Paint Coverage Calculator
Add one or more rooms, choose whether you are painting walls, ceilings, or both, and factor in doors, windows, and coats. The calculator estimates total paintable area and how many gallons of paint you should plan to buy.
This tool focuses on quick estimates, not architectural precision. Always review measurements on-site and consult product labels before making final purchase decisions.
Paint settings
Most interior paints cover roughly 300–400 sq ft per gallon on smooth walls. Use a lower value for rough or previously unpainted surfaces.
Rooms
Room 1
Length, width, and height are in feet. Doors and windows use typical sizes.
Each door is approximated as 20 sq ft and subtracted from wall area.
Each window is approximated as 15 sq ft and subtracted from wall area.
Most rooms use at least 2 coats when changing colors or painting bare walls.
Results
Total paintable area
317 sq ft
Coverage used
350 sq ft/gal
Total gallons (approximate)
0.91 gal
Recommended purchase
1 gallon.
It is usually safer to round up slightly so you have a bit of extra paint for touch-ups and future repairs.
Per-room estimated area
| Room | Surfaces | Coats | Area (sq ft) |
|---|---|---|---|
| Room 1 | Walls only | 1 | 317 |
How paint coverage really works in the real world
Paint cans often promise a neat coverage number, but real surfaces rarely behave that cleanly. A gallon labeled as covering 400 square feet might actually cover less once you account for texture, previous colors, and how heavily you load the roller. The goal of this calculator is not to nail the exact ounce of paint required, but to put you in a realistic range so you can buy confidently instead of guessing in the aisle.
Why smooth walls and rough walls behave differently
Smooth, previously painted drywall behaves very differently from rough masonry or fresh drywall that has never seen paint. Rough or porous surfaces act like a sponge and soak up more paint. That is why coverage values as low as 250 square feet per gallon are common for the first coat on raw surfaces, while premium paints on smooth walls can legitimately stretch closer to 400 square feet per gallon.
The presets in this calculator give you a fast way to reflect those realities: lower numbers for rough or unprimed surfaces, higher numbers when you are refreshing a smooth, already-painted wall. If you know your specific product and surface, the custom coverage input lets you plug in whatever number makes the most sense for your situation.
Multiple coats and why they matter so much
A single thin coat rarely gives a satisfying finish, especially when you are changing colors or covering marks. Each additional coat is effectively another full pass over the same area, which is why this calculator multiplies the paintable surface by the number of coats you choose. Going from one coat to two is not a minor tweak; it doubles the effective square footage you are asking that paint to cover.
In practice, it is common to run at least two coats on most interior projects. Strong color changes, dark-to-light transitions, or patchy walls may demand three or more. Setting coats accurately in the tool gives you a more honest picture of how much paint your project will consume.
Why subtracting doors and windows actually helps
Doors and windows reduce the amount of wall you are painting, even though they visually feel like part of the room. Ignoring them means you quietly round your estimate up, which is sometimes fine, but it can overshoot on small rooms with lots of openings. This calculator subtracts a typical area for each door and window before applying coats so tighter spaces with big windows do not get overestimated quite as badly.
Smarter paint buying: round up on purpose, not by accident
Once you have an estimated number of gallons, the next question is how to translate that into actual cans on the shelf. The tool breaks the answer into whole gallons and quarts, rounding up the remaining fraction to the nearest quart and rolling four quarts into a full gallon when needed. In reality, people often choose to round up one extra gallon beyond the bare minimum so they have paint for touch-ups, inevitable mistakes, and future repairs.
As with every tool on LifeHackToolbox, this calculator is about getting you to a clear, defensible decision fast, not running a perfect simulation. If you are deciding between two close options, it is usually safer to lean slightly high and keep the leftover paint sealed for later rather than risk running out with one wall half finished.
If you are juggling paint costs alongside other budget questions, the Hourly → Salary → After-Tax Calculator can quickly translate your income into realistic, after-tax monthly numbers so you can see how this project fits into the bigger picture.
These numbers are estimates only. Always confirm coverage information on the actual paint can and, when in doubt, talk to a paint professional at your supplier before committing to large purchases.