Siding Calculator

Background.

The installation manual published by the vinyl siding industry's trade association gives a piece of estimating advice that surprises first-time buyers: measure the walls with the windows in. Ordinary windows and doors are not usually deducted, because every one of them means cutting panels to fit around trim, and the offcuts from those cuts rarely fit anywhere else. Leaving the openings in the measurement quietly pays for that waste. Only very large openings, such as a garage door or a wide patio slider, are worth taking out.

That advice shapes how this estimate works. You give it the area of the walls, split into rectangular sections and triangular gables, plus anything oddly shaped. You subtract only the large openings, add a waste allowance, and the result is the area of siding to buy. From there it converts to the two units suppliers actually use: squares, the trade unit of 100 square feet, and cartons, whose coverage is printed on the box.

The cartons are where a tidy estimate meets reality. Nobody sells 6.09 cartons, so the count always rounds up, and the result shows how much coverage the whole cartons deliver and how much will be left over. For the example house, a simple gabled box with 1,360 square feet of wall, that means seven 200-square-foot cartons and 182.3 square feet to spare. Some of that spare will vanish into cuts; the rest is worth keeping in the garage for the day a branch cracks a panel.

The estimate covers the siding panels themselves. Starter strip, J-channel, corner posts, soffit, fascia, house wrap and flashing are ordered by length or by piece and need their own takeoff, and the way the siding is fastened and sealed is governed by the manufacturer's instructions and by the building code, not by its area.

What is siding calculator?

A siding estimate is a takeoff: a count of how much cladding material it takes to cover the outside walls of a building. It starts from the wall area in square feet, the sum of every rectangular wall section (width × height) and every triangular gable (half the base × the height), and adjusts it twice. Large openings are subtracted, and a waste allowance is added for cuts, breakage and short pieces at corners and rakes. The adjusted figure is then expressed in squares, where one square is 100 square feet or about 9.29 square metres, and in cartons, the packages siding is sold in. Carton coverage varies by product and profile, so it comes from the label rather than from any standard.

How to use this calculator.

1. 01Break the walls into rectangles: for each one, multiply width by height in feet and add the results together. Enter that total as the rectangular wall area. Measure across windows and doors as if they were not there.
2. 02Work out each triangular gable as 0.5 × base × height, where height is measured straight up from the eave line to the peak, and enter the sum. If you know the roof pitch instead, the height is half the gable's width × rise ÷ 12.
3. 03Put anything that is neither a rectangle nor a triangle, such as dormer cheeks or a bump-out you measured separately, in the custom area field.
4. 04Enter the area of large openings only, such as garage doors and sliding doors. Leave ordinary windows in the measurement; their offcuts are part of the waste.
5. 05Set the waste allowance. Ten percent is a common planning value; go higher for many gables, dormers or a steep roof, lower for long plain walls.
6. 06Enter the coverage per carton from the product label and, if you want a cost, the price per carton. The cost is shown in whatever currency that price is in.

The formula.

The material target is A = (R + G + C − O) × (1 + w/100), where R is the total rectangular wall area, G the total gable area, C any custom area, O the large-opening deduction and w the waste allowance in percent, all areas in square feet. R + G + C is the gross wall area. Subtracting O gives the net area, which cannot go below zero. Multiplying by (1 + w/100) adds the waste allowance on top of the net area, so 10% waste on 1,107 square feet adds 110.7 square feet, not 10.

The order matters. The waste allowance is applied after the openings come out, so a large garage door does not attract waste that would never be cut. From A the calculator derives three ordering figures. Squares are A ÷ 100. Cartons are A ÷ k rounded up to a whole number, where k is the coverage per carton. Ordered coverage is cartons × k, spare coverage is ordered coverage minus A, and material cost is cartons × the price per carton.

The rounding step decides more than the waste percentage often does. In the example, A ÷ 200 is 6.0885, and anything from just over 6 up to 7 still buys seven cartons. Raising the waste from 10% to 15% lifts A to 1,273.05 square feet and changes nothing about the order: seven cartons, 1,400 square feet, with 126.95 to spare instead of 182.3. Smaller cartons track the target more closely; with 100-square-foot cartons the same house needs 13, leaving 82.3 square feet over.

Two allowances can overlap. Leaving windows in the measurement already adds material for waste, and so does the percentage. Using both is a deliberate choice for complicated walls, not a requirement, and on a plain house it can tip the order into an extra carton.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Polymeric Exterior Products Association (n.d.). "How to Install Vinyl Siding: Getting Started." PEPA/VSI.
2. [2]Polymeric Exterior Products Association (2022). "2022 Vinyl Siding Installation Manual." PEPA/VSI.
3. [3]National Institute of Standards and Technology (2023). "NIST Handbook 133, Appendix E: General Tables of Units of Measurement." U.S. Department of Commerce.
4. [4]International Code Council (2024). "2024 International Residential Code, Chapter 7: Wall Covering." ICC.
5. [5]Federal Emergency Management Agency (2010). "Home Builder's Guide to Coastal Construction, Technical Fact Sheet 5.3: Siding Installation in High-Wind Regions." FEMA P-499.