FramersMath

FramersMath › Siding

Siding Calculator

Net wall area for siding, including the triangles at the gable ends, then squares to order and lap siding pieces at your exposure.

House

Walls
Perimeter: 40×30 house = 140'
Bottom of siding to soffit
0 for a hip roof
Rise per 12
Counted as 3' × 7'
Counted as 15 sq ft
Siding
Inches showing per course
10% simple, 15% many openings
Squares to order (with waste)—
Lap siding pieces—
Net siding area—
Walls—
Gables—
Doors and windows—
J-channel / casing—
Starter strip—

Estimates. Openings use typical sizes, so measure big windows and garage doors and subtract them yourself. J-channel assumes trim on the sides and top of each door (17') and all four sides of each window (14'). Corners, soffit and fascia are not included.

⚠ Check all math and verify with a licensed engineer and your local building department before cutting or building. Results are estimates.

Sources: Siding

  1. HardiePlank Lap Siding Best Practices GuideJames HardiePlank widths 5¼"–12" giving exposures of 4"–10¾" (1¼" lap), e.g. 8¼" plank at 7" exposure.
  2. Area formulasStandard mathWalls = perimeter × height; gable = ½ × width × (width ÷ 2 × pitch ÷ 12).
  3. Opening sizes and trim lengthsRule of thumbDoors 21 sq ft and 17' of J-channel; windows 15 sq ft and 14'.

Code tables are copied from the documents above and checked against a second copy where noted. Your local code, its edition and amendments govern. Check all math and verify with a licensed engineer and your building department. See the terms & disclaimer.

How to use the siding calculator

  1. Total wall length is the distance around the house. A 40' × 30' house is 40 + 30 + 40 + 30 = 140'.
  2. Wall height runs from the bottom of the siding to the soffit, not including the gable triangles.
  3. Gables: enter how many gable ends there are, their width and the roof pitch. The calculator adds each triangle for you.
  4. Doors and windows are subtracted at typical sizes.
  5. Exposure is how much of each course shows. A typical 8 1/4" fiber-cement plank is installed at about 7" exposure.

The math

Wall area = perimeter × height. A gable triangle is ½ × width × gable height, where the gable height is (width ÷ 2) × pitch ÷ 12. Subtract openings, add waste, and divide by 100 for squares. Lap siding pieces = area ÷ (exposure × piece length).

Example: a 40' × 30' house with 9' walls and two 30'-wide gables at 6/12 has 1,260 sq ft of wall plus 225 sq ft of gables. Subtract 2 doors and 10 windows (192 sq ft) for 1,293 sq ft. With 10% waste that's 14.22 squares, or 204 pieces of 12' lap siding at a 7" exposure.

Common exposures

Always follow the siding maker's instructions for laps, clearances, fastening and flashing. These are often warranty and code requirements.

FAQ

How do I calculate how much siding I need?

Multiply the house perimeter by the wall height, add the gable triangles (½ × width × gable height), subtract doors and windows, and add 10–15% waste. Divide by 100 to get squares.

How many squares of siding for a 40x30 house?

With 9' walls, two 30' gables at 6/12, 2 doors and 10 windows, it's about 1,293 sq ft net, or 14.22 squares with 10% waste.

How do I calculate gable siding?

Gable height = (gable width ÷ 2) × pitch ÷ 12. Area = ½ × width × gable height. A 30' gable at 6/12 is 7.5' tall and 112.5 sq ft.

How many pieces of lap siding are in a square?

Divide 100 sq ft by the exposure (in feet) times the piece length. At a 7" exposure, a 12' piece covers 7 sq ft, so a square takes about 14.3 pieces.

Related calculators

Disclaimer: Results are estimates for planning and layout only. Check all math yourself and verify with a licensed engineer and your local building department before cutting or building. FramersMath is not a substitute for engineered plans, span tables or code review. See the full disclaimer.