How to calculate siding
Siding is sold by the square: 100 sq ft of wall coverage. To estimate it, add up the wall area, add the gable triangles, take away the windows and doors, then add waste. This siding calculator also counts the trim that goes around the openings and corners.
Gable area = base x height / 2
Squares = (walls + gables - openings) x (1 + waste %) / 100
J-channel = window perimeters + door sides and tops
Worked example
Four walls of 40, 30, 40 and 30 ft at 9 ft high give 1,260 sq ft. Two gables with a 30 ft base and 8 ft rise add 240 sq ft. Ten 3 x 5 windows and two 3 x 7 doors take away 192 sq ft, which leaves 1,308 sq ft. With 10% waste that is 1,439 sq ft, or 14.39 squares and 8 boxes at 2 squares a box. The openings need 194 lf of J-channel, or 16 pieces at 12.5 ft.
Assumptions
- Corner posts run the average wall height. Starter strip runs the base of every wall, less door widths.
- J-channel goes around all four sides of each window and the top and sides of each door. Some installers use utility trim under windows instead.
- Soffit, fascia, house wrap and accessories are separate line items.
Building these for a living? See Bolster for roofing and siding contractors, or how digital takeoff and AutoCost turn measurements into priced estimates.
FAQ
How do you calculate siding squares?
Multiply each wall width by its height, add half the base times the height for each gable, subtract windows and doors, add 10 to 15% waste, then divide by 100. One square of siding covers 100 sq ft.
How many boxes of siding do I need?
Divide the squares by the coverage per box and round up. Most vinyl siding packs 2 squares to a box, so 14.39 squares is 8 boxes. Check the manufacturer, since some profiles pack differently.
Should I subtract windows and doors from siding?
Yes for large openings, which is how this calculator works. Some contractors skip small windows and let that area cover waste instead. Either way, add the trim around every opening.
How much waste should I add for siding?
About 10% on simple, rectangular walls and 15% or more when there are many gables, dormers or short wall sections, because each cut leaves an offcut.
Can I turn this into a siding estimate?
Yes. Add unit prices here for a material total. In Bolster, AutoCost prices siding and trim with live regional costs, and the same takeoff becomes a proposal with options the homeowner can upgrade.
