Paint Calculator: How Many Gallons Do I Need?
To find how much paint you need, multiply the paintable wall area (room perimeter times height, minus doors and windows) by the number of coats, then divide by the coverage printed for your paint. Published coverage is 350–400 sq ft per gallon for Sherwin-Williams SuperPaint and 400–450 for Benjamin Moore Regal Select. Add an allowance for waste, then round up to whole cans.
Paint calculator
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How the paint calculator works
The calculator finds the wall area, takes off the doors and windows, multiplies by the coats and divides by how far a gallon (or litre) goes. It then adds your waste allowance and rounds up to whole cans.
wall area = 2 × (length + width) × height × rooms paint area = wall area − (doors × door area) − (windows × window area) ceiling = length × width × rooms (added to paint area only if ticked) volume = paint area × coats ÷ coverage buy = round up (volume × (1 + waste ÷ 100))
- coverage
- area one gallon (or litre) covers in one coat, from the can or data sheet
- waste
- allowance in percent for roller and tray loss and touch-ups
Worked example
Given: a 12 ft × 10 ft bedroom; 8 ft walls; 1 door 3 ft × 7 ft; 2 windows 3 ft × 4 ft; 2 coats; 350 sq ft per gallon; 10% waste allowance.
- Wall area:
2 × (12 + 10) × 8 = 352 sq ft - Doors and windows:
1 × 21 + 2 × 12 = 45 sq ft - Paintable area:
352 − 45 = 307 sq ft - Area to cover, two coats:
307 × 2 = 614 sq ft - Gallons before waste:
614 ÷ 350 = 1.75 gal - With 10% allowance:
1.75 × 1.10 = 1.93 gal
Answer: 1.93 gallons, so buy 2 gallons. At 400 sq ft per gallon the same bedroom needs 1.69 gallons with the allowance, which still rounds up to 2. If you tick the ceiling box the total climbs to about 2.7 gallons, so you would buy 3.
Published coverage and coat guidance
| Product | Sq ft per gallon | Coats |
|---|---|---|
| Sherwin-Williams SuperPaint Interior Latex (source 1) | 350–400 | Bare drywall: 2 coats, or primer then 2 |
| Benjamin Moore Regal Select Interior (sources 2, 3) | 400–450 | Two recommended; repaint: 1 or 2 |
Coverage is per coat; Sherwin-Williams measures it at 4 mils wet. Other brands and lines differ. The two ranges do not overlap fully, so the calculator starts at the lower figure. Treat the data sheet of your own paint as the final word.
Assumptions and limits
- Rectangular rooms with flat walls. Odd shapes: add the rooms up as separate rectangles.
- Coverage is per coat on a smooth, previously painted or primed wall. Rough, porous or very dark surfaces and spraying use more paint.
- Door and window sizes are your inputs. Trim, baseboards and cabinets are not included.
- The waste allowance (10%) and the primer coverage start values are planning defaults, not manufacturer data. Change them.
- Benjamin Moore’s own calculator assumes two coats and no ceiling and says needs vary with application method (source 4); we follow the same convention unless you change the inputs.
- Manufacturer figures are as published on 2026-11-13 and can change when products are reformulated.
Frequently asked questions
How much paint do I need for a 12 x 12 room?
A 12 x 12 ft room with 8 ft walls has 384 sq ft of wall before doors and windows (48 ft of wall times 8 ft). Two coats is 768 sq ft. At 350 sq ft per gallon that is about 2.2 gallons, and at 400 it is about 1.9, before any waste allowance. Subtract your doors and windows, or enter them in the calculator above, to refine the number.
How many square feet does a gallon of paint cover?
It depends on the product. Sherwin-Williams lists 350–400 sq ft per gallon for SuperPaint Interior Latex, and Benjamin Moore lists 400–450 sq ft for Regal Select Interior. Those are per coat, measured at the film thickness the data sheet gives, so rough or porous walls will use more. Always use the figure on the can or data sheet of the paint you actually buy.
How many coats of paint do I need?
Plan on two. Sherwin-Williams specifies two coats of SuperPaint Interior Latex on bare drywall, and Benjamin Moore recommends two coats of Regal Select for full color development, noting that previously painted surfaces can be finished with one or two. A big colour change, such as dark to light, may need more; the calculator accepts up to four coats.
Do I need primer before painting walls?
Not always. Sherwin-Williams says SuperPaint Interior Latex can go directly over bare drywall (two coats), or over one coat of its Premium Wall & Wood Primer. Benjamin Moore describes Regal Select as self-priming on most properly prepared surfaces and points to special-purpose primers for water-stained or smoke-damaged surfaces. Tick the primer box to add a primer line to your estimate.
Should I subtract windows and doors from the paint estimate?
Yes, if they are large, because they reduce the wall area you actually paint. Enter how many doors and windows the room has and their sizes, and the calculator subtracts them from the wall area. Measure your own openings rather than guessing. If you would rather keep a safety margin, leave the openings at zero and the result will err toward buying more paint.
How much extra paint should I buy for waste?
The calculator starts with a 10% allowance, which is our planning default, not a manufacturer figure. Manufacturers publish coverage per gallon but do not publish a waste rate, because losses depend on your tools and technique. A larger allowance suits rough walls or spraying; you can set it from 0% upward. Leftover paint is useful for touch-ups.
Does the calculator include the ceiling?
Only if you tick the ceiling box. When ticked, the ceiling area (room length times width) is added to the wall area and treated as the same paint and the same number of coats. Benjamin Moore’s own calculator excludes the ceiling by default. Ceilings often use a dedicated ceiling paint, so for that run the tool again with just the ceiling.
How do I convert paint coverage from square feet per gallon to square metres per litre?
Multiply by 0.0929 (square metres per square foot) and divide by 3.7854 (litres per US gallon). So 350 sq ft per gallon is about 8.6 square metres per litre. Switch the units menu to Metric and the calculator converts the lengths and the coverage for you, using the NIST conversion factors listed in the sources.
Sources
- Sherwin-Williams, SuperPaint Interior Latex Flat A86 Series product data sheet (101.02A, 08/2026) (sherwin-williams.com) Accessed 2026-11-13.
- Benjamin Moore, Regal Select Interior Eggshell N549 technical data sheet (media.benjaminmoore.com) Accessed 2026-11-13.
- Benjamin Moore, Regal Select Interior Paint, Eggshell, product page (benjaminmoore.com) Accessed 2026-11-13.
- Benjamin Moore, paint calculator (benjaminmoore.com) Accessed 2026-11-13.
- NIST SP 811, Appendix B.8: conversion factors for units (used for feet, gallons, litres) (nist.gov) Accessed 2026-11-13.