What to measure
How to use the siding calculator
Enter the house’s length and width and the wall height from the bottom of the siding to the eaves. Gable ends are the triangles under a gable roof: enter 2 for a plain gable house or 0 for a hip roof, then enter the roof pitch so the calculator can size them. Count windows and doors, set the waste and the coverage per box from the box label, and add a price per box for the cost.
For a two-story house, enter the full wall height. For an L-shaped house, enter the overall size, or work out each wing and add them.
The formula
- Walls = 2 × (length + width) × wall height
- Each gable = ½ × width × (width ÷ 2 × pitch ÷ 12)
- Net area = walls + gables − windows × 15 − doors × 20
- Squares = net area × (1 + waste) ÷ 100; boxes = net area × (1 + waste) ÷ 200, rounded up
Worked example: 40 × 30 ft house, 9 ft walls, 6/12 gables
- Walls: 2 × 70 × 9 = 1,260 sq ft
- Gables: the gable rises 15 × 0.5 = 7.5 ft; 2 × ½ × 30 × 7.5 = 225 sq ft
- Openings: 8 × 15 + 2 × 20 = 160 sq ft
- Net 1,260 + 225 − 160 = 1,325 sq ft → × 1.10 = 14.58 squares → 8 boxes
Siding by house size
| House | Sq ft | Squares | Boxes |
|---|---|---|---|
| 30 × 24 ft, 8 ft walls, 6 windows | 878 | 9.66 | 5 |
| 40 × 26 ft, 9 ft walls, 8 windows | 1,197 | 13.17 | 7 |
| 40 × 30 ft, 9 ft walls, 8 windows | 1,325 | 14.58 | 8 |
| 50 × 30 ft, 9 ft walls, 10 windows | 1,475 | 16.23 | 9 |
| 40 × 30 ft, two-story (18 ft), 14 windows | 2,495 | 27.45 | 14 |
| 60 × 32 ft, hip roof, 12 windows, 3 doors | 1,416 | 15.58 | 8 |
Two 6/12 gables and two doors unless noted; 10% waste, 200 sq ft boxes.
Common mistakes
- Forgetting the gables. On a gable house the triangles add hundreds of square feet. They are easy to miss when you measure only up to the eaves.
- Taking off small openings exactly. The panels you cut around a small window are mostly waste, so the net saving is less than the hole. Large openings are worth subtracting; tiny ones barely matter.
- Ordering panels only. Starter strip, corner posts, J-channel and trim are sold separately and can add a good share of the cost.
- Mixing boxes of different runs. Color can shift between production lots. Order everything at once, with enough extra to finish the job.
Frequently asked questions
What is a square of siding?
100 sq ft of wall coverage. Siding is priced and ordered by the square, and vinyl siding is commonly boxed at 200 sq ft, or two squares, per box.
How do I measure a house for siding?
Measure the length of each wall and the height from the bottom of the siding to the eaves. Add the triangle on each gable end, which is half the house width times the height of the gable, then take off doors and windows.
How much siding for a 40 × 30 ft house?
With 9 ft walls, two 6/12 gables, 8 windows and 2 doors, about 1,325 sq ft to cover. With 10% waste that's 14.58 squares, or 8 boxes of 200 sq ft.
How do I calculate the gable end area?
Half the width times the gable height. On a 30 ft wide house with a 6/12 roof, the gable rises 7.5 ft, so each gable is ½ × 30 × 7.5 = 112.5 sq ft.
Should I subtract windows and doors?
Yes, for large openings. The calculator takes off 15 sq ft per window and 20 sq ft per door; measure big picture windows and garage doors and count them as several. The waste allowance covers the offcuts around each opening.
How much extra siding should I buy?
The calculator adds 10% for cuts. Raise it for many corners, gables and openings; lower it for long, plain walls.
What else do I need besides siding panels?
Starter strip along the bottom of every wall, outside corner posts at each corner, J-channel around windows and doors, and trim at the eaves and gables. The calculator gives the starter strip and outside corner lengths.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.