How to measure for siding
The reliable method: add the rectangular walls (perimeter × the smallest of three height readings), add the gable triangles, subtract the openings, then round up and add waste. This tool does the arithmetic.
Using the smallest of your three wall heights (9.9 ft) on a 140 ft perimeter, adding the gables and subtracting the openings, gives about 1,322 sq ft — round the area up and add waste, then buy by the square. Measure each wall’s length and height, add the gable triangles, subtract the openings.
1 Enter your numbers
Every siding quantity and cost estimate is built on one number: the net area to side. Get that wrong and every square, box and dollar downstream is wrong too. The reliable method has three parts. First, the rectangular walls: the perimeter of the house times the wall height. Because a settled house is rarely perfectly square, take the height in three places and use the smallest reading — it keeps you from overstating a wall that leans in. Second, the gables: each gable is a triangle, half its base times its rise, and you add every gable, dormer and triangular section. Third, the openings: subtract the doors, windows and garage doors you will not side over.
What comes out is the net area — the true surface of siding you have to cover. It is still a bare area: before you buy, round it up and add a waste factor for cuts, corners and gables (see the waste-factor calculator), then convert to squares with the how-much-siding calculator or into boxes/bundles with the boxes-by-material calculator.
Formula
Net siding area, in square feet:
net_area = perimeter × min(h₁, h₂, h₃) + Σ(½ × gable_base × gable_height) − Σopenings
with the labeled deductions door = 21, window = 15 and double garage = 112 square feet. Using the smallest of three height readings is a deliberate conservative choice — it biases the area slightly high on a leaning wall so you do not run short. The result is floored at zero and is a bare area: round up and add waste before ordering.
Worked example
Take a house with a 140 ft perimeter and three height readings of 10.0, 9.9 and 10.1 ft. The tool uses the smallest, 9.9 ft, so the rectangular walls are 140 × 9.9 = 1,386 sq ft. Add 2 gables of 30 ft base × 8 ft rise: 2 × (½ × 30 × 8) = 240 sq ft. Subtract the openings — 10 windows (150), 2 doors (42) and 1 double garage (112) = 304 sq ft. Net area = 1,386 + 240 − 304 = 1,322 sq ft. That is the surface to side; round it up and add ~10% waste before you buy.
Measure first — avoid a re-order
Measure carefully and the rest is easy. Tips: measure each straight wall run and sum them for the perimeter rather than pacing it; take the wall height at corners and mid-wall, and trust the smallest; measure a gable’s width at the eaves and its rise to the peak, and count dormers as their own small gables. Common mistakes: forgetting a gable or a bump-out; deducting every tiny window and ending up short; using a single, optimistic height on a wall that has settled; and confusing this net area with the count of squares you order (that is area ÷ 100, rounded up, plus waste). This tool measures walls only — not the roof (that is exteriorcalcs) — and it plans quantity, not moisture, flashing or structural condition; follow the manufacturer’s installation instructions and hire a pro for anything behind the siding.
Reference table
| Opening | Deduct (sq ft) |
|---|---|
| Standard door (3 × 7) | 21 |
| Standard window | 15 |
| Patio / sliding-glass door | 40 |
| Single garage door (9 × 7) | 63 |
| Double garage door (16 × 7) | 112 |
Labeled typical areas — measure your actual openings. Deduct only full openings; skip small ones if you would rather carry the extra as waste.