Siding Calculator

The formula

Siding takeoff is an area problem with three adjustments. The sequence is:

Gross area = perimeter × wall height + gable area

Gable area (each) = 0.5 × base width × rise

Net area = gross area − openings

Order area = net area × (1 + waste %)

Squares = order area ÷ 100

A square is 100 square feet and it is the unit siding is actually sold and quoted in. Every supplier, estimator and installer talks in squares, so it is worth getting comfortable with: 1,800 square feet of wall is 18 squares.

The final step converts squares into purchasable units, which depends on the product. A box of vinyl siding covers two squares, or 200 sq ft, in some lines and one square in others — check the label, since this calculator uses 100 sq ft per box. A box of fiber cement or engineered-wood lap holds a set number of planks and typically covers about 49.2 sq ft. Cedar bevel comes in bundles nearer 32 sq ft. Board and batten is sold as 4 ft × 8 ft panels, 32 sq ft each.

A worked example

A simple two-storey rectangular house: 140 ft of wall perimeter, 9 ft high, two gables 28 ft wide with a 7 ft rise, two doors, ten windows, vinyl siding, 10% waste.

• Wall area: 140 × 9 = 1,260 sq ft
• Gables: 0.5 × 28 × 7 = 98 sq ft each, × 2 = 196 sq ft
• Gross: 1,260 + 196 = 1,456 sq ft
• Openings: (2 × 21) + (10 × 15) = 192 sq ft
• Net: 1,456 − 192 = 1,264 sq ft
• With 10% waste: 1,390 sq ft, which is 13.9 squares
• Vinyl at 100 sq ft per box: 14 boxes

Measuring the walls

Measure from the bottom of the starter strip to the underside of the soffit, not floor to ceiling — the difference is the rim joist and it is usually a foot or more per storey. On a two-storey wall, measure the whole height in one go rather than adding two storey heights and hoping.

For the perimeter, walk the building and record each wall separately, including returns, bump-outs and the sides of any porch or garage that gets siding. Skip the walls that do not get sided: a shared wall, a masonry base, or anything below a water table. Where a wall is a different height from its neighbour, treat it as its own rectangle and add the areas rather than averaging.

Gables are triangles, so half base times rise. If you cannot safely measure the rise, calculate it from the roof pitch: rise equals half the span multiplied by the pitch over twelve. A 28 ft span on a 6/12 roof has a rise of 14 × 6 ÷ 12 = 7 ft. Dormers, shed roofs and clipped gables each get measured as their own shape and added.

What the waste factor really covers

Ten percent is the working default for a rectangular house with straightforward walls. Push it to 15% when the walls are cut up with many windows, when the gables are steep, when the runs are short relative to the plank length, or when the pattern has to line up around corners. Fifteen to twenty percent is normal for board and batten, where the panel module rarely matches the wall width and every cut-off is scrap.

Some estimators skip the opening deduction entirely on houses with average window counts, on the grounds that the offcuts around openings are largely unusable anyway. Deducting the openings and then adding a realistic waste factor gets to nearly the same number, but it makes the assumption visible instead of hidden.

Ordering

Order all field material in one delivery from the same production run. Vinyl and fiber cement colours vary slightly between batches, and a wall pieced together from two lots can show a visible shift in raking light. Add one extra box beyond the calculated number for future repairs and store it in the attic or garage — matching a discontinued colour five years later is nearly impossible, and one box is cheap insurance.