Exterior Paint Calculator with Doors and Windows
Exterior painting estimates go wrong in one of two directions. Count the walls and forget the openings, and you buy far more paint than the siding will ever absorb. Subtract every window and door as if the siding behind them were missing, and you come up short with a half-painted gable end. The trick is subtracting openings the way a painter actually does, then adjusting for the texture of the siding.
Step 1: gross wall area
Walk the house and measure each wall separately, length times height from the foundation line to the soffit. Do not include the roof gables at the same time, because their triangular area is easier to figure on its own.
The two deductions above are working averages. A standard single-hung window is often 3 feet by 5 feet on the outside, or about 15 square feet; a 3/0 by 6/8 entry door is close to 21 square feet including the frame. Count a window with a wide surround and shutters a little higher, a small bathroom window lower. Precision here matters less than consistency, because these are small fractions of the wall.
Step 2: how much to subtract for openings
Here is where most estimates go wrong, and it is worth understanding why the calculator does not simply remove every opening in full.
| Opening | Actual area | Recommended deduction | Why the difference |
|---|---|---|---|
| Standard window | ~15 sq ft | 15 sq ft | Close to actual; trim and sill are painted separately |
| Large picture window | 30-45 sq ft | Half the area | Surround, flashing and casing still get painted |
| Exterior door, solid | ~21 sq ft | 21 sq ft | The slab usually gets painted, so count it back in |
| Sliding glass door | ~42 sq ft | 30 sq ft | Frame and track surround are still painted |
| Garage door, 16 ft | ~112 sq ft | 0 if painted | A painted garage door is a large surface of its own |
Treat a painted garage door or painted entry slab as a separate line rather than a deduction. Both are large flat surfaces that take real paint, and if you subtract them from the wall you will double-count the savings.
Step 3: match coverage to the siding
Coverage is the variable that moves the answer most, because exterior paints are engineered for specific substrates and textures. The figures below are per gallon, per coat, from real-world application rather than the best case on the label.
| Siding type | Coverage per gallon | Notes |
|---|---|---|
| Smooth lap or hardboard siding | 300-350 sq ft | Ideal; primer and paint both go further |
| Beaded or rough-sawn lap | 200-275 sq ft | Texture doubles the surface area in practice |
| Stucco, smooth finish | 200-300 sq ft | Very absorbent; a masonry primer helps |
| Cedar shake or shingle | 150-250 sq ft | Deep texture; often brushed or sprayed and back-brushed |
| Brick or concrete block | 150-250 sq ft | Porous and irregular; elastomeric products are common |
| Vinyl or aluminum siding | 350-400 sq ft | Smooth and sealed; the best coverage of any substrate |
Step 4: coats and the one-body-shop rule
Two coats is the standard recommendation for an exterior repaint and non-negotiable for a color change. A single coat of exterior paint thinned for spray application will not deliver the film thickness that resists weather. Buy all buckets from the same batch, because exterior color matching is done at the factory and an odd bucket can flash noticeably on a large sunny wall. Spraying is fast but consumes 15 to 25 percent more paint than brushing and rolling, so if your painter sprays, add that waste factor in.
A worked example
A single-story house, 60 feet by 30 feet, with 9 foot walls. The gross wall area is 2 x (60 + 30) x 9, or 1,620 square feet. It has 14 windows at roughly 15 square feet and two doors at 21, or about 252 square feet of openings, leaving 1,368 square feet of paintable siding. On smooth lap siding at 320 square feet per gallon per coat, one coat takes 4.3 gallons, two coats take 8.6, and rounding up with a 10 percent cushion gives 10 gallons. If that same house were cedar shake at 200 square feet per gallon, the total would climb to about 14 gallons, which is exactly why the calculator asks for siding type rather than assuming one number.
What the calculator still cannot see
- Failing paint and bare wood. Areas that need scraping, sanding, and priming are extra labor, often the largest line in an exterior job.
- Caulking and rot repair. Every joint and gap should be caulked before the finish coat; soft trim has to be replaced first.
- Trim, fascia and soffits. These are painted separately and often in a contrasting color; measure them on their own.
- Access. Two-story walls, steep lots, and roof valleys that need a ladder platform add labor, not paint.
From gallons to a bid
Once you have the gallon count, turn it into a price with a per-gallon material figure and the labor rate for exterior work. The Painting Cost Calculator does the labor and material math, and the Paint Calculator handles interior rooms if this is part of a larger project. Every other estimator in the category is listed on the painting tools index.