Field Guide
How Much Siding Do I Need? Squares, Gables & Trim
Siding is ordered by the square (100 sq ft of wall). Measure each wall, add the gables, take off the doors and windows, add waste, and convert to cartons or planks.
1. How to Calculate Siding (The Math Explained)
Here's the exact process this calculator uses, with a 30 ft gable end wall, 9 ft high, a 7.5 ft gable and two windows as the example.
-
1
Measure the wall
Multiply the length by the height, from the bottom of the siding to the eaves.
Wall = 30 ft × 9 ft = 270 sq ft -
2
Add the gable
A gable is a triangle: width × height ÷ 2.
Gable = 30 ft × 7.5 ft ÷ 2 = 112.5 sq ft -
3
Subtract openings and add waste
Take off about 15 sq ft per window and 21 sq ft per door.
Area = 270 + 112.5 − 2 × 15 = 352.5 sq ft With 10% waste = 387.75 sq ft = 3.9 squares Vinyl cartons = 387.75 ÷ 200 = 1.9 → 2 cartons
2. Siding Types & Coverage
| Siding | Sold as | Coverage |
|---|---|---|
| Vinyl | Cartons | Usually 2 squares (200 sq ft) per carton; check the label. |
| Fiber cement lap, 8¼ in | 12 ft planks | 7 in exposure: 7 sq ft per plank, about 14.3 per square. |
| Fiber cement lap, 6¼ in | 12 ft planks | 5 in exposure: 5 sq ft per plank, 20 per square. |
3. Corners, Window Trim & Starter
The calculator gives the main trim lengths, with the same waste allowance as the siding.
Outside corners: one post or pair of boards per outside corner, the full wall height.
Window & door trim: J-channel or casing around each opening: about 16 ft for a 3 × 5 ft window and 17 ft for a 3 × 7 ft door.
Starter strip: along the bottom of every wall to lock in the first course.
Pro tip: measure the gable height
You can work out a gable's height from the ground: half the wall width × the roof pitch ÷ 12. A 30 ft wide gable under a 6/12 roof rises 15 × 6 ÷ 12 = 7.5 ft.
4. Frequently Asked Questions
How do I calculate how much siding I need?
Multiply the distance around the house by the wall height, add the gables, and subtract the openings. A house 140 ft around with 9 ft walls is 1,260 sq ft; take off 2 doors and 12 windows (222 sq ft) and you have 1,038 sq ft. With 10% waste that's 1,142 sq ft, or 11.4 squares.
What is a square of siding?
A square is 100 sq ft of wall, the same unit used for roofing. Siding is priced and ordered by the square, and most vinyl siding cartons hold 2 squares.
How do I calculate the siding for a gable end?
A gable is a triangle: width × gable height ÷ 2. The gable height is half the width times the roof pitch ÷ 12, so a 30 ft wide gable under a 6/12 roof is 15 × 6 ÷ 12 = 7.5 ft high, and its area is 30 × 7.5 ÷ 2 = 112.5 sq ft.
How many boxes of vinyl siding do I need?
Divide the area to buy by the carton coverage, which is usually 2 squares (200 sq ft). 1,142 sq ft ÷ 200 = 5.7, so 6 cartons. Check the label; some profiles pack differently.
How many pieces of fiber cement siding do I need?
An 8¼ in lap plank, 12 ft long, set at a 7 in exposure covers 7 sq ft, or about 14.3 planks per square. 1,142 sq ft ÷ 7 = 163.1, so 164 planks. A 6¼ in plank at a 5 in exposure covers 5 sq ft.
Should I subtract windows and doors from siding?
Yes, for an accurate order. This calculator subtracts 21 sq ft per door, 15 sq ft per window and 112 sq ft per 16 × 7 ft garage door. Some installers skip small windows on purpose to leave a little extra for cuts.
What trim do I need for siding?
The main pieces are outside corner posts or boards (one per corner, the height of the wall), J-channel or casing around windows and doors, and starter strip along the bottom. You may also need inside corners, utility trim under windows and soffits, and soffit and fascia; add those from your plans.