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*Nominal and Actual Sizes*The Nominal size of a board is the way you talk about the size. When talking about a 1x6 board, 1" and 6" is the Nominal size. The actual size is 3/4" x 51/2".The nominal size of a 2x8 is 2" x 8", but the actual size is 11/2 x 71/2. To determine accurate coverage, the actual size of the board is used. The actual size is the actual coverage of the board. So for example a 1x6 tongue and groove board gives you an actual coverage value of 51/4". It's nominal size is 6" and the actual size is 51/4". (It's overall actual size is 51/2" but once the togue is in the groove, it actually covers only 51/4") Now apply the formula of Nominal divided by Actual = 6 / 5.25 = 1.14 . This means that you would have to order 14% more of this board to cover what a real 6" board would have covered. Example You have to cover 140 square feet using 6" bevel siding. You could easily figure that 6" is 1/2 of a square foot  so right away you know you'll need twice as much or 280 lineal feet of 6" material to cover 140 sq ft. But the 6" really isn't 6". When you lap over the next board, you only have a 41/2" exposure or actual value. N/A = 6 / 4.5 = 1.333 meaning you have to order 33% more. 280 x 1.333 = 373 lft to order. Add a percentage for cutting waste and you've got it. This Nominal divided by Actual applies in many places. If you knew that it took 4 bundles of perfection shingles to cover 100 sq ft at 51/2" exposure and you had to cover 250 sq ft, you could easily figure it will take 2.5x4 = 10 bundles. But now if you had to adjust to match a 7" exposure, you would take your 5.5 / 7 = .78 meaning you only need 78% of those 10 bundles, or 8 bundles. Keep it simple and divide it up by the actual inches. You have a deck that's 17 feet wide. You'll be using 5/4x6 cedar which actually measures 51/2". You'll leave an 1/8" space, so it's actually 55/8" of value for each board. 17 feet x 12" = 204 total inches of deck to cover using 55/8, you have 204 / 5.625 = 36.27 or 37 boards.


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