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Generator Hut

Paint Calculator

How many gallons a room needs, with doors and windows deducted.

Your measurements

Add up the length of every wall you are painting.

Each door is treated as 21 sq ft.

Each window is treated as 15 sq ft.

Check the can. Most interior latex covers 350–400 sq ft per coat.

Paint required

2.5gallons

Buy 3 gallons

Gross wall area60 ft perimeter × 8 ft high
480 sq ft
Deducted for openings1 door, 2 windows
−51 sq ft
Net area to paint
429 sq ft
Total area across all coats2 coats
858 sq ft
Paint requiredat 350 sq ft per gallon
2.45 gallons
Round up to
3 gallons

Primer is not included. Bare drywall, patched repairs and any dark-to-light colour change need a separate primer coat — calculate it as one additional coat.

Ceilings are not included. Add the floor area as a separate calculation if you are painting the ceiling too.

Buying one container larger than the exact figure is usually worth it. Touch-ups years later never match a fresh mix, and a partly used can stores fine.

Measure the perimeter of the room and the height of the walls, tell it how many doors and windows to deduct, and it works out the gallons. The coverage rate is editable because it is printed on the can and it varies — a one-coat premium paint and a contractor-grade emulsion can be a hundred square feet per gallon apart.

Why use this tool?

Deducts the openings

Doors and windows are subtracted at 21 and 15 square feet, so you are not buying paint for a surface that is not there. On a small room with two windows and a door that is most of a gallon.

Perimeter, not four separate walls

Add the wall lengths together once and enter a single figure. Four walls of different lengths take one measurement pass rather than four calculations.

Coverage you can override

The 350 sq ft default is typical for interior latex, but the number on your can is the one that counts. Change it and everything recalculates.

Shows every step

Gross area, deductions, net area, total across coats and the final gallons are all itemised, so you can check the figure rather than trust it.

How this paint calculator works

Wall area is the room's perimeter multiplied by the wall height. That single multiplication replaces measuring each wall separately, and it is exact for any room with vertical walls regardless of how many corners it has.

From that gross figure the calculator subtracts a fixed 21 square feet per door and 15 per window. Those are standard approximations — a 3 by 7 foot door and a typical double-hung window — and they are close enough that the waste in a partly used can dwarfs the error.

The net area is then multiplied by the number of coats and divided by the coverage rate. The result is the honest decimal figure, with the rounded-up whole gallons shown beneath it, because paint is sold in cans and 2.3 gallons means buying three.

How to use it

  1. Step 1: Measure the perimeter

    Walk the room measuring along the base of each wall you intend to paint, and add the lengths together. Skip any wall you are leaving alone.

  2. Step 2: Enter the wall height

    Floor to ceiling. If you have a chair rail and are painting only above or below it, use that height instead.

  3. Step 3: Count the openings

    Enter how many doors and windows fall within the walls you are painting. Leave both at zero for a garage or a stairwell with neither.

  4. Step 4: Set coats and coverage

    Two coats is standard over an existing similar colour. Read the coverage figure off the can you plan to buy and enter it — this is the input that moves the answer most.

Example usage

A standard bedroom
A 12 by 14 foot room has a 52 foot perimeter. At 8 foot walls that is 416 square feet gross, less one door and two windows leaves 365, and two coats at 350 sq ft per gallon needs 2.09 gallons — so three one-gallon cans, or a two-gallon plus a quart if you are confident in your measurements.
A stairwell with no openings
Tall walls and no windows to deduct make stairwells hungrier than their floor plan suggests. A 30 foot perimeter at 16 feet is 480 square feet, more than the bedroom above despite occupying a fraction of the floor.
Changing from dark to light
Set coats to three rather than two. A primer coat plus two finish coats is the realistic figure for covering a strong colour, and calculating two will leave you short at exactly the wrong moment.

Frequently asked questions

Does this include the ceiling?

No. It calculates walls only. For the ceiling, run the calculator again using the room's floor area — enter the length as the perimeter and set the height to 1, or simply divide the floor area by your coverage rate.

Does it include primer?

No. Primer is a separate product with its own coverage rate. The simplest approach is to treat a primer coat as one extra coat and calculate it separately using the primer's own figure.

How much paint does a gallon actually cover?

Most interior latex claims 350 to 400 square feet per coat on a smooth, previously painted surface. Textured walls, bare drywall and porous surfaces drink considerably more — closer to 250 to 300 is realistic on new plaster.

Should I buy exactly the calculated amount?

Round up rather than down, and consider one extra quart. Touch-ups years later never match a fresh mix of the same colour, so a sealed remainder from the original batch is worth having.

How do I handle a room with a sloped ceiling?

Split it. Calculate the rectangular portion normally, then add the triangular gable ends separately as half the base times the height, and add that area to the net figure.

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