Siding square footage calculator

Wall area in siding squares, for plain walls and gable ends. The gable triangle comes from your roof pitch, not a guessed peak height — and the calculator shows you what the usual peak-height shortcut over-orders.

01 How the math works

Siding square footage is wall area, one wall at a time. A plain wall is width × height. A gable end is that same rectangle up to the eave line, plus the triangle above it — and a gable is a rectangle with a triangle on top, which is the one shape most siding calculators quietly treat as just a taller rectangle.

The triangle's height is not a separate measurement. It is set by the roof pitch: the gable rises (width ÷ 2) × (pitch rise ÷ 12) above the eave. A 30 ft wide gable at a 6/12 pitch rises 7.5 ft (2.29 m) to the ridge. Give the calculator the pitch and it derives the triangle for you.

Siding is estimated and sold by the square — 100 ft² (9.29 m²), the same unit roofers use. Take the net area, add a waste factor, divide by 100, and round up to the next whole square. Per-box coverage varies by profile, so squares are the number that always holds; convert to boxes off the carton at the end.

02 Worked example

Take a gable end: 30 ft wide, 9 ft to the eave, over a 6/12 roof (9.14 m × 2.74 m). The rectangle below the eave is 270 ft² (25.08 m²). The gable rises 7.5 ft (2.29 m), so the triangle is ½ × 30 × 7.5 = 112.5 ft² (10.45 m²). Gross wall: 382.5 ft² (35.54 m²).

Deduct one 3×4 ft window — 12 ft² (1.11 m²) — and the net is 370.5 ft² (34.42 m²), or 3.71 squares. With a 10% waste factor that is 4.08 squares, which rounds up to 5 squares to order.

Now the shortcut most people reach for: full width × peak height. The ridge is 16.5 ft (5.03 m) up, so that reads 30 × 16.5 = 495 ft² (45.99 m²) gross — 112.5 ft² more than the wall actually has, because it counts the gable triangle twice. Net of the window and inflated 10%, the shortcut orders 6 squares. That is a full square — 100 ft² of siding — bought for a wall that does not exist.

03 When this calculator is wrong

The result uses a flat 10% waste factor, which holds for a long, mostly rectangular elevation. On a cut-up house — several gables, dormers, corners, and a course cut on the rake at every one — real waste runs closer to 15% or more, because every rake and corner turns a full course into offcuts. That is opinion O1 on this site: the standard 10% is calibrated for production work on simple geometry. The exception is a plain rectangular ranch wall, where 10% is right and sometimes generous.

• The gable is only as right as the pitch. The triangle scales with the roof: a 30 ft gable rises 7.5 ft at 6/12 (triangle 112.5 ft²) but 15 ft at 12/12 (triangle 225 ft² / 20.90 m²) — double the gable for the same wall width. Measure the pitch or read it off the plans; do not eyeball it.
• It deducts openings at full area. That is honest math, but siding waste eats most small openings anyway — the cut-around scrap roughly offsets a window you carefully subtracted. Common estimating practice leaves openings under about 10 ft² in the wall and lets the waste factor cover them; deduct only the doors, the picture windows, and the garage.
• One wall at a time. A gabled house has two gable ends plus the long eave walls between them. Run each wall, sum the net areas, then convert the total to squares once — rounding every wall up on its own over-orders.
• Squares are not boxes. The order figure is in squares (100 ft²). A box of vinyl covers a profile-specific area — often about 200 ft² (2 squares), but read the carton — and fiber-cement lap is sold by the plank off its exposure. Convert squares to the box or plank count on the label, and round up.
• Trim is a separate take-off. Starter strip, J-channel, corner posts, and soffit are linear-foot items measured off the perimeter and openings, not the field area this calculator returns.

04 What to do with the result

Add up every wall — both gable ends and the eave walls — before you convert anything. Net the openings that are worth netting (doors, big windows), sum the areas, divide by 100, add the waste factor, and round up to the next whole square. Then take the squares to the carton: divide by the box's stated coverage and round up to whole boxes. If the siding is a color-matched vinyl or a prefinished fiber cement, buy it in one lot run — a re-order rarely matches, the same way a paint batch doesn't. Order the trim off a separate perimeter take-off; the field-area number here does not include it.

05 Common questions

How do I calculate the square footage of a gable?

A gable end is a rectangle plus a triangle. The rectangle is width × eave height; the triangle is ½ × width × gable rise, where the gable rise is (width ÷ 2) × (pitch rise ÷ 12). A 30 ft gable at 6/12 rises 7.5 ft, so the triangle is 112.5 ft² on top of the 270 ft² rectangle.

How many squares of siding do I need for a house?

Add up the net area of every wall and divide by 100 — that is the square count. Then add a waste factor (10% for a simple elevation, 15% or more for a cut-up one) and round up. A 30 ft gable end at 6/12 nets about 3.71 squares; a whole house is the sum of all its walls.

Do you subtract windows and doors from siding square footage?

Subtract the large ones — doors, picture windows, the garage opening. Leave the small ones (under about 10 ft²) in the area and let the waste factor cover them, because the siding you cut around a small window becomes scrap that roughly cancels the opening.

How much extra siding should I buy for waste?

10% for a long, mostly rectangular wall. 15% or more for a house with several gables, dormers, and corners, where a course is cut on the roof angle at every rake. Buy at the higher end if the siding is a matched lot you cannot easily re-order.

What is a square of siding?

A square is 100 ft² (9.29 m²) of wall coverage — the estimating and sales unit for siding, the same square roofers use. It is not a box: per-box coverage depends on the profile, so squares are the unit that stays constant.