About Siding Calculator (Squares, Planks & Trim)
The siding calculator counts the material to reside a house with horizontal lap siding: squares and planks from the net wall area, housewrap rolls, starter strip, and corner trim. Enter each wall elevation as length × height (up to 20), subtract the doors and windows, and pick an exposure — the weather-exposed height of each course. The presets follow common trade practice for fiber-cement lap planks: an 8.25-inch plank hung with a 1.25-inch headlap exposes 7 inches, and a 6.25-inch plank exposes 5 inches. Manufacturers publish their own minimum-lap tables, so check the wrapper before ordering; a custom exposure from 3 to 12 inches covers other products.
Coverage is figured as course-equivalent lineal feet: the net area in square feet times 12, divided by the exposure in inches, then increased by waste — 10% for simple rectangular walls, 15% for complex elevations with gables, dormers, and many cuts. Dividing by the plank length (12 or 16 ft) and rounding up gives planks to buy. Housewrap is deliberately counted from the GROSS area, openings included, because the wrap is installed across the openings and cut back to the window and door nailing flanges. Treat everything as a buying estimate, not a spec.
How It Works
- Add each wall elevation as length × height in feet, up to 20 walls. Gable ends can be entered as an equivalent rectangle (half the gable height).
- Add the door and window openings as width × height; their area is subtracted from the sided (net) area but not from the housewrap area.
- Pick the exposure: the 8.25-in plank preset (7-in exposure), the 6.25-in preset (5-in), or a custom value between 3 and 12 inches from your product’s lap table.
- Choose the plank length (12 or 16 ft) and the waste preset — 10% simple for plain rectangular walls, 15% complex for gables, dormers, and many cut-ups.
- Enter the number of trimmed corners (inside plus outside) and their height; corner trim is counted in 10-ft sticks.
- Read the results: net and gross area, siding squares, course-equivalent lineal feet, planks to buy, housewrap rolls, starter strip, and corner sticks. A chart compares the plank count for 12-ft versus 16-ft stock.
Worked Example
Two 40 × 10 ft walls give 800 ft² gross. A 3 × 7 door (21 ft²), a 3 × 5 window (15 ft²), and a 4 × 6 window (24 ft²) subtract 60 ft², so the net sided area is 740 ft² — 7.4 squares. With the 8.25-in plank preset (7-in exposure) and simple 10% waste, the siding is 740 × 12 / 7 × 1.1 = 1,395.4 lineal feet, which is ceil(1,395.4 / 12) = 117 twelve-foot planks. Housewrap is counted from the gross 800 ft²: ceil(800 / 1,215) = 1 roll. Starter strip runs the wall bases, 40 + 40 = 80 LF, and four 10-ft corners need ceil(4 × 10 / 10) = 4 corner sticks.
Formulas
- Wall area
A_gross = sum(l_i x h_i); A_net = A_gross - sum(openings); squares = A_net / 100- Exposure presets
exposure = plank width - 1.25 in headlap: 8.25 in -> 7 in; 6.25 in -> 5 in- Siding LF and planks
LF = A_net x 12 / exposure x (1 + waste); planks = ceil( LF / plank length )- Housewrap (gross area)
rolls = ceil( A_gross / 1215 ft2 ); 1215 = 9 x 150 roll x 0.9 overlap factor- Starter strip and corners
starter = sum(l_i) LF; corner sticks = ceil( corners x corner height / 10 ft )
Standards & References
- Exposure presets follow common lap-siding trade practice (plank width minus a 1.25-in headlap) — manufacturer installation instructions govern the minimum lap and exposure
- Housewrap counted per water-resistive barrier practice (IRC R703.2 requires a WRB behind the siding); roll coverage and the 0.9 overlap factor are representative
Frequently Asked Questions
How much siding do I need per square foot of wall?
Work in course-equivalent lineal feet: net wall area in ft² × 12 ÷ exposure in inches, plus waste. At a 7-inch exposure each square foot of wall takes 12/7 ≈ 1.71 LF of plank, so a 740 ft² net area needs 740 × 12 / 7 × 1.1 ≈ 1,395 LF with 10% waste — 117 twelve-foot planks. A smaller exposure means more courses and proportionally more material.
What is a square of siding?
A square is 100 square feet of finished wall — the unit siding is priced and sold by. Divide the net wall area (gross minus doors and windows) by 100: two 40 × 10 walls with 60 ft² of openings are 740 ft², or 7.4 squares. Suppliers usually round to the half or whole square and waste is added on top, so the plank count in this calculator is the number to actually buy.
Why is housewrap figured on the gross area instead of the net area?
Because the wrap goes on before the windows and doors are flashed: you run it straight across every opening, then cut it back and lap it into the rough opening so it tucks behind the nailing flanges. Skipping the openings would leave the count short and break the drainage laps. This calculator therefore divides the GROSS wall area — openings included — by the effective 1,215 ft² of a 9 × 150 ft roll (1,350 ft² less a 10% overlap allowance).
What exposure should I use for lap siding?
Exposure is the plank width minus the headlap. Trade practice for fiber-cement lap is a 1.25-inch minimum headlap, so an 8.25-inch plank exposes 7 inches and a 6.25-inch plank exposes 5 inches — the two presets here. Manufacturers publish the governing figure in their installation instructions, and some regions or exposures call for more lap; enter a custom exposure from 3 to 12 inches to match your product.
How much waste should I add for siding?
Use 10% (simple) for plain rectangular walls where most offcuts start the next course. Use 15% (complex) for elevations with gables, dormers, bays, and many openings — angled gable cuts in particular throw off unusable triangles. Waste covers cutting loss and breakage; factory-primed planks damaged in handling also come out of the same allowance.
How do I estimate siding for gable ends?
A gable is a triangle: width × (peak height above the eave) ÷ 2. The easy way in this calculator is to enter the gable as an equivalent rectangle — the full width times HALF the peak height — which gives the same area. Then switch the waste preset to complex (15%), because every gable course ends in an angled cut that rarely reuses.