- Enter board width (in) (above 0). It starts at 5.5.
- Enter gap between boards (in) (0 or more). It starts at 0.125.
- Enter board length (ft) (above 0).
- Enter waste allowance (%) (between 0 and 100). It starts at 10.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Deck length (board direction) | A number, in ft | > 0 |
| Deck width | A number, in ft | > 0 |
| Board width | A number, in in | > 0, starts at 5.5 |
| Gap between boards | A number, in in | ≥ 0, starts at 0.125 |
| Board length | A number, in ft | > 0 |
| Waste allowance | A number, in % | ≥ 0, ≤ 100, starts at 10 |
Formula and method
Formula
rows = ⌈deck width × 12 ÷ (board width + gap)⌉; boards = ⌈rows × length × (1 + waste) ÷ board length⌉
IEEE-754 double precision; shown to the decimals indicated.
Real board widths are smaller than nominal sizes (a 5/4×6 board is about 5½ in). Plan geometry only; confirm quantities with your supplier and local code before ordering.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Deck length (board direction) = 16; Deck width = 12; Board width = 5.5; Gap between boards = 0.125; Board length = 12; Waste allowance = 10.
| Result | Value |
|---|---|
| Boards needed | 39 |
| Rows of boards | 26 |
| Linear feet (before waste) | 416 |
Example 2
Inputs: Deck length (board direction) = 20; Deck width = 10; Board width = 3.5; Gap between boards = 0.25; Board length = 8; Waste allowance = 5.
| Result | Value |
|---|---|
| Boards needed | 84 |
| Rows of boards | 32 |
| Linear feet (before waste) | 640 |
The first line is the headline result (boards needed). Below it the deck Boards Calculator lists rows of boards and linear feet (before waste), which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
The count
12 ft of width is 144 inches; each board plus gap takes 5.625 inches, so 144 ÷ 5.625 = 25.6, rounded up to 26 rows. 26 rows of 16 ft is 416 linear feet; with 10% waste 457.6 ft, divided by 12 ft boards gives 38.1, rounded up to 39.
Choosing lengths
Boards run across joists, and butt joints should land on a joist. Pick a board length that avoids short offcuts: for a 16 ft deck, 16 ft boards give no waste from seams. Gap size depends on the material, as wood swells and composites expand with heat.
How the result changes with deck length
Holding every other input at the first worked example, this table changes only deck length. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Deck length | Boards needed | Rows of boards | Linear feet (before waste) |
|---|---|---|---|
| 8 | 20 | 26 | 208 |
| 16 | 39 | 26 | 416 |
| 32 | 77 | 26 | 832 |
| 64 | 153 | 26 | 1,664 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Deck length (board direction): entering "0" shows "Deck length (board direction) must be greater than 0." beside that field and no result.
- Board width: entering "0" shows "Board width must be greater than 0." beside that field and no result.
Choosing between the Deck Boards Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Deck Stain Calculator](/construction/deck-stain-calculator/) | deck length, deck width and coats | Stain needed |
| [Baluster Calculator](/construction/baluster-calculator/) | railing length, baluster width and maximum gap | Balusters needed |
| [Roof Pitch Calculator](/construction/roof-pitch-calculator/) | i know the and value | Pitch |
| [Tile Calculator](/construction/tile-calculator/) | area to tile, tile length and tile width | Tiles needed |
| [Aquarium Volume Calculator](/construction/aquarium-volume/) | length, width and water height | Capacity |
Background: construction estimates
Material quantities are geometry plus a margin. Calculate the exact volume or area, add an allowance for waste, spillage, cuts and uneven ground (calculator pages commonly mention 5 to 10 percent, a rule of thumb that varies by material and job) and round up to whole units. One cubic yard is exactly 27 cubic feet.
Suppliers often have minimum order sizes and may charge for small loads, so confirm quantities, product yields and local code with your supplier or a qualified estimator before ordering.
Assumptions and limitations
- Pick board lengths that avoid short offcuts
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for deck length (board direction) and board width to confirm this.
Frequently asked questions
How much gap between deck boards?
About 1/8 inch for kiln-dried boards; follow the manufacturer's advice for composites.
How much waste should I allow?
Around 10% for straight layouts, 15% for diagonals.
Do joists and fasteners count?
No, only boards are estimated.
What does the Deck Boards Calculator calculate?
Number of deck boards for a deck surface. It returns Boards needed, Rows of boards and Linear feet (before waste).
What formula does the Deck Boards Calculator use?
rows = ⌈deck width × 12 ÷ (board width + gap)⌉; boards = ⌈rows × length × (1 + waste) ÷ board length⌉. Real board widths are smaller than nominal sizes (a 5/4×6 board is about 5½ in).
What do I need to enter in the deck Boards Calculator?
You enter Deck length (board direction) (ft), Deck width (ft), Board width (in), Gap between boards (in), Board length (ft) and Waste allowance (%).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Pick board lengths that avoid short offcuts.
What is an example of a deck Boards result?
With deck length (board direction) 16, deck width 12, board width 5.5, gap between boards 0.125 and board length 12, the boards needed is 39.
Can I rely on this calculator for a decision?
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work. For this calculator in particular: Pick board lengths that avoid short offcuts.
Related calculators
- Deck Stain Calculator — Gallons of stain for a deck surface.
- Roof Pitch Calculator — Roof pitch as rise per 12 inches, angle, slope percent and rafter length factor.
- Tile Calculator — Number of tiles (and boxes) for an area.
- Aquarium Volume Calculator — Capacity and filled weight of a rectangular aquarium.
- Flooring Calculator — Flooring area with waste, and boxes if you know the coverage per box.
Sources
- OpenStax, Prealgebra 2e (Rice University): perimeter, area, volume, circumference and the Pythagorean theorem
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B conversion factors (international foot 0.3048 m, inch 25.4 mm, US gallon 3.785411784 L)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.