How much paint do I need for a 12x12 room?

A 12 ft × 12 ft room with 8-foot ceilings, one door, and two windows has roughly 334 square feet of net paintable wall area. At 375 sq ft per gallon coverage, that requires about 1.78 gallons for two coats — round up to 2 gallons for your purchase.

How many square feet does one gallon of paint cover?

Most interior wall paints cover 350–400 square feet per gallon for a single coat on primed drywall. Always check your specific paint can's label, since coverage varies by brand, sheen (flat covers slightly more than gloss), and the texture of the surface being painted.

Do I really need two coats of paint?

In most cases, yes. A single coat rarely achieves full, even opacity, especially over a different existing color, patched drywall, or new drywall. Two coats is the standard recommendation from nearly all paint manufacturers for professional-looking, durable results, particularly for significant color changes.

How do I calculate paint for a whole house?

Calculate each room separately using its own perimeter, height, doors, and windows, since coverage needs vary by room size and opening count. Sum the gallons needed across all rooms, then add roughly 10% extra for touch-ups, trim work, and unexpected surfaces like closets or hallways.

Does paint color affect how much I need?

Yes. Dramatic color changes — especially painting a dark wall a light color, or vice versa — typically require a full third coat or a tinted primer first, since standard two-coat coverage may not fully hide the underlying color. Same-family color changes usually need only two coats.

How much extra paint should I buy for touch-ups?

Buy an extra quart, or round your gallon calculation up rather than down, to have leftover paint for scuffs and touch-ups. Store it in a sealed, labeled container in a temperature-stable location so it stays usable for future repairs.

Paint Calculator

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Calculate paint needed for a room

Length (ft)

Width (ft)

Height (ft)

Doors

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Standing in the paint aisle trying to guess how many gallons a room needs is a gamble that usually ends in either an extra trip back to the store mid-project or a leftover gallon gathering dust in the garage. Paint coverage depends on wall area, the number of coats, and how much space doors and windows take out of the total — variables that are easy to eyeball wrong, especially in rooms with vaulted ceilings or lots of trim.

Our paint calculator does the math for you. Enter your room's dimensions, ceiling height, and the number of doors and windows, and it calculates the net paintable wall area, then converts that into gallons based on standard paint coverage rates and how many coats you plan to apply. The result is a shopping list number you can trust before you're standing at the checkout counter.

This calculator works for a single room, an entire house's interior walls, or an exterior paint job — just adjust the dimensions and coverage rate for the surface you're painting.

Why Paint Calculator Matters

Paint is one of the more expensive materials per square foot in a home improvement project, especially with quality interior and exterior paints now commonly priced $40–$70 per gallon. Buying one gallon too many on a two-coat job can waste $50–$100, while buying too few means an interrupted painting session, a color-matching risk if you buy from a different batch later, and lost time driving back to the store mid-project with wet brushes waiting.

Accurate coverage estimates also affect the quality of the finished job. Painters who run short often thin their remaining paint to stretch it further, resulting in a thin, uneven second coat that shows lap marks and requires an additional coat to fix — which defeats the purpose of trying to save money in the first place. Calculating gallons needed up front, including a correct count for two coats (the standard for most color changes, especially light-to-dark or dark-to-light transitions), ensures consistent coverage and a professional-looking finish on the first attempt.

The Paint Calculator Formula, Explained

Wall area (sq ft) = (Perimeter × Height) − Door/Window area; Gallons needed = Wall area × Coats / Coverage per gallon

Start by calculating the room's total wall area: multiply the perimeter (the sum of all wall lengths) by the ceiling height. Then subtract the area taken up by doors and windows, since you don't paint through them — a standard interior door is about 20 square feet and a standard window is about 15 square feet, though you should measure actual openings for accuracy.

Divide the net paintable area by your paint's coverage rate, which is typically 350–400 square feet per gallon for one coat on primed drywall (check the specific product label, since coverage varies by paint brand, sheen, and surface texture). Multiply by the number of coats you plan to apply — most projects need 2 coats for full, even coverage, especially when changing colors significantly.

Always round the final gallon count up to the nearest whole gallon, since paint is sold in fixed can sizes and running even slightly short mid-coat creates visible touch-up seams.

How to Use the Paint Calculator: Step by Step

Measure the room's perimeter and height
Add up the length of all four walls to get the perimeter, then measure the ceiling height. Multiply perimeter × height to get the total gross wall area.
Subtract doors and windows
Count the doors and windows in the room and subtract their combined area (roughly 20 sq ft per door, 15 sq ft per window, or measure exact openings) from the gross wall area.
Check your paint's coverage rate
Look at the paint can label for the manufacturer's coverage rate, typically 350–400 sq ft per gallon; use 375 sq ft as a reasonable default if unknown.
Calculate gallons and coats needed
Divide net wall area by coverage per gallon for a one-coat estimate, then multiply by 2 for standard two-coat coverage, and round up to the nearest whole gallon.

Paint Calculator Examples: Real-World Scenarios

Standard Bedroom, Two Coats

A 12 ft × 12 ft bedroom with 8-foot ceilings, one door, and two windows needs two coats of a new color.

Perimeter:48 ft (2 × (12+12))

Height:8 ft

Openings:1 door (20 sq ft) + 2 windows (30 sq ft) = 50 sq ft

Coverage:375 sq ft/gallon

Calculation

Gross area = 48 × 8 = 384 sq ft; Net = 384 − 50 = 334 sq ft; 1 coat = 334 / 375 = 0.89 gal; 2 coats = 1.78 gal

Result

Buy 2 gallons of paint. This provides a small surplus for touch-ups, which is standard practice for any two-coat interior job.

Larger Living Room, Two Coats

A 15 ft × 20 ft living room with 9-foot ceilings, two doors, and four windows is being repainted with a lower-coverage matte paint.

Perimeter:70 ft (2 × (15+20))

Height:9 ft

Openings:2 doors (42 sq ft) + 4 windows (60 sq ft) = 102 sq ft

Coverage:350 sq ft/gallon

Calculation

Gross area = 70 × 9 = 630 sq ft; Net = 630 − 102 = 528 sq ft; 1 coat = 528 / 350 = 1.51 gal; 2 coats = 3.02 gal

Result

Buy 4 gallons (or a 3-gallon combination) to safely cover both coats without running short mid-application; a single 1-gallon can is not enough on its own for the second coat.

Common Mistakes to Avoid

Forgetting to subtract door and window area, which inflates the paint estimate and leads to buying more than necessary — though slight overbuying is safer than underbuying.
Assuming one coat is enough — most color changes, and nearly all light-to-dark or dark-to-light transitions, require two coats for even, opaque coverage.
Using the coverage rate for a smooth wall on a textured surface (like stucco or heavy orange-peel drywall texture), which absorbs significantly more paint and can require 20–30% more product than the can label suggests.

Tips & Tricks

Buy an extra pint or quart beyond your calculated gallons for future touch-ups — paint formulas and batch tinting can shift slightly over time, making exact color matches harder years later.
Ceilings and trim are painted separately and use different coverage rates and often different paint types (flat for ceilings, semi-gloss or satin for trim) — calculate them independently from wall paint.

A correct gallon estimate saves money, prevents mid-project store runs, and ensures your walls get the full, even coverage a fresh paint job deserves. Once you've planned your paint, check our drywall calculator for material needed to repair the walls first, or our tile calculator if the room includes a tiled accent or backsplash.

Paint Calculator — Frequently Asked Questions

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Authoritative References

NACHI — Interior Paint and Finish Inspection Guide

nachi.org — Home inspector reference on interior surface and finish standards

EPA — Lead-Safe Painting Practices

epa.gov — Official guidance on safe painting practices in older homes

External links open in a new tab. OmniCalc.us is not affiliated with these organisations.

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