Width and gap are separate
Enter the actual width of one board, not its nominal product name or the width plus a gap. The calculation adds the gap itself. Read both dimensions from the chosen product and installation plan.
Board quantities before the quote
Deck calculator
Board rows = (deck width + gap) ÷ (actual board width + gap), rounded up.
Use matching units in that formula. Boards run along the entered deck length; the worksheet counts whole pieces per row, then applies your chosen allowance. Enter the board width without the gap.
Boards and joists visibleGap and allowance explicitUS and metric inputsNo login or usage limit
01 Measure02 Enter product facts03 Count
Rows = (deck width + board gap) ÷ (board width + board gap), rounded up · pieces per row = deck length ÷ board length, rounded up · boards = rows × pieces per row, then the entered allowance · joists = deck length ÷ joist spacing, rounded up, plus one end joist
Check the layout assumption
Count rows, then whole boards
This deck estimator models a single rectangle with straight boards running along its length and joists across its width. Each row receives enough whole boards to span that length. It does not arrange joints, reuse offcuts or calculate a diagonal pattern.
Allowance applies to boards
Rows × whole pieces per row gives the base board count. Your allowance increases that count before rounding up again. It does not increase the measured area or the joist count.
Joists are a baseline count
The worksheet rounds deck length ÷ entered joist spacing up, then adds one end joist. Extra supports, blocking, beams, posts and joint details are not in that count.
For the row formula, convert deck width and board width to the same units as the gap. Pieces per row = deck length ÷ board length, rounded up. Use the same units for deck length and joist spacing when checking the joist count.
Measurement reference: Decks.com board and material guide distinguishes nominal and actual board sizes and identifies spacing and layout as inputs. Checked October 2, 2026. Its default dimensions, waste rates and installation advice are not substituted into this worksheet.
A count you can check
A 12 ft × 10 ft rectangle
Example inputs, not an installation specification: 12 ft deck length, 10 ft width, boards 5.5 in wide and 12 ft long, a user-chosen 0.25 in gap, 16 in joist spacing and 10% allowance. Boards run along the 12 ft direction.
- Rows: convert 10 ft to 120 in. (120 + 0.25) ÷ (5.5 + 0.25) = about 20.91, rounded up to 21 rows.
- Boards: 12 ÷ 12 gives one whole board per row. 21 × 1 × 1.10 = 23.1, rounded up to 24 boards. That is 288 linear ft of purchased boards, not a cut list.
- Joists: convert 12 ft to 144 in. 144 ÷ 16 = 9 spacing intervals; add one end joist for 10 baseline joists. The allowance is not applied to them.
The measured area stays 120 ft². If you replace the example board length with 8 ft, each row needs two whole boards: 42 before allowance, 47 after the same 10%. This conservative row-by-row count does not share offcuts between rows.
For a deck material calculator workflow, take these quantities to your actual plan and supplier. A price estimate still needs quoted rates and the parts not counted here.
Before ordering
Deck calculator FAQ
How do I use a deck board calculator?
Enter the rectangle's length and width, actual board width and length, separate gap, joist spacing and your chosen allowance. The worksheet counts rounded-up rows and whole pieces per row, then applies the allowance to the boards.
Should board width include the gap?
No. Enter only the actual width of one board. The row formula adds the gap separately; entering board width plus the gap would count it twice. Use actual product dimensions, not just the nominal size name.
Which way do the boards run?
Boards run along the entered deck length, with rows across its width. Joist spacing is counted along the length. To compare a rotated rectangle, swap its length and width and check the result against the framing plan.
How does the board gap change the count?
Rows equal deck width plus one gap, divided by actual board width plus the gap, rounded up. All three values must use matching units when checking the formula. Enter the gap from your installation plan, not a generic default.
Does this deck estimator reuse offcuts?
No. It rounds pieces per row up and counts each row independently. Shorter boards can therefore produce a larger count. The worksheet does not optimize cuts, stagger joints or model diagonal layouts.
Is this a complete deck building calculator?
No. It counts boards and baseline joists for one rectangle. Beams, posts, blocking, fasteners, stairs, railings and structural span checks are not calculated; use the project documents for those parts.
Does this include a deck cost calculator?
No. This page returns quantities, not a currency total. Apply your own board and framing quotes separately, then include the labor, delivery and other project costs that your quote covers.
Can I use this as a Trex deck cost calculator?
You can enter the confirmed dimensions and installation gap for your selected boards to estimate quantities. This is not a Trex price tool: it has no live product prices, brand-specific fastener count, warranty check or complete installation cost.
Do I need to log in or register?
No. This free deck calculator runs in your browser without login, registration or a usage limit.
Connect the project
Keep related material quantities separate
Use the fence calculator for fence panels and posts, or the paver calculator for patio units and base layers. Each page keeps its own measured inputs and formulas visible.