Siding is estimated in squares – the trade’s packaging unit, where one square covers 100 square feet of wall. The math is wall area divided by 100, once you have measured the walls and subtracted the big openings. Here is the formula, a chart, and worked examples.
The quick answer
Squares of siding = wall area in square feet ÷ 100, rounded up. Wall area is each wall’s length x height, added up, minus the large openings like garage doors, with gables counted as triangles. For cutting waste and lap losses, a 10 percent example allowance works like this: multiply the area by 1.10 first, then divide by 100 and round up.
The formula
Wall area (sq ft) = sum of (wall length x wall height) – large openings + gable triangles (base x rise ÷ 2). Squares = wall area ÷ 100, rounded up. With an allowance, apply it before rounding: squares = area x 1.10 ÷ 100, rounded up.
Squares by wall area
Area-based minimum counts divide the wall area by 100; the allowance column multiplies by 1.10 before dividing and rounding up. The 10 percent is an example, not a universal waste rate.
| Wall area (sq ft) | Squares (area minimum) | Squares (with 10% allowance) |
|---|---|---|
| 500 | 5 | 6 |
| 800 | 8 | 9 |
| 1,000 | 10 | 11 |
| 1,500 | 15 | 17 |
| 2,000 | 20 | 22 |
| 2,500 | 25 | 28 |
Worked examples
Ranch house, 30 x 40 ft footprint
Perimeter: 140 ft. Walls 9 ft high: 140 x 9 = 1,260 sq ft. Subtract the big openings – a 16 x 7 garage door (112 sq ft), the entry door and windows at about 48 sq ft more – leaving about 1,100 sq ft. 1,100 ÷ 100 = 11 squares as the area-based minimum. With the 10 percent example allowance: 1,100 x 1.10 = 1,210, and 1,210 ÷ 100 = 12.1, so 13 squares. Gables, if any, add on top.
Gable end, 24 ft wide with a 6 ft rise
A gable is a triangle: 24 x 6 ÷ 2 = 72 sq ft, or 0.72 square. Two matching gable ends add about 1.44 squares – round up when you order.
Tips before you buy
- Measure wall by wall, including bump-outs and dormers as their own rectangles and triangles. A footprint perimeter is only a shortcut for a plain rectangle.
- Subtract the big openings like garage doors and large windows. Many estimators leave small windows in as a partial waste offset – pick one convention and stay consistent.
- The 10 percent figure is an example allowance, not a universal waste rate. Starter strips, laps, corners and angled cuts all eat material, and complex walls can need more.
- Panels and accessories are bought separately: vinyl panels come in lengths such as 12 ft and are sold by the square (often two squares to a carton), while starter strip, J-channel, corners and trim are separate pieces with their own counts.
- Squares measure coverage, not pieces. The same math works for vinyl, aluminum and other panel sidings sold by the square – check how your product is packaged.
Sources
- The Spruce: vinyl siding cost and the 100 sq ft square – the trade’s per-square packaging
- Ask Me DIY: how to estimate vinyl siding – 12 ft panel lengths and two squares per carton convention
- Example siding product sold per square – per-square packaging in practice
Related calculators and guides
- Square footage calculator – work out each wall first
- How many bundles of shingles do I need – roofing uses the same 100 sq ft square
- How many bricks do I need – if the walls are masonry instead
- How much baseboard do I need – the interior trim equivalent
Estimates only. This is general guidance, not professional advice. Confirm details with your supplier, installer or local building department.