One house, one order, every figure shown. This worked example takes a 45 by 32 ft one-story house from the tape measure to the store receipt: elevation math, gable triangles, opening deductions, a primer line for 180 sq ft of bare patched wood, and separate body and trim orders.
How much paint does this house order?
13 gallons ordered: 10 gallons of body paint, 1 gallon of primer, and 2 gallons of trim paint. The exact need is 10.55 gallons. Container rounding and a stated 10 percent allowance account for the other 2.45.
What does the measurement record show?
The siding measurement guide owns this method: measure each elevation as its own rectangle, add the gable triangles, and subtract the windows and doors.
| Line | Math | Area |
|---|---|---|
| Wall body | 154 ft perimeter x 9 ft height | 1,386 ft² |
| Gables | 2 x (32 ft base x 5 ft height / 2) | 160 ft² |
| Openings | 12 windows x 15 ft² + 2 doors x 20 ft² | -220 ft² |
| Net siding | 1,386 + 160 - 220 | 1,326 ft² |
The record also carries two separate lines: 180 sq ft of bare patched wood that needs primer, and 180 sq ft of trim on its own line. The wall rectangles stop at the eave line, and the triangles start there. Measuring gable walls to the peak would double-count the 160 ft².
How many gallons does the body paint take?
Divide the net area by the label coverage rate, multiply by the coats, add the allowance, and round up to containers.
Body gallons = (net siding area / label coverage) x coats
The chosen paint lists 250 to 400 sq ft per gallon. This weathered lap siding uses 300, the mid-low of the range, as the stated example rate. The Behr Premium Plus exterior data sheet states that range and notes that porous, rough, textured, or previously unpainted surfaces need more paint than smooth ones.
Per coat: 1,326 / 300 = 4.42 gallons. Two coats: 8.84 gallons exact. With a 10 percent allowance: 8.84 x 1.10 = 9.72 gallons. Order 10 gallons: two 5-gallon pails.
What does the primer line cover?
The primer line covers the 180 sq ft of bare patched wood and nothing else. A repaint over sound, previously painted siding needs no primer. Bare wood and repairs do, per the siding paint and primer guide. The primer data sheet lists 300 to 400 sq ft per gallon; the example uses 350.
One coat: 180 / 350 = 0.51 gallons exact. With a 10 percent allowance: 0.51 x 1.10 = 0.57 gallons. Order 1 gallon.
What goes on the trim line?
Trim needs its own line because the trim paint differs in color and sheen from the body paint. The 180 sq ft of trim runs through the same formula separately.
Per coat: 180 / 300 = 0.6 gallons. Two coats: 1.2 gallons exact. With a 10 percent allowance: 1.2 x 1.10 = 1.32 gallons. Order 2 gallons: two 1-gallon cans.
How do you check the arithmetic?
Run the record against three independent checks before buying.
- Body coverage check: 10 gallons at 300 sq ft per gallon over 2 coats covers 10 x 300 / 2 = 1,500 sq ft. That clears the allowance-adjusted need of 1,326 x 1.10 = 1,458.6 sq ft.
- Primer coverage check: 1 gallon at 350 sq ft per gallon covers 350 sq ft, which clears 180 x 1.10 = 198 sq ft.
- Rounding check: 9.72 became 10, 0.57 became 1, and 1.32 became 2. Every line moved up. A line that rounds down is a shortage.
The three paths agree. The ordered 13 gallons cover the exact 10.55 with the stated allowance and real container sizes, and the 2.45-gallon surplus is fully accounted for.
Which mistakes change the order?
- Using the top of the label range. Running the body line at 400 sq ft per gallon drops the order to 8 gallons, which leaves weathered siding short.
- Priming the whole house. The primer line covers the 180 sq ft of bare patches, not the full 1,326 sq ft. Priming everything orders about 5 gallons instead of 1.
- Skipping the gable triangles. The 160 sq ft of gable ends is more than half a gallon per coat.
- Pooling body and trim before rounding. The trim paint is a different sheen and color. A shared 12-gallon order can still leave the trim line short.
Safety and professional scope
Quantity planning does not replace the product data sheet, the project specification, or a qualified painter. Follow the label for surface preparation, recoat times, and temperature limits. Lead paint on pre-1978 siding and work above one story carry their own safety rules and, in many jurisdictions, licensing requirements.
Sources and scope
- Behr Premium Plus Exterior Flat technical data sheet: 250-400 sq ft/gal coverage range with porosity and application limits; supports the 300 sq ft/gal example rate.
- KILZ 2 Latex Interior/Exterior Primer technical data sheet: 300-400 sq ft/gal coverage range with texture and porosity limits; supports the 350 sq ft/gal example rate.
- Benjamin Moore Element Guard Exterior Paint product page: 300-400 sq ft/gal coverage; 1 or 2 recommended coats.
- How to Measure for Exterior Siding Paint: Behr measuring method, gable rule, opening deductions, and trim rule. It is the measurement record this example starts from.
- Choosing Paint and Primer for Exterior Siding: primer rules by material.
- How Much Paint to Buy for Exterior Siding: order math, overage practice, and container rounding.
Source scope: The manufacturer pages support only the listed products and their stated coverage ranges and limits. The 10 percent overage is contractor practice, not a manufacturer specification. The 300 and 350 sq ft/gal figures are stated example assumptions. These sources do not select a product, coverage rate, coat system, overage, or container size for a reader's project.
Review note: Saleem Sial owns the research and editorial record. Source verification, independent calculation fixtures, build validation, link crawl, schema review and rendered QA form the internal publication gate. Waseem Sial, External Reviewer and Engineer, is listed for ongoing external review; no completed review date is claimed.
Next, use the siding measurement guide to record your own net area, the paint and primer guide to choose products, or the paint coverage guide to read the label range. The Paint Calculator runs this order math with the rates you enter.
