How to measure a house for siding
- Walls: add up the length of every wall to be sided (the perimeter) and multiply by the wall height from the bottom of the siding to the eaves.
- Gables: each triangular gable end is width × height ÷ 2. Add dormers and bump-outs the same way.
- Openings: subtract doors and windows. A typical exterior door is about 21 sq ft (3 × 7 ft); windows vary, so measure the big ones.
- Add waste and convert to the unit your siding is sold in.
Siding formula
Gable area = Gables × Width × Height ÷ 2
Squares = (Wall area + Gable area − Openings) × (1 + waste) ÷ 100
Worked example: a two-story box with gables
A house with a 140 ft perimeter and 9 ft walls has 1,260 sq ft of wall. Two gables, each 30 ft wide and 7.5 ft high, add 2 × 30 × 7.5 ÷ 2 = 225 sq ft. Subtracting 2 doors (42 sq ft) and 10 windows (150 sq ft) leaves 1,293 sq ft. With 10% waste that is 1,422 sq ft, or 14.2 squares — 8 boxes of vinyl siding at 2 squares per box.
Squares, boxes and boards
- Squares: siding is priced and estimated by the square (100 sq ft), like roofing.
- Vinyl boxes: vinyl panels are commonly packed 2 squares (200 sq ft) per box. Check the carton — some profiles differ.
- Lap boards: each plank covers its length × its exposure (the visible face). A 12 ft fiber-cement board at 7 in exposure covers 12 × 7 ÷ 12 = 7 sq ft.
| Lap board | Typical exposure | Coverage per 12 ft board | Boards per square |
|---|---|---|---|
| 6¼ in plank | 5 in | 5.0 sq ft | 20 |
| 7¼ in plank | 6 in | 6.0 sq ft | 17 |
| 8¼ in plank | 7 in | 7.0 sq ft | 15 |
| 9¼ in plank | 8 in | 8.0 sq ft | 13 |
Exposure is set by the manufacturer's required overlap (often about 1¼ in). Use the exposure from the product's installation instructions.
How much extra siding to order
Ten percent is a common starting point for a simple rectangular house. Allow 15% or more for many gables, dormers, corners and short wall sections, or for diagonal and shake patterns. If you don't subtract small windows, you build in some extra automatically.
Trim and accessories
Order these separately, by the linear foot or piece:
- Outside and inside corner posts — the height of each corner.
- Starter strip — the length of the bottom edge.
- J-channel — around windows and doors, and along roof lines on gables.
- Soffit and fascia — the length of the eaves and rakes.
- House wrap, flashing tape and fasteners.
Tips
- Buy all siding in one order so the color and lot match.
- Install over a weather-resistive barrier and flash every window and door before siding.
- For gable roof framing, the rafter calculator gives gable height from span and pitch: height = span ÷ 2 × pitch ÷ 12.
Frequently asked questions
How do I calculate how much siding I need?
Multiply the wall perimeter by the wall height, add gable triangles (width × height ÷ 2), subtract doors and windows, add 10% waste and divide by 100 for squares.
How many square feet are in a square of siding?
One square of siding covers 100 sq ft of wall.
How many squares are in a box of vinyl siding?
Most vinyl siding boxes contain 2 squares (200 sq ft), but check the carton for your profile.
How many lap siding boards do I need per square?
About 15 boards of 12 ft at 7 in exposure, or 17 boards at 6 in exposure.
How much extra siding should I buy?
About 10% for simple houses and 15% or more for complex walls with many gables, corners and openings.
Sources & standards
This calculator's formulas and default values are based on the following codes, standards and published data. Codes are adopted and amended locally, so check with your building department for permitted work. See how we build and check our calculators.
- Vinyl Siding Institute (VSI), Vinyl Siding Installation Manual and ASTM D3679 (rigid PVC siding).
- Fiber-cement manufacturers’ installation instructions: lap exposure and overlap requirements; ASTM C1186.
- International Residential Code (IRC) R703: exterior wall coverings and water-resistive barriers.