Siding calculator: how much siding you need, counted by exposure

About 15.6 squares for a 30 × 50 ft one-story ranch with 9 ft walls, two gables on a 6/12 roof, two doors, six windows and a garage door, with 10% waste. The net wall area is 1,421 sq ft, which is close to the floor area by coincidence, not by rule. A two-story house with 1,500 sq ft of floor has a smaller footprint but twice the wall height, so it needs more siding. Floor area never tells you the siding; measure the perimeter and the wall height, add the gables and subtract the openings, or enter them in the calculator.

A square of siding is 100 sq ft of wall coverage. It is the unit siding is sold and boxed in, and the same unit roofers use for shingles. To find squares, divide the area to cover by 100 after adding waste: the default house has 1,577.4 sq ft to buy, or 15.8 squares. A square is a measure of wall covered, not of material, so the number of boards or panels in a square depends on the exposure. For 6 in lap boards it is 16.7 boards 12 ft long; for double 4 in vinyl it is about 12.1 panels.

It depends on the exposure and the piece length. Lap boards 12 ft long at a 6 in exposure take 16.7 boards per square, at 7 in it is 14.3 and at 4 in it is 25. Double 4 in vinyl panels 12 ft 6 in long cover 8.28 sq ft each after the 1 in end lap, so about 12.1 panels per square; double 5 in panels need about 9.7. Use the formula 100 ÷ (exposure in feet × length in feet), and round the total up, not each square.

Many boxes of horizontal vinyl siding are rated at 2 squares, 200 sq ft, but the rating is printed on the box and differs by brand, profile and panel length, and the number of panels in a box changes with the panel length and exposure. Divide your squares by the box rating and round up: the default house needs 15.8 squares, so 8 boxes at 2 squares each. Enter the rating from the box you are buying in the squares-per-box field.

A box rated at 2 squares holds enough panels to cover 200 sq ft of wall once installed. For double 4 in panels 12 ft 6 in long that is about 24 panels, and for double 5 in panels about 19. The panel count is printed on the box, and boxes of the same brand can hold different numbers by profile. Coverage is the installed area, after the panels lock together and overlap at the ends, so you do not need to deduct the laps yourself. Buy whole boxes from one production run so the color matches across the house.

Yes, subtract them, then add a waste allowance. The calculator takes off 21 sq ft per door (3 × 7 ft), 15 sq ft per window (3 × 5 ft) and 112 sq ft per garage door (16 × 7 ft). Some guides skip the deduction for small windows to cover waste, but that hides the waste in the measurement and makes it hard to know how much you allowed. On the default house the openings are 51 sq ft. For large picture windows or sliders, measure them and subtract the real size from a known wall area. The trim around each opening is added back as J-channel.

Ten percent is the usual allowance and the calculator's default. A plain rectangular house with few openings can get by on 5 to 10%; houses with many corners, dormers, steep gables or bump-outs need 15% or more. Add the waste once, to the net area, and do not also round every wall up or skip the opening deduction, or you count it twice. For the default house 10% waste moves the order from 239 to 263 lap boards. Keep a few spare pieces for repairs, because colors and profiles change over the years.

Exposure, or reveal, is the height of each course that shows after the next course laps over it. It equals the board width minus the lap. A 7 1/4 in fiber-cement plank laps 1 1/4 in and shows 6 in; a cedar 1×8 bevel board is 7 1/2 in wide and shows 6 1/2 in with a 1 in lap. Exposure sets the board count: at 6 in a square of wall takes 200 linear feet of board. Using the full board width instead of the exposure undercounts the default house by 45 boards. Vinyl profiles name their exposure: double 4 means 8 in per panel.

At least 1 1/4 in for fiber-cement and at least 1 in for wood, unless the maker says otherwise. IRC Section R703.10.2 requires fiber-cement lap siding to be lapped a minimum of 1 1/4 in. Section R703.5.3 sends horizontal wood lap siding to the manufacturer's recommendations and, where there are none, requires a lap of not less than 1 in, or 1/2 in if the siding is rabbeted. Engineered wood and hardboard follow their own instructions. Vinyl panels lock together, and their end joints overlap about 1 in. The calculator shows the lap for the board you pick and warns when it falls under the minimum.

Multiply half the gable width by the rise from the top of the wall to the ridge. A 30 ft gable with a 7 ft 6 in rise is ½ × 30 × 7.5 = 112.5 sq ft. If you know the roof pitch, the rise is half the width times the pitch ÷ 12, so a 6/12 roof over a 30 ft gable rises 7.5 ft. Add both sloped edges for rake trim: 2 × √(15² + 7.5²) = 33.5 ft per gable. Gable courses get shorter toward the peak and every board needs an angled cut, so gables use more waste than the walls. Enter the number of gables, the width and the rise in the calculator.

For lap siding, about one nail per stud per course. The default house takes 2,367 nails with studs 16 in on center and 1,578 at 24 in. Horizontal vinyl is nailed at least every 16 in under IRC R703.11.1.3, so the same house in double 4 in vinyl needs about 1,775 nails, plus nails for every corner post, starter strip and J-channel. Use corrosion-resistant siding nails long enough for the penetration the instructions call for; vinyl needs at least 1 1/4 in into the sheathing and framing. Buy by count or by the pound from the nail box's count per pound.

Add the trim around every opening and both sloped edges of every gable. The calculator uses 17 ft per door, 16 ft per window and 30 ft per garage door, so the default house's one door and two windows need 49 ft, and its two 30 ft gables add 67.1 ft of rake trim. Divide by the piece length and round up: 10 pieces at 12 ft 6 in. Measure a large window or a slider and use its real perimeter. Starter strip and corner posts are separate runs, counted along the base of the walls and at each corner.

Divide the wall area by the width of one board plus its gap, then add one batten per joint. In feet: area × 12 ÷ (board width + gap) gives the linear feet of boards. With 1×10 boards (9 1/4 in) and 1/2 in gaps each board covers 9 3/4 in, so the default house needs 1,941.4 linear feet: 162 boards 12 ft long with waste. The battens run the same linear feet, since every gap gets one. Buy boards at least as long as the wall height to avoid vertical end joints. For battens over sheet panels, divide the area by the batten spacing instead.

Measure each wall's length and height, each gable's width and rise, and count the openings, then enter them in the calculator. The full measuring walk-through is in how to measure for siding.

Common fiber-cement planks are 12 ft long and 5 1/4, 6 1/4, 7 1/4, 8 1/4, 9 1/4 and 12 in wide. Their maker's coverage chart lays them at 4, 5, 6, 7, 8 and 10 3/4 in of exposure, so each laps the course below by 1 1/4 in, the IRC minimum for fiber-cement. The 8 1/4 in plank at 7 in is one of the most common; it takes 14.3 planks per square and 226 planks for the default house. Pick the plank in the calculator and it fills in the exposure. Check the chart in the instructions for the exact product.