Measure across the boards, not along them
Enter the finished width to be covered perpendicular to the board lengths. Measure the actual board face width at several points and use the width appropriate to your material. This calculator compares two ways to fill that cross-section: full-width boards with equal gaps, or fixed gaps with a narrower final board.
Both layouts place the first and last board against the entered boundaries. If your installation requires a perimeter clearance, deduct it before entering Deck Clear Width; the tool does not add wall clearances automatically. It does not calculate board lengths, butt joints, framing, support spacing or structural capacity.
Equal gaps: a complete 12 ft example
The starting example uses 144 in of clear width, boards measuring 5.5 in and a target gap of 1/4 in. Equal mode chooses 25 full-width boards. Because boards occupy both edges, there are 24 interior gaps, not 26 end-and-interior gaps.
25 × 5.5 = 137.5 in of board faces leaves 144 − 137.5 = 6.5 in for the gaps. Divide by 24 to obtain 13/48 in, approximately 0.2708 in per gap. The result is close to the target, but it is not exactly 1/4 in. The display rounds to the nearest 1/16 in; the underlying positions retain greater precision.
The first board starts at zero and its center is 2.75 in. The final board starts at 138.5 in, its center is 141.25 in and its far edge is 144 in. Check the far edge against the overall width rather than expecting the last center to equal 144.
Equal mode is a geometric fit. Its recommendation to keep full-width boards does not establish that the solved gap is allowed by your board manufacturer. If the allowed gap differs, use the fixed-gap comparison or revise the planned coverage width.
Fixed gaps: why the default last board is rejected
With the same 144 in width and an exact 1/4 in gap, 25 full boards cover 25 × 5.5 + 24 × 0.25 = 143.5 in. Reaching the far boundary requires another gap and one more board. The remaining board would be only 144 − 25 × (5.5 + 0.25) = 0.25 in wide.
That is below the example Minimum last-board width of 1 in, so the fixed-and-rip option is rejected. The minimum is your input, not a universal safe cutting width. Do not reduce it merely to make an error disappear; verify permitted cuts, fastening and edge support for the actual product.
A fixed-gap layout that fits the chosen minimum
Change Deck Clear Width to 143 in, keep 5.5 in boards, a 1/4 in target and a 1 in minimum, then select Use fixed gaps + rip last board. The result is 25 boards: 24 full-width boards and one 5 in final board.
| Part of the cross-section | Width |
|---|---|
| 24 full boards × 5.5 in | 132 in |
| 24 gaps × 1/4 in | 6 in |
| Last board | 5 in |
| Total | 143 in |
The last board begins at 138 in and its center is 140.5 in. The full-width Equal alternative at 143 in would use 25 boards with gaps of 5.5 / 24, approximately 0.2292 in. Read the comparison before choosing; the results and materials follow the selected option. These are arithmetic demonstrations, not approval of a particular decking product or installation.
Interpret purchases and fasteners carefully
Boards to buy rounds the installed board count plus your waste percentage up to whole boards. A ripped board still counts as one board to purchase; the calculator does not assume its offcut can be reused. This is a count across the deck, not a complete stock-length or square-footage order. Check board lengths, joints, trimming and usable offcuts separately.
The screw estimate assumes two fasteners per board at each support line you enter, plus waste. The support-line count is supplied by you; the tool does not design the framing. Hidden clips, starter clips and proprietary fixing patterns require their own takeoff. The materials list intentionally excludes foundations, Posts, concrete and Gate hardware.
Transfer and retain your plan
Running measurements start at the left boundary of the entered clear width. Use the left-edge or center column consistently and measure from one origin to avoid accumulated rounding. Copy plan link preserves all inputs. Print plan gives a summary; Print marking strips provides a full-size cross-section reference. For strips, print at 100% and physically check the six-inch calibration line before transferring marks.
For material-dependent gap decisions, read the deck board gap guide and the current instructions for your chosen board and fixing system. The formula page explains rounding and fixed-gap calculations. No layout output certifies installation suitability.