Formula
A square is 100 square feet of siding. Wall area is width times height. A rectangular house uses the perimeter, twice the length plus twice the width. A gable is half the base times the rise. Openings come out of the gross area, then waste goes back on. The net cannot fall below zero.
order = (walls + gables − openings) × (1 + waste ÷ 100)
squares = order ÷ 100
panels = ceil(order ÷ (length × exposure ÷ 12))
starter = ceil((perimeter − door widths) ÷ piece)
J = openings + rakes, then waste, then the piece length
Vinyl double 4 is a 12 ft panel with 8 inches exposed, 8.0 sq ft. Double 5 is 10 inches exposed, 10.0 sq ft. A box in those presets is 2 squares. Fiber-cement lap at 8.25 inches wide and 7 inches exposed is 7.0 sq ft a plank. At 5 inches exposed a 12 ft plank is 5.0 sq ft. A 4 × 8 panel is 32 sq ft. Only the exposed face is coverage. The sheet in the bundle replaces every one of these.
Starter stops at a door, so the door widths come off the perimeter. J-channel around a door is the head and the two legs, not the sill. Windows go all the way around. Each gable adds two rakes. Corner posts are the corner count times the wall height divided by the post length, rounded up. Soffit is the eave run times the overhang, plus waste, divided by the panel. F-channel and fascia are reported as that eave run.
Worked examples
40 × 30 ft, 9 ft walls, two 30 ft gables with a 6 ft rise, 2 doors, 12 windows, 10% waste. Walls 1,260 sq ft, gables 180, gross 1,440. Openings 220. Net 1,220. Order 1,342 sq ft, 13.42 squares, order 14. Vinyl boxes 7. Double-4 panels 168. Starter 12. J-channel 26. Corners 4. Soffit 6 panels.
The same house in fiber cement at 7 in exposure. 192 planks. There is no 200 sq ft vinyl box in that count.
Reference
Double 4 is 8.0 sq ft a panel. Double 5 is 10.0. Fiber cement at 7 in exposure is 7.0 sq ft a 12 ft plank, about 14.3 planks a square. At 5 in exposure it is 5.0 sq ft. A 4 × 8 sheet is 32 sq ft.
Questions
How many squares of siding do I need for a 2,000 sq ft house?
Floor area is not wall area. A 40 × 30 ft house is 1,200 sq ft of floor and, at 9 ft walls with two gables, about 1,440 sq ft of wall before openings. After doors, windows, and 10% waste the example on this page is 13.42 squares, so you order 14.
How many pieces of siding are in a square?
A square is 100 sq ft. A vinyl double-4 panel at 8 sq ft covers 100 ÷ 8 = 12.5 panels per square. A box of that vinyl is often 2 squares, 200 sq ft. The label on the box wins.
How much does a box of vinyl siding cover?
The double-4 and double-5 presets here use a 200 sq ft box, which is 2 squares. The 40 × 30 example orders 1,342 sq ft, which is 7 boxes.
How much waste should I add?
Ten percent is the default for simple walls. Fifteen percent is a reasonable start when there are many corners and gables. The field accepts 0 to 30 percent. Buy the extra from the same batch so a repair matches.
How do I calculate siding for a gable?
A gable is a triangle: half the base times the rise. A 30 ft gable with a 6 ft rise is 90 sq ft. Each sloping edge is the square root of (base ÷ 2) squared plus the rise squared. The roof pitch calculator turns a pitch into that rise.
How many pieces of J-channel do I need?
Add the opening perimeters. A door is the width plus twice the height, with no piece across the sill. Windows go all the way around. Rakes can be included. Divide by the piece length after trim waste. The example is 26 pieces of 12.5 ft.
How much fiber-cement siding is in a square at a 7 inch exposure?
A 12 ft plank with 7 inches exposed covers 7 sq ft. One square is 100 ÷ 7, about 14.3 planks. The exposure printed by the fiber-cement maker, including the tables sold with Hardie-style laps, replaces this assumption.
Sources
- Vinyl Siding Institute, Vinyl Siding Installation Manual. Panel courses, starter strip, J-channel, and outside corner posts are the trim types that manual describes. A published panel is often 12 ft, and a double-4 exposure of 8 inches is the coverage used here. The carton label wins.
- Fiber-cement exposure and coverage are taken from the kind of table a manufacturer prints for lap siding, including the published 7 inch exposure on an 8.25 inch plank. The name on the bundle is the authority. “Hardie” is not a product recommendation.
- Ten percent waste for simple walls and about 15 percent when the wall is broken by corners and gables are estimating allowances, not a code minimum. Buy repair pieces from the same batch. Battens on a panel wall are the board and batten calculator.