Measure each wall elevation as its own rectangle, add the gable triangles, and subtract the windows and doors. That net siding area is the measurement record the paint quantity step works from.
How do you measure the siding area?
Walk each elevation of the house and measure its length, then measure the wall height from the foundation to the eave line. Multiply the two for that elevation, and add the elevations together.
For a plain rectangular house the shortcut gives the same total: Behr's method measures the perimeter and multiplies by the height from the foundation to the roofline. Use one unit for every measurement, and keep exact figures until the final total.
How do you measure gables?
Measure the gable base and the vertical height from the base to the peak. Multiply and divide by two for each gable, then add the total to the body measurement.
Gable area = gable base × gable height ÷ 2
Do not measure gable walls to the peak when you record the elevation. The wall rectangle stops at the eave line, and the triangle starts there.
Which openings do you subtract?
Subtract the windows and doors, because they take no body paint. Measure each opening's width and height and multiply for its area.
For standard openings, Behr's published tip uses 15 ft² per window and 20 ft² per door. Measure large openings individually instead: garage doors, sliders, and picture windows are far bigger than those averages, and a guess here can move the total by a whole wall.
Does the siding profile change the measurement?
The flat-plane area is the starting record, but it understates the true painted surface of lap siding, shakes, and stucco. Each lapped course adds its exposed face plus the butt edge, and every ridge and pit adds surface a flat tape line never touches.
Behr states that porous, rough, textured, or previously unpainted surfaces need more paint than smooth ones. So record the flat-plane area as measured, then use a coverage rate matched to the profile and the selected product at the quantity step. Benjamin Moore's Element Guard exterior lists 300 to 400 ft² per gallon across wood, vinyl, composite, and stucco siding. Do not invent a multiplier for the profile. The product's published rate carries the surface condition.
Worked example: 40 ft by 30 ft one-story house
A rectangular house measures 40 ft by 30 ft with 8 ft walls. Two gable ends sit on the 30 ft sides, each rising 4 ft. The house has 8 standard windows and 2 standard doors.
- Perimeter: 2 × (40 + 30) = 140 ft.
- Wall body: 140 × 8 = 1,120 ft².
- Gables: 2 × (30 × 4 ÷ 2) = 120 ft².
- Openings: 8 × 15 = 120 ft² of windows, plus 2 × 20 = 40 ft² of doors, for 160 ft².
- Net siding area: 1,120 + 120 − 160 = 1,080 ft².
- Metric check: 1,080 × 0.09290304 = 100.34 m².
The example uses Behr's 15 ft² and 20 ft² opening averages as fixtures, not a reader's project quantities. Measure the reader's own openings where they are nonstandard.
What do you measure separately?
Keep siding, trim, doors, and gutters on separate quantity lines. They use different paint and different coverage rates, and one combined area hides that.
| Line | How to measure |
|---|---|
| Siding body | Per-elevation length × height, plus gables, minus openings |
| Trim | Total trim length × 0.5 ft (Behr's rule for 6 in average trim width) |
| Entry doors | Each door's width × height |
| Gutters | About 1 ft² per linear foot (Behr's rule) |
Common exterior siding measurement mistakes
- Measuring gable walls to the peak, which double-counts the triangle on top of the rectangle.
- Using interior room dimensions, which miss the wall thickness and the eave line.
- Mixing units: feet for walls, inches for windows, then multiplying without converting.
- Rounding each elevation before adding, which can drop whole square feet from the total.
- Applying the 15 ft² and 20 ft² averages to garage doors, sliders, and picture windows.
- Rolling trim into the siding area, which mixes two different paint lines.
- Treating the flat-plane area as the painted surface on shakes or stucco, then ordering against a smooth-surface coverage rate.
Safety and professional scope
Measure upper elevations from the ground. Run the tape along the foundation line or use a laser distance meter for the height. Do not set a ladder just to read a dimension. Keep clear of overhead service lines when measuring near the eave.
This worksheet plans the measurement record. It does not select the paint product, set the coat count, or approve the surface preparation. The product data sheet and the project specification control those decisions.
Sources and scope
- Behr, How Much Exterior Paint Do I Need?: 250 to 400 ft² per gallon per coat; perimeter × height excluding gables; 15 ft² per window and 20 ft² per door averages; gable width × height ÷ 2 each; trim length × 0.5 ft; 1 ft² per linear foot of gutter; rough, textured, and unpainted surfaces need more paint. Accessed 2026-10-24.
- Benjamin Moore, Element Guard exterior paint product record: 300 to 400 ft² per gallon; 1 or 2 recommended coats; listed for wood, vinyl, composite, and stucco siding. Accessed 2026-10-24.
- Sherwin-Williams, How to Use Paint Calculator: typical gallon covers 350 to 400 ft²; primer covers 200 to 300 ft². Accessed 2026-10-24.
- NIST Handbook 44, 2026, Appendix C: square-foot to square-metre relationship for the worked-example conversion. Accessed 2026-10-24.
Scope: the manufacturer pages support the stated coverage ranges, the measurement method, and the opening and trim rules. The worked example uses calculation fixtures, not project quantities. No texture multiplier is given because no source in this pack publishes one; the product's own coverage rate carries the surface condition.
Review responsibility: Saleem Sial owns the published calculation, source, and editorial checks. Waseem Sial, External Reviewer and Engineer, remains listed for ongoing external review; no completed external-review date is claimed.
Next, use the paint coverage guide to check the product coverage rate for the selected paint, plan the room-by-room interior ledger with How Much Paint Do I Need?, or read the paint coats and primer guide before choosing the coat system. The Paint Quantity Planning hub holds the published tools.
