Article — Paint Calculator
Paint Calculator: Gallons of Paint for Any Room
A standard 12 by 14 foot bedroom with 8-foot ceilings needs about 2.7 gallons of paint for two coats, after deducting one door and one window. The math: wall area equals 2 × (length + width) × height, minus 20 square feet per door and 15 square feet per window, multiplied by the number of coats, divided by 350 square feet per gallon (the industry coverage standard for interior latex on smooth walls), with 10% added for waste and touch-ups.
This calculator handles those steps automatically and switches between US imperial (feet, gallons) and metric (meters, liters) units. The default coverage figure of 350 sq ft per gallon comes from manufacturer wet-film thickness specifications and matches what most premium interior latex paints actually deliver on a smooth, previously-painted surface.
How the paint calculator works
The calculator multiplies room perimeter by ceiling height to get the total wall area, subtracts opening allowances for doors and windows, multiplies by the number of coats, and divides by the coverage rate to get the volume of paint needed. A 10% waste factor on top accounts for roller and brush absorption, tray residue, drips, and the touch-up batch every project needs three weeks after completion.
The headline output is gallons (or liters). Stats below show wall area, opening deductions, paintable area, total surface covered (including coats), volume with and without waste, and the number of one-gallon cans (or 5-liter tins) to buy. Cans always round up — half a gallon left over for touch-ups is much better than running out mid-wall and creating a visible color seam from a new batch.
Paint coverage rates explained
Coverage is rated in square feet per gallon (or square meters per liter). The 350 sq ft per gallon figure used as the default is the industry standard for one coat of interior latex on a smooth, previously-painted wall, applied with a 3/8-inch nap roller at the manufacturer-specified wet-film thickness of 4 mils. That works out to 8.6 m² per liter — premium European emulsions often quote 10 m²/L for the same product class.
Real-world coverage drops below 350 sq ft/gal for textured walls, new drywall (which sucks up the first coat), masonry, and exterior surfaces. It rises above 400 for premium one-coat formulas (Benjamin Moore Aura, Sherwin-Williams Emerald) used on smooth previously-painted walls. Always check the back of the can — the manufacturer's number for that specific product is more accurate than a generic estimate.
Interior latex 350 sq ft/galPremium one-coat 400-450 sq ft/galDrywall primer 250-300 sq ft/galExterior latex 250-350 sq ft/galMasonry / stucco 150-250 sq ft/galMetric standard 10 m²/LHow many coats of paint
Two coats is the answer for almost every paint job. One coat is acceptable only when repainting the exact same color on the same surface with a premium product. Color changes always need two coats. Light paint over dark walls typically needs three coats, or a tinted primer plus two finish coats — going from navy to white in one coat will streak no matter how good the paint is.
New drywall is the other case where coat count goes up: it absorbs the first coat unevenly, and untreated joints show through. The proper sequence is one coat of PVA primer plus two finish coats. Same applies to bare wood trim, fresh patching compound, and water-stained ceilings. The calculator's coats input lets you set 1 through 5 — three is the right choice when going light over dark or covering stains.
Paint calculator room by room
The same formula scales from a powder room to a great room. A 5 × 7 ft bathroom with 8 ft ceilings has 192 sq ft of wall and needs about 1.2 gallons of paint for two coats — buy two quarts and one gallon, or just one gallon if you accept running close. A 14 × 18 ft master bedroom needs about 3.3 gallons, so a one-gallon plus a 2-gallon (or a 5-gallon bucket with leftover for an accent wall later).
Open-plan living areas with cathedral ceilings change the math because wall height varies. Measure to the highest point and use that height — overbuying a quart is much cheaper than scaffolding back in for a missed strip near the peak. Stairwells with vaulted ceilings often hide 50+ sq ft of wall you forget to count until you are halfway through and notice the wall keeps going up.
The 350 sq ft per gallon coverage standard dates to 1930s ASTM testing protocols developed for oil-based paints. Modern latex formulations are physically capable of 500+ sq ft per gallon at very thin films, but coverage at that rate is poor (the dry color reads as a tint, not a finish coat). Manufacturers calibrate "spread rate" to deliver acceptable hide at 350 sq ft/gal, which is why the number has been stable for ninety years.
Interior vs exterior paint coverage
Exterior paint covers 100 to 150 fewer square feet per gallon than interior latex. Exterior surfaces are rougher (rough-sawn siding, textured stucco, masonry), they absorb more paint, and the formulations themselves are thicker because they need to resist UV, water, and freeze-thaw cycles. Plan on 250 to 350 sq ft per gallon for exterior latex on wood siding, 150 to 250 for masonry and stucco.
The trim-versus-body calculation matters too. Exterior trim and fascia eat paint disproportionately — a 2,000 sq ft body might need 8 gallons, but the trim around windows, doors, and rafter tails on the same house easily takes another 2 gallons. Plan the trim separately, especially if you are using a different sheen (eggshell for body, semi-gloss for trim is standard).
Paint cost by brand and grade
Standard interior latex runs $25 to $45 per gallon at Home Depot and Lowe's (Behr, Glidden, Valspar Signature). Premium brands push into the $50 to $80 range (Benjamin Moore Regal Select, Sherwin-Williams Cashmere). One-coat premium formulas hit $80 to $120 per gallon (Aura, Emerald). UK and European premium designer paints (Farrow & Ball, Little Greene) start at £95 per 5-liter tin, which converts to roughly $120 per gallon.
A typical bedroom repaint runs $75 to $250 in paint depending on grade, plus $30 to $80 in supplies for the first room (rollers, brushes, painter tape, drop cloths, tray, edger pad). Per gallon, premium paint costs 2 to 3 times more than the bottom of the line, but the better hide often saves a third coat, and the durability difference shows after 5 to 7 years.
- Standard coverage = 350 sq ft per gallon (8.6 m² per liter)
- Premium one-coat = 400 to 450 sq ft per gallon
- Door deduction = 20 sq ft each (1.86 m²)
- Window deduction = 15 sq ft each (1.39 m²)
- Waste allowance = 10% for roller absorption and touch-ups
- Two coats = standard for most color changes and new walls
- Standard interior latex = $25 to $45 per gallon (US 2024-2026)
- 5-gallon bucket = covers 1,750 sq ft at standard rate
Common paint calculator mistakes
The first mistake is forgetting to multiply by the number of coats — calculating one-coat coverage and ordering that amount leaves you a gallon short. The second is skipping the waste factor, which produces tight orders that run dry just before the last wall. The third is using exterior coverage rates for interior paint (or vice versa), which throws the math off by 30 to 40%.
Paint colors are mixed at the store from base + tints. Two cans tinted to the same formula on different days can have visible color differences once on the wall — slightly different humidity, different tint volumes, or different operators all show up. For projects over 2 gallons, ask the store to "box" the cans together (pour into a 5-gallon bucket and stir) to homogenize before painting. For projects above 5 gallons, the store should do this automatically.
The fourth mistake is buying cheap paint to save $80 and then needing a third coat to cover, which uses 50% more paint and ends up costing more. The fifth is ignoring primer on bare drywall — even self-priming paint needs a true PVA primer on fresh joint compound to keep gloss differences (flashing) from showing up. The sixth is starting a project with one gallon "to see if it's enough" — a second batch tinted weeks later will not match.
For very large projects, the cost-per-square-foot of paint matters more than the cost-per-gallon. A $90 one-coat premium formula at 450 sq ft/gal is 20 cents per square foot per coat; a $30 standard latex at 350 sq ft/gal is 8.6 cents per square foot per coat — but the standard product often needs three coats where the premium needs two, so installed cost can swing the math back toward premium.