Decking is bought in packs that cover a stated area, but it is installed as long boards that run in one direction only. Those two facts pull against each other on every job. A deck of 5 m by 3 m is 15 square meters on paper, yet the order that finishes it depends on how many 140 mm boards fit across the 3 m run and on how cleanly the stock board lengths divide into the 5 m run. Enter the footprint, the coverage printed on the pack label and a waste allowance, and the answer comes back in whole packs, because no merchant splits a pack at the counter.
Measuring a deck footprint before you price packs
- Fix the board direction first. Stand at the house wall and decide which way the boards will run, because that one choice fixes every other number. Boards normally run parallel to the wall, so the dimension measured away from the house is the run the boards have to cross. Tool: Chalk line.
- Take the long run in three places. Pull a tape along the deck length at the house wall, at the middle and at the outer edge, and write all three readings in meters to two decimal places. Enter the largest of the three in the length field. Tool: Tape measure.
- Take the crossing run the same way. Repeat those three readings across the deck width, from the house face out to the rim board. Frames that are out of square by 20 mm to 30 mm are common on older decks, so the largest reading is the one your boards must reach. Tool: Tape measure.
- Read coverage from the pack you will buy. Turn the pack over, find the coverage figure printed as square meters per pack, and check whether the label states it with or without a fitting gap. Enter that number in the coverage field exactly as printed on the label. Tool: Pack label.
- Keep stairs and fascia off the footprint. Measure stair treads, fascia strips and riser boards on a separate sheet in linear meters, and leave them out of the deck area. They are cut from full board lengths and will not draw down evenly from a pack quoted in square meters. Tool: Notepad.
- Match the waste percent to the layout. Use 10 percent for boards laid square across a rectangular frame, and raise it to 15 percent or more for a diagonal layout, a picture frame border or a deck broken by posts. Enter the number without a percent sign.
| Nominal size | Dressed thickness, dry | Dressed width, dry | Width plus 5 mm gap | Board run per square meter |
|---|---|---|---|---|
| 1 x 4 | 19 mm (3/4 in) | 89 mm (3-1/2 in) | 94 mm | 10.64 m |
| 5/4 x 4 | 25 mm (1 in) | 89 mm (3-1/2 in) | 94 mm | 10.64 m |
| 1 x 6 | 19 mm (3/4 in) | 140 mm (5-1/2 in) | 145 mm | 6.90 m |
| 5/4 x 6 | 25 mm (1 in) | 140 mm (5-1/2 in) | 145 mm | 6.90 m |
| 2 x 6 | 38 mm (1-1/2 in) | 140 mm (5-1/2 in) | 145 mm | 6.90 m |
| 2 x 8 | 38 mm (1-1/2 in) | 184 mm (7-1/4 in) | 189 mm | 5.29 m |
Source: National Institute of Standards and Technology, Voluntary Product Standard PS 20-20, American Softwood Lumber Standard.
Dressed sizes are the dry values in Table 3 of PS 20-20. The last two columns are arithmetic from those widths at a 5 mm fitting gap; widen the gap and the run per square meter falls with it.
Why pack coverage and board coverage disagree
A pack label reports square meters, but a deck is built from boards of a fixed dressed width. Under PS 20-20 a nominal 6 inch board leaves the mill at 140 mm, or 5-1/2 inches, when it is dry, and at 143 mm when it is sold green and unseasoned. Lay those boards with a 5 mm fitting gap and each one occupies a 145 mm module, so one square meter of finished deck absorbs 6.90 linear meters of board rather than the 7.14 meters the bare 140 mm width suggests. That is a gap of about 3.5 percent before a single offcut. Merchants are not consistent about which figure they print. Some quote the bare area of the boards inside the pack, others quote the area those boards cover once gapped. A pack of five boards at 3.6 m and 140 mm holds 2.52 square meters of bare board and covers 2.61 square meters once a 5 mm joint is counted, so a label reading 2.4 square meters is describing neither. Count the boards in one pack and check the arithmetic yourself before the order goes in.
Five by three meters at ten percent waste
| Field | Value |
|---|---|
| length | 5 |
| width | 3 |
| coverage | 2.4 |
| waste | 10 |
The footprint is 5 m by 3 m, so the bare area is 15 square meters. A 10 percent allowance lifts that to 16.5 square meters, and dividing by the 2.4 square meters printed on the pack gives 6.875 packs, which rounds up to seven. Rounding happens once, at the end, so no intermediate figure is rounded twice. Length and width move raw need directly, and pack coverage moves it inversely. Reducing coverage about 8.3 percent from 2.4 to 2.2 square meters raises the raw requirement about 9.1 percent to 7.5 and crosses the boundary to eight packs; raising coverage to 2.6 leaves the order at seven. The waste percent has a wide plateau here: every allowance from 0 through exactly 12 percent produces a raw need of no more than 7 packs, while any value above 12 percent crosses the boundary and orders 8. The largest real uncertainty depends on deck measurement, layout, and the product label.
Result: 7 packs.
Board length drives offcut loss, not deck area
Waste percent is an area idea, while offcut loss is a length problem. PS 20-20 sets standard lumber lengths in multiples of 0.3048 m, one foot, or 0.6096 m, two feet, and metric merchants usually stock decking in steps such as 2.4 m, 3.0 m, 3.6 m, 4.2 m and 4.8 m. A deck that runs 5.0 m along the house takes none of those cleanly. Cut from 4.8 m boards you fall 200 mm short on every row. Pair a 2.4 m board with a 2.6 m piece instead and the 2.6 m piece comes out of a 3.0 m board, leaving a 400 mm tail that is scrap unless another row can use it. A 3 m width at a 145 mm module needs 21 rows, so those tails add up to 8.4 m of board on the ground, close to three and a half 2.4 m boards. A 10 percent allowance on 15 square meters buys back only 1.5 square meters, which is not the same thing at all. Lay the stock lengths against the deck length on paper first, and decide whether a butt joint over a joist beats paying for a longer board.
Joist spacing decides whether the boards can span
The pack count says nothing about whether the boards can carry a person. Grading rules for exterior patio decking, set by the National Lumber Grades Authority and reported by the American Wood Council, cover 5/4 inch to 2 inch thicknesses at 4 inches and wider for flatwise loading, and only where the span does not exceed 16 inches on center. That figure is a ceiling rather than a target. Turn the boards to a 45 degree diagonal and the clear span between joists grows by a factor of about 1.41, so a frame built at 16 inches on center is quietly asking each board to span 22.6 inches. Holding that diagonal span inside the 16 inch limit means joists at about 11 inches on center, while 12 inches on center is the spacing site practice settles on and it still leaves a span of about 16.9 inches. Price those extra joists with the framing rather than here. The load case is settled the same way by published guidance. DCA 6 sizes deck members for a 40 psf live load with a 10 psf dead load, which is the figure the IRC requires, while ASCE 7 requires 100 psf for decks and balconies, so the governing load depends on which code the project is designed to. None of that changes the pack arithmetic, but it decides whether the deck being priced is the deck that should be built.
Common mistakes
- Entering the finished deck area in the length field and leaving the width at its default value
- The calculation multiplies length by width, so 15 square meters typed into the length box is multiplied by the 3 m width the field already holds, returning a 45 square meter deck and roughly three times the packs the job needs. Type the two site measurements separately, in meters, and let the multiplication happen inside the calculation rather than on your notepad.
- Copying the pack coverage from an old delivery note or a competitor label
- Coverage per pack is set by the board length, the dressed width and whether the merchant counts the fitting gap, and all three change between suppliers and between production runs. Read the coverage from the exact pack you intend to buy, then check it by multiplying board length by dressed width and by the number of boards in the pack.
- Using the same waste percent for a straight layout and a diagonal one
- A 45 degree layout turns both ends of every board into an angled offcut, and that loss climbs with the length of the diagonal instead of staying proportional to the deck area. Keep 10 percent for boards laid square to the joists, and move to 15 percent or more for diagonals, picture frame borders and decks broken by posts.
- Leaving stair treads, fascia and riser boards inside the deck footprint
- Those parts are cut from full board lengths and are consumed in linear meters, so folding them into a square meter figure understates the number of boards the job needs. List treads and fascia in linear meters on a separate sheet and convert them into whole boards using the stock length you are actually buying.
- Ordering the rounded up pack several weeks after the main delivery
- Pressure treated pine and composite boards shift in color between production batches, and a late pack can sit visibly lighter or darker in the middle of a finished deck. Take the rounded up pack with the first delivery and keep the surplus for stair treads, replacements and the boards that split at a fastener.
Ordering packs the way a merchant sells them
Merchants sell whole packs, so the answer is rounded up once, at the very end. A deck of 5 m by 3 m at 10 percent waste needs 16.5 square meters of coverage and 6.875 packs, which becomes seven. The seventh pack carries 0.125 packs of surplus, about 0.3 square meters, and it is the cheapest insurance on the job, because a replacement pack ordered three weeks later rarely matches the first delivery for color. Pressure treated pine and composite boards both shift shade between production runs, so ask for one batch on one delivery. Keep the surplus for stair treads and for the two or three boards that always split at a fastener. Regional habit matters as well. North American yards price decking by the lineal foot and by the individual board, so a coverage figure per pack may not exist at all, and the run per square meter in the table above is the number to convert from. When the deck sits high enough above grade to need a guard, or when the frame hangs from a house wall, the ledger, footings and guards are engineering questions for a licensed builder rather than a quantity estimate.
Frequently asked questions
Why does the tool round 6.875 packs up to seven?
Packs are not split at the trade counter, so a fraction of a pack has to become a whole one. Fifteen square meters plus a 10 percent allowance is 16.5 square meters, and 16.5 divided by 2.4 is 6.875. Rounding down to six packs covers 14.4 square meters, which is 0.6 square meters short of the bare 15 square meter deck and about 4.3 linear meters of 140 mm board. The rounding is applied once, at the end of the calculation, so no intermediate value is rounded twice.
How much waste should I allow for a diagonal deck layout?
Raise the waste percent from 10 to at least 15 for a 45 degree layout, because every board meets the rim at an angle and both ends become an offcut rather than a square cut. On a 5 m by 3 m deck that moves the requirement from 16.5 to 17.25 square meters, which pushes the order from seven packs to eight. A picture frame border adds its own perimeter boards, and those are better counted separately in linear meters.
Does the pack coverage figure include the gap between boards?
It depends on the merchant, and the label rarely says which convention it uses. Some quote the bare area of the boards in the pack, which for five boards of 3.6 m at a 140 mm dressed width is 2.52 square meters. Others quote the area covered once a fitting gap is allowed, which at 5 mm per joint is 2.61 square meters for that same pack. The difference is about 3.5 percent, enough to turn a seven pack order into an eight pack order on a larger deck.
Can I use this for composite boards as well as sawn timber?
Yes, provided you enter the coverage the composite manufacturer prints rather than a figure carried over from a timber pack. Composite boards are sold in fixed lengths with a stated square meter coverage, and their dressed widths do not have to follow the 140 mm that PS 20-20 sets for a nominal 6 inch sawn board. Composite also asks for a wider fitting gap in hot climates, and a wider gap means fewer square meters covered per pack.
What joist spacing do these boards need underneath them?
Grading rules for exterior patio decking cover 5/4 inch to 2 inch thicknesses at 4 inches and wider, for flatwise loading, at spans no greater than 16 inches on center. Boards laid at 45 degrees cross a longer clear span, roughly 22.6 inches over joists set at 16 inches, so tighten the framing to 12 inches on center for a diagonal, which brings the span back to about 16.9 inches, or to 11 inches on center to stay fully inside the 16 inch limit. The count returned here covers deck boards only, and the extra joists have to be priced with the framing.
Related calculators
Sources
- Voluntary Product Standard PS 20-20, American Softwood Lumber Standard National Institute of Standards and Technology. Table 3 supplies every dressed size in the board table, including 140 mm for a nominal 6 inch width and 25 mm for 5/4 thickness, and clause 5.3.1 sets the one foot and two foot length steps. Accessed 2026-09-06.
- In the DCA6 you specify a live load = 40 psf. I thought the live load requirement for decks and balconies was 100 psf? American Wood Council. Confirms the 40 psf live load and 10 psf dead load that DCA 6 sizes deck members for, and records that ASCE 7 requires 100 psf while the IRC requires 40 psf. Accessed 2026-09-06.
- Where can I get span tables and span table information for lumber? Where can I get decking span tables? American Wood Council. Reports the National Lumber Grades Authority rule that exterior patio decking covers 5/4 inch to 2 inch thicknesses at 4 inches and wider for flatwise loading at spans not over 16 inches on center. Accessed 2026-09-06.
This is a planning result, not a supplier quote; sellers use different pack, bag, board, roll, and bundle conventions, so confirm the exact product label and selling unit before you order.