How the siding calculator works
Siding is priced and packed in squares: one square covers 100 ft² of wall, which is 9.29 m². A carton of standard double 4 in vinyl panels holds two squares. To reach that number the calculator treats the house as rectangles plus triangles, removes the openings, adds a waste margin, and then lists the trim that frames the panels.
Squares = (wall length × height + gables × ½ × base × rise − openings) × (1 + waste) ÷ 100 ft²Each window is counted as 15 ft² (1.4 m²) and each door as 21 ft² (1.9 m²). Starter strip runs along the bottom of every wall, so its length equals the wall length. J-channel surrounds windows (about 16 ft or 4.8 m per window), the sides and head of doors (17 ft or 5.1 m) and the sloping rakes of each gable. Trim is counted in 12 ft 6 in (3.81 m) lengths and outside corner posts in 10 ft (3.05 m) lengths.
Worked example
A single-storey house has 40 m of wall, 2.6 m high: 104 m². Its two gables are 8 m wide and rise 2.5 m, adding 2 × ½ × 8 × 2.5 = 20 m². Eight windows and two doors remove 15 m², leaving 109 m² of net wall. With 10% waste the order is 119.9 m², or 12.91 squares, so seven two-square cartons of panels. Starter strip: 40 ÷ 3.81 = 11 pieces. J-channel: 38.4 m round the windows, 10.2 m round the doors and 18.9 m of gable rakes make 67.5 m, or 18 pieces. Four corners at single-storey height take four posts.
Panel sizes and coverage
| Product | Exposure | Panel length | Panels per square |
|---|---|---|---|
| Vinyl double 4 in (D4) | 8 in | 12 ft 6 in | about 12 |
| Vinyl double 5 in (D5) | 10 in | 12 ft | 10 |
| Vinyl vertical board and batten | about 12 in | 10 ft or 12 ft | 8–10 |
| Fibre-cement lap plank 8.25 in | 7 in | 12 ft | about 14.3 |
| Wood clapboard 6 in | 4.5–5 in | random lengths | about 250 linear ft |
Always use the exposure, not the full board width, because each course overlaps the one below.
Tips and frequent errors
- Forgetting the gables. On a typical house the triangles add 15–25% to the wall area and use more panels because of angled cuts.
- Subtracting small openings. Vents, meter boxes and small bathroom windows are faster to count as wall; the offcuts are wasted anyway.
- Nailing vinyl tight. Vinyl expands and contracts with temperature. Nail in the centre of the slots and leave about 1 mm (1/32 in) between the nail head and the panel so it can slide; leave a gap at J-channels and corners.
- Skipping the other trim. Also plan undersill trim below windows, soffit and fascia at the eaves, and flashing tape around openings behind the J-channel.
- Buying from different runs. Colour can vary between production batches; order the full quantity plus a spare carton at once.
Siding on tall walls requires scaffolding or proper staging; follow local building rules on water-resistive barriers and fire separation near boundaries.
Frequently asked questions
How many squares of siding do I need for a 1,500 sq ft house?
Floor area does not tell you wall area. A 1,500 ft² single-storey house of about 30 × 50 ft has 160 ft of wall; at 8 ft high that is 1,280 ft², plus gables, minus openings, usually 13 to 15 squares with waste.
How many square feet are in a square of siding?
One square is 100 ft², about 9.29 m². A carton of double 4 in vinyl siding usually covers two squares.
How much J-channel do I need?
Count about 16 ft for each average window, 17 ft for each door and the full length of every gable rake. Divide the total by 12.5 ft to get the number of pieces.
Should I subtract windows and doors when measuring for siding?
Subtract large openings such as doors, patio doors and big windows. Small openings can be ignored because the offcuts around them are rarely reusable.
How much extra siding should I buy?
Add 10% for a plain rectangular house and 15% or more when there are several gables, dormers or angled walls.
Last reviewed: 2026-09-27 · How we calculate