Recommended Free Tools
PC Slower Than It Used to Be?
Free scan · Windows 10 & 11
Outdated Drivers Are Slowing You Down
Recommended Free Tools
Free scan · exact hardware match
Windows Errors? Fix Them Before They Spread
Free scan · no reinstall
ThatPainter is reader-supported. When you buy through links on our site, we may earn an affiliate commission. Learn More
To estimate paint, measure each paintable surface in square feet, subtract doors and windows when you know their dimensions, multiply the remaining area by the number of coats, and divide by the product label’s coverage per gallon. Round the result up to the can sizes you can buy. Add a practical allowance for rough, porous or highly textured surfaces, difficult color changes and touch-ups. Treat primer as its own coat and calculate it with the primer’s coverage rate.
As an Amazon Associate I earn from qualifying purchases.
The paint-gallon formula
The basic calculation is:
Gallons needed = (paintable area × number of coats) ÷ coverage per gallon
For a more cautious estimate that includes an allowance:
#1 Best Overall
- COMPLETE 5-PIECE BRUSH SET FOR EVERY PAINTING TASK — Includes 1 Inch Flat, 1.5 Inch Angle, 2 Inch Flat, 2 Inch Angle Stubby, and 2.5 Inch Angle brush giving you every size professional painters use daily. Get all 5 for less than the price of a single brush from hardware store brands — professional grade performance at a fraction of the cost. Perfect for walls, trim, cabinets, doors, furniture, and detailed cutting-in work on any project.
- ZERO BRISTLE LOSS — GUARANTEED BY PRECISION ADHESIVE BONDING — Every bristle is locked in place with precision-set adhesive that keeps them in the brush — not in your paint job. The coated steel ferrule provides structural support while the adhesive bond ensures clean performance on every stroke. Premium SRT synthetic filaments load more paint and release it evenly for a smooth streak-free finish on walls, ceilings, and trim. Works with latex, oil-based paint, primer, sealer, and wood stain.
- INCLUDES THE ANGLE STUBBY MOST SETS LEAVE OUT — Professional painters know the 2 Inch Angle Stubby is the go-to brush for tight cabinet interiors, window sashes, inside corners, and detailed trim where standard angle brushes are too long. Combined with the 1.5 and 2.5 inch angle sash brushes you have precision cutting-in covered at every scale — baseboards, crown molding, door casings, and ceiling lines.
- FLAT BRUSHES FOR BROAD SMOOTH COVERAGE ON ANY SURFACE — The 1 and 2 inch flat brushes deliver even coverage on walls, furniture, shelving, doors, fences, decks, and wood surfaces. Ideal for applying primers, stains, varnishes, polyurethane, and specialty coatings. Solid wood handles provide all-day comfort and control for both professional contractors and weekend DIY homeowners tackling interior or exterior painting projects.
- TRUSTED BY PROFESSIONAL PAINTERS AND CONTRACTORS NATIONWIDE — Pro Grade delivers professional-grade results at a price that makes sense for crews who go through brushes on every job. Ideal for property managers, remodeling contractors, apartment turnover crews, maintenance teams, and commercial painting operations. Backed by thousands of verified reviews from working painters. Washable and reusable for multiple projects.
Gallons needed = (wall area − measured openings) × coats ÷ coverage × (1 + waste allowance)
Use the coverage printed on the exact paint can or technical data sheet you plan to buy. Coverage varies by formulation, sheen, surface and application method, so a generic “one gallon always covers” rule can leave you short.
For example, 800 square feet with two coats at 400 square feet per gallon is (800 × 2) ÷ 400 = 4 gallons. If the label says 350 square feet per gallon instead, the same job requires (800 × 2) ÷ 350 = 4.57 gallons; purchase 5 gallons rather than rounding down.
Measure every surface before calculating
1. Measure each wall
Measure wall width and wall height in feet. Multiply each pair to get that wall’s area, then add all walls together.
Wall area = width × height
For a rectangular room, you can multiply the room’s perimeter by the wall height:
Wall area = (length + width + length + width) × height
Measure unusual sections separately. A half wall, stairwell, dormer or gable should be broken into rectangles and triangles rather than estimated as a single rectangle. Add the pieces after calculating each one.
Rank #2
- Great Value Pack Artist Paint Brushes: Soucolor 20 brushes in 10 sizes professional paintbrushes set — no shedding, holds paint well, cleans easily, Acrylic, watercolor, oil, gouache, Halloween pumpkin decoration & face painting, rock painting, ceramics. Fantastic high quality gifts for creative, artistic siblings to share!
- Awesome Set of Paint brushes: Professional paint brushes can cover all of your needs. The multi-purposed brushes are ideal for Watercolor, Oil, Gouache, enamel, Acrylic Painting, Body, Nail, Face Painting, Miniature, Model, Wood, Glass, Cup, Ceramic, Fine work, Art class, Craft Art Painting, DIY Art Crafts etc.
- Soft and long-lasting: The bristles are soft and strong, with nickel metal ferrules and blue wooden handles. After cleaning, the bristles will become soft and fluffy again and will not fall off.
- Easy to clean: it’s very convenient to Wash well in soap and water. Retain shape after drying. the brushes hold up very well. Nice selection to beginners and professionals. Keep them clean and they last a long time.
- Halloween Pumpkin Ceramic Crafts Supplies: A Great painting gift for family, friends, kids, girls, women, student, classmate, teacher. For using in school, travel or used in the home and office.
2. Subtract doors and windows
Measure the width and height of every door and window, multiply each pair, and add those opening areas. Subtract the total from the wall area only when you have reliable dimensions. If an opening is irregular or you cannot measure it accurately, leave a conservative margin instead of subtracting an optimistic guess.
Paintable wall area = total wall area − measured door and window area
Do not subtract trim automatically. If you will paint the casing, baseboard or sill, that trim is a separate paintable surface and must be measured and calculated separately.
Windows Errors? Fix Them Before They Spread
Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallOutdated Drivers Are Slowing You Down
One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware match3. Calculate ceilings, trim and doors separately
Ceiling area is usually length multiplied by width. Doors, baseboards, crown molding, cabinets and exterior sections should each have their own calculation. Manufacturer calculators commonly use different coverage assumptions for walls, ceilings, trim and exterior areas, so combining everything into one room number can be misleading.
Use the exact coverage on the paint label
Coverage figures are guides, not guarantees. Texture, porosity, application method, color and the condition of the existing coating can change how much paint a gallon actually spreads.
| Published coverage guidance | Qualification | How to use it |
|---|---|---|
| 350–400 sq. ft. per gallon | Sherwin-Williams; year not stated on the cited page | Use the lower end when the surface is absorbent, textured or the color change is demanding. |
| 400–450 sq. ft. per gallon for one coat | Benjamin Moore, 2012 | Multiply the one-coat area by your planned number of coats before dividing by coverage. |
Because these are manufacturer guidelines, the label for your selected product takes priority over a general chart. A high-build coating, specialty finish or exterior product may specify a different spread rate. Read whether the stated coverage assumes one coat and whether it applies to a smooth, previously painted surface.
Count every coat, including primer
Topcoat coats
Multiply the paintable area by the number of finish coats you expect to apply. Two coats are common when changing color, covering a dark wall with a lighter color or seeking a uniform sheen, but the product directions and the existing surface determine the actual requirement.
Rank #3
- 【All sizes of art brushes】: The 10 sizes of multi-functional brushes have various shapes, making the mixing color easier, which is very suitable for artists, students, teenagers, children and painters at all levels. Wonderful Present ideas for friends and family who like painting!
- 【general brush set】: Professional paint brush can meet all your needs. Multifunctional brush is very suitable for watercolor, oil, gouache, acrylic painting, body, nail, face painting, miniature, model, ceramics, arts and crafts painting, etc.
- 【high quality process】: It is designed with anti falling bristles, anti-rust nickel ferrule and solid wood handle. You can use the brush to focus on the painting and always be creative.
- 【easy to maintain】: After using the brush, it is very convenient to clean the brush with warm soapy water and reshape the tip of the brush. Air dry the brush and place it on top of the top for long use.
- 【100% satisfaction guarantee - buy now】1 packs of set brushes, you can also get friendly customer service, any questions feel free to contact us, within 24 hours to help you! There is no risk in buying our paint brush kit.
Primer
Calculate primer as a separate product and a separate coat:
Primer gallons = (primer area × primer coats) ÷ primer coverage per gallon
Do not use the finish paint’s coverage rate for primer unless both labels explicitly give the same rate. Preparation products can have different spread rates, and a porous substrate may absorb more primer than a sealed wall.
Color and hiding power
A strong color change can require an additional coat or a tinted primer. Count each planned application in the formula. If you are uncertain whether one or two coats will hide the old color, price the project using two coats and confirm the recommendation on the label or with a paint-store representative.
The Tool Desk
Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Worked calculations
Simple 800-square-foot project
Suppose the measured paintable area is 800 square feet and you plan two coats.
- At 400 square feet per gallon:
(800 × 2) ÷ 400 = 4 gallons. - At 350 square feet per gallon:
(800 × 2) ÷ 350 = 4.57 gallons.
The first label gives an exact 4-gallon result. The second requires 5 gallons because 4.57 cannot be rounded down without risking an incomplete color batch or an unpainted section.
Published room example
A manufacturer guide uses a 12-by-10-foot room with 8-foot walls, one door and two windows. The gross wall area is 352 square feet. After the listed openings are removed, the guide identifies 302 square feet of paintable area and recommends two gallons for two coats. Your own door and window dimensions may differ, so reproduce the measurement rather than copying the quantity.
Rank #4
- Bates paint brushes come in a convenient 6 pieces with 3 flat and 3 angled bristle styles, including (1”, 1.5”, 2”) flat and (1.5”, 2”, 2.5”) angled bristles, perfect for tackling any surface painting or staining job.
- The sturdy natural bristles of each brush are designed to hold more paint, creating a smoother finish with fewer streaks and saving you time on the job.
- The lightweight wooden handles provide extra comfort and ease during long painting processes like walls, trims, cabinets, and doors.
- These versatile brushes are ready-to-use, easy to clean, and can be reused multiple times, making them economical as well as reliable tools for any painting project.
- Bates paint brushes can be used for both oil-based paints as well as latex stains and are ideal for home or commercial projects such as decks, fences or DIY painting jobs.
Room with a ceiling and trim
For a 12-by-10-foot room, the ceiling is 120 square feet. Calculate the walls after subtracting measured openings, then calculate the ceiling with the ceiling paint’s coverage. Measure baseboards and door casing separately if they will receive a different product. Add the gallons for each product only after each area has been calculated; do not assume one gallon of wall paint can be substituted for trim or ceiling material.
Primer plus two finish coats
Assume 600 square feet of wall area, one primer coat rated at 300 square feet per gallon and two finish coats rated at 400 square feet per gallon:
- Primer:
(600 × 1) ÷ 300 = 2 gallons. - Finish paint:
(600 × 2) ÷ 400 = 3 gallons.
Buy the primer and finish paint as separate quantities. If the surface is very porous, add an allowance to the primer calculation rather than silently reducing the finish-paint quantity.
When to add an allowance
Coverage decreases when a surface absorbs paint or makes application uneven. Plan an allowance when any of these conditions apply:
- Unpainted drywall, bare wood, masonry or another porous substrate.
- Heavy texture, stippling, brick, concrete block or a rough exterior.
- A dramatic change between old and new colors.
- Spraying or back-rolling, where transfer and overspray can increase consumption.
- Many corners, small sections or repairs that make roller loading less efficient.
- A need to reserve paint for future touch-ups.
Use the lower end of the label’s coverage range first. If you still expect significant loss, apply an explicit allowance to the formula. For example, 4 calculated gallons with a 10 percent allowance becomes 4 × 1.10 = 4.4 gallons, which still rounds up to the purchasable container combination that provides at least 4.4 gallons. The correct percentage depends on the product, surface and application; it is not a universal manufacturer guarantee.
Free tools Windows power users keep installed
One-click scans. No signup required.
On a smooth, previously painted wall in a similar color, the label rate may be a reasonable planning basis. On a first coat over a thirsty surface, the label’s best-case number is less likely to match real consumption.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Round up to real container sizes
Never round a fractional result down. If the calculation is 2.2 gallons, you need containers totaling at least 2.2 gallons. A practical purchase might be two 1-gallon cans plus a smaller container if the product is sold that way, or a larger can if that is the available size.
Best Value
- 2 Inch Angled Paint Brush for Precision Cutting-In – Designed for clean, sharp lines around trim, baseboards, ceilings, corners, and detailed edge work. The perfect size for control and coverage.
- Value 3-Pack for Pros & DIY Projects – Includes three professional 2-inch angled sash brushes — ideal for contractors, remodelers, and homeowners tackling multiple rooms or repeat paint jobs.
- Smooth Results on Walls, Trim & Cabinets – Great for interior wall painting, cabinet edges, furniture projects, and trim work where a smooth finish and crisp control matter most.
- Works with Latex Paint, Oil-Based Paint & Stains – Premium synthetic filaments hold more paint, reduce streaks, and provide a clean finish with latex paints, oil coatings, and wood stains.
- Works with Latex Paint, Oil-Based Paint & Stains – Premium synthetic filaments hold more paint, reduce streaks, and provide a clean finish with latex paints, oil coatings, and wood stains.
For a single color, buying enough paint from one batch can reduce visible variation between cans. Mix cans together in a clean larger bucket (often called boxing the paint) before rolling, following the product instructions. Keep a sealed, labeled remainder for touch-ups, including the color name, sheen and room where it was used.
Container sizes differ by product and retailer. Your final step is therefore to convert the calculated gallons into the available cans, not to assume every brand sells the same sizes.
What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
A repeatable worksheet
- List every wall, ceiling, door, trim run and exterior section that will receive paint.
- Record width and height for each rectangular surface; split irregular shapes into measurable pieces.
- Multiply dimensions and add the pieces to obtain gross area for each category.
- Measure every door and window that will not be painted and subtract those openings from the relevant wall totals.
- Write down the exact product, finish and coverage rate from each label.
- Enter the planned number of finish coats. Enter primer coats separately if primer is required.
- Multiply each area by its coats and divide by that product’s coverage per gallon.
- Apply a clearly stated allowance when texture, porosity, color change or application method warrants it.
- Round every product up to the purchasable can combination, preserving enough for a continuous color batch and touch-ups.
- Before checkout, compare the result with the product label and confirm unusual surfaces with a local paint-store representative.
Common calculation mistakes and fixes
Using floor area as wall area
A 12-by-10-foot floor is 120 square feet, but its walls are based on perimeter times height. Using 120 square feet for the walls can understate the requirement by several gallons. Calculate vertical surfaces from their own dimensions.
Forgetting the second coat
A coverage figure normally describes one coat. Multiply by two before dividing when two coats are planned. Do not divide the area by coverage and then assume the result already includes both applications.
Subtracting standard opening sizes without measuring
Doors and windows vary widely. Subtract only measured openings. If dimensions are unknown, keep the estimate conservative and retain the allowance.
Combining all products into one gallon number
Wall paint, ceiling paint, trim enamel and primer can have different rates and purposes. Keep separate line items so a wall-paint surplus is not mistaken for sufficient primer or trim coating.
Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsTrusting a calculator’s precision
Manufacturer calculators provide estimates. Behr explicitly labels its calculator results as estimates, and other manufacturers likewise describe coverage as guidance. A result such as 3.18 gallons does not mean the job can be completed with exactly 3.18 gallons; surface condition and can sizes control the purchase.
Rounding down to avoid leftovers
Running out halfway through a wall can create a visible lap or color mismatch. Round up, keep the same batch together where possible and store the remainder for repairs.
How to decide whether your estimate is conservative enough
Review the estimate against five questions:
- Did you measure every wall rather than multiply floor dimensions?
- Are door and window deductions based on actual measurements?
- Does the coverage rate come from the exact product label?
- Are primer, ceiling, trim and exterior areas calculated separately?
- Have you allowed for texture, porosity, color change and touch-ups?
If any answer is no, correct that item before buying. If all five are yes, the calculation is a sound planning estimate, but it remains an estimate until the product is applied. Recheck the final quantity against the label and ask the local paint counter about unusual substrates or colors.
Quick reference formulas
| Purpose | Formula |
|---|---|
| One wall | width × height |
| Rectangular room walls | perimeter × wall height |
| Opening area | opening width × opening height |
| Net wall area | gross wall area − measured openings |
| Product quantity | (net area × coats) ÷ label coverage per gallon |
| With allowance | (net area × coats ÷ coverage) × (1 + allowance) |
The dependable method is always the same: measure, subtract only what you can verify, separate surfaces and products, count every coat, use the exact label coverage, allow for real-world losses and round up to the cans available.
Quick wins for a faster PC:
Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
