Paint calculator.

Every paint calculator on the internet tells homeowners that premium paint stretches further. It does not. Sherwin-Williams Emerald publishes 350 to 400 square feet per gallon. Benjamin Moore Aura publishes 350 to 400 square feet per gallon. Sherwin-Williams ProMar 200, a contractor-grade budget line, publishes 350 to 400 square feet per gallon. The coverage band is identical because coverage is a function of film thickness, not quality, and all three paints are specified at 4 mils wet.

Premium paint is worth the money for a different reason. It hides in fewer passes. A high-pigment, high-resin formula like Aura or Emerald covers a color change on the first coat where budget flat needs two coats and maybe a touch-up. That reduces labor hours, which on a professional job is where the real cost lives. For a homeowner priming a nursery on a Saturday, the gallon count is the same; the afternoon is shorter with premium paint because the roller holds more paint and the wall is done sooner.

This calculator reflects published coverage, not sales copy. Budget tier rounds conservatively to 350 square feet per gallon, standard to 375, premium to 400. The spread exists inside the manufacturer-published band, not outside it. A gallon that claims to cover 500 square feet is a gallon that was mixed thinner than 4 mils, and a wall painted at 3 mils will not hide a color change regardless of what the label says.

Two coats is the right answer almost all the time. One coat hides on a color refresh where the new paint is the same color as the old paint and the only job is to clean up scuff marks and heel drag. One coat does not hide a color change. It does not hide new drywall. It does not hide a patched and sanded area. It does not hide trim overlap at the ceiling line, because the roller cannot reach the corner and the brush strokes print through the first pass.

The practical rule for a color change is two coats of finish paint over whatever the existing surface is, plus a primer coat if the color shift is severe or the substrate is new. Dark brown over off-white needs three coats without primer or two coats over a tinted primer. Bright red over eggshell needs three coats without primer or two coats over a gray-shaded primer. This calculator exposes the primer toggle for exactly this decision: flip it on for new drywall or dramatic color change, leave it off for same-color or minor shift refresh.

The classic homeowner mistake is buying one coat of paint, finishing the first pass, realizing the color is streaky, and then driving back to the store for a second gallon. The second gallon will be the same product but a slightly different batch. Batch-to-batch color variation on latex paint is small but visible on a wall under raking light, which is why the store employee rings up both gallons from the same batch when they are intermixed in the store. Buy all the gallons at once from the same production run and the color is consistent across the wall.

Textured walls are the single most common reason paint jobs run short. Orange peel texture adds roughly 10 to 20 percent to the wall surface because every bump is more painted area than a flat sheet of drywall. Knockdown texture adds more, sometimes 25 percent or more depending on how deep the knockdown is. None of this shows up in the length times width times height geometry, which computes the sheetrock area, not the painted area.

There is no manufacturer-published multiplier for texture, because texture depth varies so much that any single number is marketing, not engineering. The practical fix is the waste slider. For a smooth previously painted wall, leave the waste at 5 percent; that covers normal drips, roller loss, and the last-wall-of-the-day shortage that every job runs into. For light orange peel, bump it to 10. For heavy orange peel or knockdown, bump it to 15 or even 20. The slider is honest about what it does: it orders more paint per square foot than the geometry says, because the geometry is lying.

Primer is a different consideration. New drywall needs a PVA primer coat before any finish paint. Polyvinyl acetate primer is specifically formulated for new gypsum board: it seals the paper face, locks down any lifted fibers from sanding joint compound, and gives the finish paint a uniform porosity to roll out on. Skipping primer on new drywall means the joint compound absorbs more paint than the paper face, so the finish coat dries unevenly and the mud lines print through the topcoat. PVA primer is cheap, runs about 300 square feet per gallon on new board, and takes one coat. This calculator adds a line for primer gallons when the primer toggle is set to new drywall or color shift; the primer gallons are separate from the finish paint gallons and separate from the coat count. Primer is never a finish coat; a finish coat is never primer.

Homes built before 1978 may contain lead-based paint. The EPA Renovation, Repair and Painting rule, codified at 40 CFR Part 745 Subpart E, regulates how paint is disturbed in those homes. It applies to any renovation, repair, or painting that disturbs more than 6 square feet of interior painted surface or 20 square feet of exterior painted surface in target housing or child-occupied facilities. Target housing means housing constructed prior to 1978, with narrow exceptions for zero-bedroom dwellings and housing for the elderly or disabled unless a child under 6 resides there.

The rule is not a suggestion. Work practice standards require containment, certified renovators, and specific cleanup procedures. Firms performing the work must be EPA Lead-Safe certified. Homeowners doing work on their own residence are exempt from the firm certification requirement but are not exempt from the lead exposure, and the practical consequence of sanding old paint without containment is contaminated dust throughout the living space. Children and pregnant women are the high-risk population.

The first practical step for a pre-1978 home is a lead test. Lead test kits from a hardware store cost under 20 dollars and can rule lead out on most surfaces in 30 seconds. If the test is positive, the next step is a certified firm or a test by an EPA Lead-Safe certified inspector who can map the affected surfaces and scope the work properly. If the test is negative, ordinary prep and paint practices apply.

This calculator surfaces the RRP rule as an in-widget warning whenever the pre-1978 checkbox is flipped. The warning links directly to the eCFR text at ecfr.gov, which is the canonical regulation source. Homeowners and contractors who skip the rule because the calculator did not mention it are a liability the project will not carry.

An airless sprayer applies paint to a wall with a pressurized nozzle that atomizes the fluid into a fan of tiny droplets. Those droplets travel from the tip to the wall, and on the way, some of them miss. Overspray drifts sideways past the edge of the panel, bounces off the wall and into the air, or lands on the drop cloth instead of the substrate. Line priming eats another half-gallon to a gallon at the start of every session just to fill the hose. A rental Graco airless has 25 feet of hose that holds about 0.6 gallons of paint, and none of that paint hits a wall until the next day when the machine is cleaned.

The net material loss on a rental airless runs 20 to 40 percent versus a roller for the same wall. HVLP spray guns are gentler on material, typically 10 to 20 percent loss because the lower air pressure atomizes less aggressively, but rental market is dominated by airless, and the homeowner picking up a Graco at the big-box tool desk is going to run that machine airless unless the rental counter specifically hands them an HVLP setup.

This calculator handles that loss with a method toggle. Roller is the default because most DIY interior repaints use a roller. Switching the method to sprayer multiplies the total gallon count by 1.35 on top of the existing waste slider. Waste and sprayer loss are separate concerns: waste covers the inherent material loss of any application method (drips, roller-loading inefficiency, last-wall-of-the-day shortage), while the sprayer multiplier covers the application-specific loss that only happens when the material is atomized. A user spraying a textured wall bumps the waste slider to 15 percent and the sprayer multiplier stacks on top, so the total multiplier is 1.15 times 1.35, or about 1.55. That extra buy is not paranoia; it is the difference between finishing the job and driving back to the store mid-wall.

The classic failure mode is a homeowner who rents an airless for a whole-house repaint, reads a generic paint calculator, buys the gallon count it prints, and runs out halfway through the first coat on the third room. The fix is to size the job for the method. Roller user: keep the default. Sprayer user: flip the method to sprayer before buying paint.

Most paint calculators on the internet accept one room at a time. A homeowner doing a whole-house repaint runs the calculator six times, once per room, writes down the gallon count for each, and adds them up. That sum is wrong in a way that over-buys paint, not under-buys. Here is why.

Take six small rooms, each needing 1.2 gallons of paint. Running the calculator once per room rounds each one up to 2 gallons, because half-gallons do not exist at the retail counter. The user buys 12 gallons total. The correct answer is to sum the paintable square footage across all six rooms first, then apply coats and waste and coverage to the sum, and round up once at the end. Six times 1.2 gallons is 7.2 gallons, which rounds to 8 gallons. The single-room-at-a-time approach over-buys four gallons.

The over-buy is not a small concern. At standard-tier pricing, four extra gallons of interior latex is about 200 dollars of retail, which is often the entire paint budget for a small bedroom job. The leftover paint is not wasted in the long run because touch-up paint is useful for years, but ordering four extra gallons up front is not what the user asked for.

This calculator fixes the trap in multi-room mode. Set the primary room as usual, then click 'Add another room' and enter the next room. The calculator accumulates paintable square footage across all rooms in the job, applies coats, waste, and method multipliers to the running total, and rounds up once at the very end. A three-bedroom-plus-hall-plus-living-room repaint that would have ordered 12 gallons separately becomes an 8 or 9 gallon order combined.

One subtlety: the math about interior doorways is sometimes misunderstood. Each room's walls have their own 21 square foot hole for a doorway, and subtracting that hole in each room is correct because the doorway genuinely removes wall area from both sides of the wall. This is not double-counting. The multi-room fix is about rounding, not about shared openings. The calculator still subtracts every door and window per room, as it should.

Retail paint is priced and sized around the gallon. Quarts exist, but they are reserved for trim, touch-ups, and small accent walls; quart pricing per square foot is roughly double the gallon pricing, so any project that fits a gallon should buy a gallon. The one exception is touch-up paint from the original batch, which is worth saving for scuff repair even if the job only needed four gallons and the fourth gallon is mostly full at the end.

The calculator rounds up to the next whole gallon for a reason. A 2.1 gallon calculation is a 3 gallon order. Trying to buy 2 gallons and hoping the last wall stretches is how jobs end on a Sunday afternoon with one wall at an obviously lighter shade. A gallon of finish paint is cheap compared to the labor of driving to the store, and a gallon of unused paint stored properly is the cheapest touch-up resource a home will ever have.

Store the leftover with a tight lid and the paint stays usable for two to three years. Write the room name on the can with a marker. Resist the urge to pour leftovers into a smaller container for storage; the paint dries out faster in a partially full can than a smaller full can because of the air volume above the surface. Flip the sealed can upside down once and then store it right side up; the paint forms a thin film across the top that seals the rest of the gallon against air, which works surprisingly well and is how professional painters store touch-up paint.

One final note on price. The shelf price for interior latex swings hard by region and by retailer. Home Depot Behr pricing is usually the low end of the national range; Sherwin-Williams store pricing is midrange and variable by contractor discount; Benjamin Moore pricing is the high end at the store counter. This calculator uses mid-band national values and scales them by a regional materials index when a ZIP is entered. The resulting number is a materials estimate, not a quote. Labor is the other half of a professional paint job and is not included in the output.