About the Paint Calculator
The Paint Calculator estimates gallons needed for any room with per-surface breakdowns — walls, ceiling, trim — since each takes a different finish (eggshell/satin walls, flat ceiling, semi-gloss trim) and a different paint product. Inputs: room dimensions, ceiling height, door/window deductions (21 sq ft per standard door, 12 sq ft per window), coat count, and waste factor. Output: an optimized shopping list rounded to quart / gallon / 5-gallon can sizes.
It is built for DIY painters refreshing a single room before a move, whole-house repainters generating a shopping list across multiple rooms, exterior-painting homeowners (where one mid-project gallon shortage means an emergency Home Depot run), and contractors generating bid takeoffs. Multi-room mode sums materials across rooms with different ceiling heights or trim packages.
All calculations run locally in your browser. Room dimensions, paint type, and shopping list never leave your device. The page makes no network call after first load.
Standard coverage is 350 sq ft / gallon on smooth primed surfaces with one coat — in practice, plan on ~175 sq ft / gallon for two coats, which is what almost every job actually needs. Rough surfaces (stucco, popcorn ceilings, raw drywall) drop coverage to 250–300 sq ft. Always prime when covering dark colors with light, painting new drywall or patched spots, painting over glossy surfaces, or switching between oil and latex. Paint-and-primer-in-one is fine for like-color repaints; not a substitute for dedicated primer on bare surfaces. Buy 5-gallon buckets when projects exceed 3 gallons — the per-gallon discount is 25–35%.
Standard door = 21 sq ft (3×7). Standard window = 12 sq ft (3×4).
Add up to 8 rooms and get a combined shopping list.
Pick a base color to see complementary, analogous, and triadic palettes.
How Much Paint Do I Need? The Complete Formula
The standard formula for estimating paint is straightforward: calculate the total paintable surface area in square feet, then divide by the coverage rate on the can. For walls, multiply the perimeter of the room (2× width + 2× length) by the ceiling height, then subtract the area of every door and window. A standard interior door removes roughly 21 square feet and a typical window removes about 12 square feet. Multiply the net wall area by the number of coats and add a 10% waste factor for spills, touch-ups, and roller nap absorption.
For ceilings, multiply room width by length. Trim area is estimated by adding the perimeter’s linear footage and multiplying by the trim width (usually 3–6 inches converted to feet). This calculator automates every step and rounds up to the nearest purchasable can size so you buy exactly what you need.
Paint Coverage Rates by Finish Type
Coverage rates vary by sheen level because higher-gloss paints contain more binders and fewer solids. Flat and matte finishes cover the most area — typically 350–400 square feet per gallon. Eggshell and satin offer moderate coverage at 350–375 square feet per gallon with better moisture resistance. Semi-gloss and gloss cover 300–350 square feet per gallon but provide superior durability and scrub resistance, making them ideal for kitchens, bathrooms, and trim.
Surface texture significantly affects actual coverage. Smooth drywall absorbs less paint than textured knockdown or orange-peel finishes, which can reduce coverage by 15–25%. If your walls are heavily textured, add an extra 20% to the estimate.
When Do You Need Primer?
Primer is not always necessary, but skipping it in certain situations causes poor adhesion and uneven color. Always prime when painting over new drywall, when covering dark colors with a lighter shade, and after patching or repairing walls. Stained surfaces from water damage or smoke also require a stain-blocking primer. When painting the same color or a similar shade over properly prepped walls, you can usually skip primer and go straight to two coats.
How to Estimate Paint for Odd-Shaped Rooms
For L-shaped rooms, split the space into two rectangles and calculate each separately. Rooms with cathedral ceilings have more wall area above the standard 8-foot line — measure the tallest point and average it with the shortest. Bay windows and alcoves add extra wall segments; measure each flat panel individually. For stairwells, measure the wall height at its tallest point and use that for the full perimeter calculation.
Gallon vs. Quart: When to Size Down
If the calculator shows 1.3 gallons, purchasing one gallon and one quart is more economical than two full gallons. Most stores sell quarts at roughly 35–40% of the gallon price. However, if the total falls between 1.6 and 2.0 gallons, buying two gallons is cheaper per square foot. Keep in mind that leftover paint is valuable for touch-ups — a spare quart stored properly can last 5–10 years unopened.
Paint Calculator FAQ
- How many square feet does a gallon of paint cover? Most interior latex paints cover 350–400 square feet per gallon on smooth surfaces with a single coat.
- How many coats do I need? Two coats is the industry standard. One coat may suffice for same-color repainting; three coats are recommended for drastic color changes.
- Should I subtract closet doors? Yes. Any opening larger than 8 square feet should be deducted from paintable wall area.
- Does texture affect paint quantity? Heavily textured surfaces increase consumption by 15–25%. Raise the waste factor slider to 15–20%.
- Can I paint over wallpaper? It is possible but not recommended. Removing the wallpaper first and priming produces far better results.
Looking for related tools? Try our Renovation ROI Calculator to estimate the return on your painting project, or explore all Home & Real Estate tools.
Frequently Asked Questions
How much paint do I need per square foot?
One gallon of paint covers approximately 350 square feet with one coat on smooth, primed surfaces. For two coats, plan on 175 sq ft per gallon. Rough textures (stucco, rough drywall) cut coverage to 250 to 300 sq ft. A standard 12x12 room typically needs 2 gallons for walls plus 1 gallon for ceiling and trim.
Do I need primer?
Primer is required when: covering dark colors with light, painting new drywall or patched spots, painting over glossy surfaces, or switching between oil and latex. Modern paint-and-primer-in-one products work for repainting similar colors over existing paint, but they don't replace dedicated primer on bare surfaces.
How much paint do I need for an exterior house?
A typical 2,500 sq ft two-story home has about 1,500 to 2,000 sq ft of exterior siding. At 300 sq ft per gallon on rough siding with two coats, that's 10 to 14 gallons for the body plus 2 to 4 gallons for trim. Dark or textured siding needs more.
What's the coverage of a 5-gallon bucket?
5 gallons covers roughly 1,750 sq ft with one coat on smooth surfaces or 875 sq ft with two coats. That's enough for the walls of a 400 to 500 sq ft room (two coats) or an open-plan living-dining space. 5-gallon buckets save 25 to 35 percent per gallon over single gallons.
Do I need different paint for bathrooms?
Bathrooms and kitchens benefit from mildew-resistant paint with a satin or semi-gloss finish. These finishes handle moisture and scrubbing better than flat or eggshell. Specialty 'bath and kitchen' paint lines cost 5 to 10 dollars more per gallon but last significantly longer in wet environments.
