Siding Calculator

How Siding Is Measured and Sold

Siding is measured in squares, where 1 square equals 100 square feet of wall area. The calculator totals your wall area plus gable area (triangles at the ends of a sloped roof), subtracts the area of windows (default 15 sq ft each) and doors (default 21 sq ft each), applies a 10 percent waste factor, and divides by 100 to give squares. Order in whole squares because partial squares almost always come packaged as full boxes.

Vinyl, Fiber Cement, Wood, or Engineered Wood?

Vinyl is the cheapest, requires no painting, and lasts 20 to 30 years. Downsides: can fade in direct sun, warps in house fires, and looks plastic up close. Fiber cement (James Hardie, Allura) is made from cement and cellulose fiber. It costs more, needs periodic painting, and is heavy to install, but it resists rot, fire, and insects and lasts 30 to 50 years. Natural cedar wood siding looks best but costs the most to install and maintain. Engineered wood (LP SmartSide) is a middle ground — cheaper than fiber cement and paintable, with better impact resistance than vinyl.

Why Waste Factors Matter

Corners eat the most waste. A miter-cut lap siding corner loses 2 to 6 inches per course. Openings waste the cuts around windows and doors. Complex rooflines with multiple peaks and dormers waste the most — 15 percent is not unreasonable. A simple rectangular ranch with a shallow gable and 6 windows uses very close to 10 percent waste; a Victorian with 3 gables, bay windows, and 15 windows easily uses 20 percent. When in doubt, order an extra square and return it if you don't need it.

Planning Around Trim and Flashing

This calculator estimates only field siding. Trim (J-channel, corner posts, starter strip, window surrounds, soffit, fascia) is a separate order. For a 30 square field job, plan an additional 200 to 400 linear feet of J-channel, 60 to 120 feet of corner post, 60 to 120 feet of starter strip, and window trim sized to each opening. Flashing at windows, doors, and penetrations is critical for water management and must be installed before siding goes up — do not skip it.

Frequently Asked Questions

How much does siding cost per square?

Vinyl: 500-1,200 dollars installed; fiber cement: 800-1,500 installed.

Can I install siding myself?

Vinyl yes — it's DIY-friendly. Fiber cement is heavy and needs 2 people minimum plus a carbide blade.

How long does siding last?

Vinyl 20-30 years, fiber cement 30-50 years, cedar 15-25 years with maintenance.

Do I remove old siding first?

Usually yes — it's the only way to inspect sheathing for rot and install house wrap properly.

What's the best time of year to install siding?

Spring and fall. Vinyl gets brittle below 40°F; fiber cement cuts produce silica dust that needs dry weather.