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Paint Coverage Calculator: Calculate How Much Paint You Need

How many gallons of paint a room needs

ConstructionBy Numora construction teamReviewed by Numora architecture teamUpdated 

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Assumptions
m
m
m
m²
Gallons needed
3

Single coat — double for two

You'll need about 3 gallons of paint to cover 417 sq ft of wall, applying 2 coats (standard).

Paintable wall area417
Total coverage needed834
Exact gallons (before rounding)2,38

Paint coverage varies by brand, color, and surface. Deep reds and yellows often need an extra coat. Textured walls can reduce coverage by 20%. Always round up to avoid a mid-project trip to the store.

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⚡ The answer

A paint coverage calculator estimates the gallons of paint needed to cover the walls of a room. It computes total wall area, subtracts standard allowances for doors and windows, multiplies by the number of coats, and divides by the coverage rate printed on the paint can. The result rounds up to whole gallons.

Quick takeaway

This calculator provides a precise estimate of the total gallons of paint required to cover the walls of a rectangular room. It meticulously factors in key variables such as the room's length, width, and ceiling height, along with the desired number of paint coats. Crucially, it also accounts for deductions for non-paintable areas like doors and windows, using standard average dimensions. This tool is invaluable for both homeowners embarking on DIY projects and professional contractors, enabling them to accurately budget for paint purchases, minimize waste, and ensure they have sufficient supplies to complete their painting projects efficiently and without unexpected delays.

What is a paint coverage?

Use this paint coverage calculator to accurately figure out how many gallons of paint you'll need for any room or wall. This tool simplifies project planning by computing the total paintable square footage based on your room's length, width, and ceiling height, then subtracting standard allowances for doors and windows. You can customize the number of coats and the paint's coverage rate per gallon, ensuring a precise estimate. Our calculations assume the standard manufacturer rate of 350–400 sq ft per gallon for one coat, a figure commonly published on technical data sheets from leading brands like Sherwin-Williams and Behr, helping you avoid over- or under-buying.

The formula

Gallons = ceil( (2 × (L + W) × H − doors × 21 − windows × 15) × coats ÷ coverage )
  • L, W, H — room length, width, and ceiling height in feet
  • 21 — average interior door area (3 × 7 ft)
  • 15 — average window area (3 × 5 ft)
  • coverage — paint coverage per gallon in sq ft (typically 300–400)

Source: Standard Architectural Paint Coverage Calculation.

Worked examples

1A 14×12 bedroom with 9-ft ceilings

Inputs
length: 14width: 12height: 9doors: 1windows: 2coats: 2coverage: 350
Walkthrough

The room has 468 sq ft of total wall (perimeter 52 × height 9), minus 21 for the door and 30 for two windows, leaves 417 sq ft paintable. Two coats is 834 sq ft of coverage, which at 350/gallon is 2.38 gallons — rounding up, buy 3 gallons. Leftover covers touch-ups or the inevitable 'we should paint the closet too.'

2A large open-plan living room

Inputs
length: 22width: 16height: 10doors: 2windows: 4coats: 2coverage: 350
Walkthrough

Perimeter 76 × 10 ft ceiling = 760 sq ft. Minus 42 for two doors and 60 for four windows = 658 sq ft paintable. Two coats = 1,316 sq ft, at 350/gallon = 3.76 gallons — buy 4. For rooms over 600 sq ft paintable, consider a 5-gallon bucket; the per-gallon price is typically 15–25% lower.

3Small hallway with high ceilings and one door

Inputs
length: 10width: 4height: 10doors: 1windows: 0coats: 2coverage: 400
Walkthrough

A hallway with length 10 ft, width 4 ft, and height 10 ft has a perimeter of 28 ft. Total wall area is 280 sq ft. Subtracting 21 sq ft for one door leaves 259 sq ft paintable. For two coats, this is 518 sq ft of coverage. At 400 sq ft/gallon, you need 1.295 gallons, which rounds up to 2 gallons. Always ensure you have enough for two full coats, especially in high-traffic areas.

How to use this calculator

  1. Room length (default: 14)
  2. Room width (default: 12)
  3. Ceiling height (default: 9)
  4. Number of doors (default: 1)
  5. Number of windows (default: 2)
  6. Number of coats (default: 2)
  7. Read the result. Use the worked examples below to sanity-check against a known scenario.

Common mistakes and edge cases

Assuming one coat

For most color changes — including primer under a new color — two coats is standard. Going bold-over-white or dark-over-light usually needs three. Budgeting one coat almost always leaves you short.

Ignoring ceiling and trim

The calculator only covers walls. Ceilings typically need another 1–2 gallons for a room this size; baseboards and door frames take quarts of a different (semi-gloss) paint. Budget them separately.

Using new-paint coverage for a major color change

Covering a dark red with white needs at minimum one primer coat plus two topcoats — three separate purchases. The formula doesn't know about primer, so add a gallon of tinted primer when making a big shift.

Not accounting for surface texture

Highly textured walls (e.g., stucco, heavy knockdown) absorb more paint and can reduce coverage by 20-30%. Always factor this in by either reducing your coverage rate input or adding an extra coat.

Forgetting about waste and touch-ups

Even with careful application, some paint is lost to rollers, brushes, and spills. It's always wise to have a little extra for touch-ups down the line. Rounding up to the next whole gallon usually covers this.

Frequently asked questions

How do I measure an irregular room?
Break it into rectangles. Calculate each rectangle separately and add the paintable areas. For vaulted ceilings, measure both the normal wall area and the triangular area above; add them before dividing by coverage.
Is it cheaper to buy 5-gallon buckets?
Usually yes — about 15–25% less per gallon. Worth it if you need 3 or more gallons. Below that, individual gallons are more flexible for touch-up color matching.
How much paint do I need for trim and ceilings?
Trim: roughly 1 quart per 100 linear feet of baseboard plus door/window frames. Ceiling: treat it like a flat rectangle (length × width), two coats. A typical 14×12 bedroom ceiling takes about 1 gallon.
What if my paint can states a different coverage rate?
Always use the coverage rate printed on your specific paint can. Our default of 350 sq ft/gallon is a common average, but actual rates vary by brand and paint type. Inputting the correct value ensures the most accurate calculation.
Should I buy extra paint for touch-ups?
Yes, it's highly recommended. Even after rounding up, having a small amount of leftover paint (e.g., a quart or half-gallon) is invaluable for future touch-ups, especially if the paint color might be discontinued or difficult to match later.
How does surface texture affect paint coverage?
Textured surfaces, like stucco or heavily spackled walls, absorb more paint than smooth surfaces. This can reduce the effective coverage rate by 10-30%. If painting a textured wall, consider increasing the number of coats or reducing the 'Paint coverage per gallon' input.
Can this calculator be used for exterior painting?
While the basic area calculation is similar, exterior painting often involves different surface types (siding, brick, wood), different paint types (exterior-grade), and potentially more complex architectural features. This calculator is primarily designed for interior rectangular rooms. For exterior projects, consult specific exterior paint guides.
What's the difference between dry-to-touch and recoat time?
Dry-to-touch means the paint surface is no longer wet, but it's not fully cured or ready for another coat. Recoat time (usually 2-4 hours for latex) is the minimum time you should wait before applying the next coat to ensure proper adhesion and finish.

Paint Coverage glossary

Coverage rate
Square feet per gallon a paint can cover. Printed on the can. Interior latex is typically 300–400 sq ft; deeper colors and textured walls are at the low end.
Primer
A prep coat that improves adhesion and hides the previous color. Needed when making a major color change or painting over new drywall.
Flat / eggshell / satin / semi-gloss
Sheen levels. Flat hides imperfections best but is hard to clean; semi-gloss wipes down easily but shows every ding. Living areas usually use eggshell or satin.
VOCs
Volatile Organic Compounds. Chemicals released into the air from paint, which can be harmful. Low-VOC or Zero-VOC paints are healthier options.
Curing time
The time it takes for paint to fully harden and reach its maximum durability, which is often much longer than the dry-to-touch time (can be weeks).
Roller nap
The thickness of the roller cover's fibers. Shorter naps (1/4"-3/8") are for smooth surfaces, longer naps (1/2"-3/4") for textured surfaces.

How we built this calculator

Methodology

Wall area for a rectangular room is the perimeter times the ceiling height. Perimeter is twice the sum of length and width, because there are two pairs of opposite walls. Multiplying by height gives total wall surface.

This calculator was written by Numora construction team and reviewed by Numora architecture team before publication. Both names link to full bios with verifiable credentials.

Formula source
Standard Architectural Paint Coverage Calculation
Last reviewed
2026-04-24
Reviewer
Numora architecture team
Calculation runs
Client-side only
NC
WRITTEN BY
Numora construction team
NA
REVIEWED AND APPROVED BY
Numora architecture team
In this review:
  • Verified the formula matches Standard Architectural Paint Coverage Calculation (1.0).
  • Confirmed the rounding rule applied by the engine: Final gallon count is rounded up to the nearest whole gallon to ensure sufficient paint for the project and account for minor variations or touch-ups.
  • Recomputed all 3 worked examples by hand and confirmed the results match the engine.
  • Confirmed all 4 cited sources resolve to current pages on the issuing institution.
  • Validated all 2 test cases pass within the declared tolerance.

Reviewed on 2026-04-24 · Next review: 2027-04-24

See editorial policy

Sources & references

Every numeric assumption traces to a primary source.

  1. Sherwin-Williams: Paint Coverage Calculator & How to Measure for PaintUSA
  2. Behr: How Much Paint Do I Need?USA
  3. Benjamin Moore: How to Estimate PaintUSA
  4. Dulux: Paint CalculatorUK
  5. Numora Editorial Policy. numora.net/editorial-policy