Room paint estimator
Paint Calculator for Rooms (Free)
A room paint calculator multiplies the room perimeter by wall height, subtracts door and window area, adds the ceiling when selected, applies the number of coats and divides by the paint coverage rate. The output shows both calculated volume and whole gallons to purchase.
This calculator is a planning estimate. Verify the actual site, product specifications and applicable local requirements before ordering or building.
How the calculation works
Gross wall area = two times room length plus room width, multiplied by wall height. Paintable area = gross wall area - openings + optional ceiling. Gallons = paintable area multiplied by coats, divided by coverage per gallon.
Worked example
A 12 by 14-foot room with 8-foot walls has 416 square feet of gross wall area. Subtracting one typical door and two typical windows leaves 365 square feet. Two coats at 350 square feet per gallon calculate to 2.09 gallons, so the purchase estimate is three gallons.
Reference table
| Input | Result | Use |
|---|---|---|
| 10 x 10 x 8 ft | 320 sq ft walls | gross |
| 10 x 12 x 8 ft | 352 sq ft walls | gross |
| 12 x 14 x 8 ft | 416 sq ft walls | gross |
| 14 x 16 x 9 ft | 540 sq ft walls | gross |
| 16 x 20 x 9 ft | 648 sq ft walls | gross |
Reference rows are examples, not product orders. Enter the measured project values in the calculator for a usable estimate.
How to use the result
- Measure finished surfaces, not rough framing, and record every obstruction.
- Enter the real product dimensions when the tool offers an editable width, length, repeat or coverage field.
- Compare the calculated result with the supplier label and round purchase quantities up only after checking package size.
- Mark the proposed layout on the wall before cutting, attaching or ordering custom pieces.
Frequently asked questions
How much area does one gallon of paint cover?
The calculator defaults to 350 square feet per gallon, but the selected product label should replace that assumption. Surface texture and application method affect coverage.
Should I subtract doors and windows?
Yes. The tool uses typical opening areas from the entered counts. For unusual openings, calculate wall area separately with exact measurements.
How many coats should I plan?
Two coats are a common planning assumption for a color change or even finish. Primer, deep colors and porous surfaces can change the requirement.
Does the estimate include the ceiling?
Only when the ceiling option is selected. The ceiling area is room length multiplied by room width.
Why does the tool round gallons up?
Paint is purchased in container sizes, and stopping short mid-wall can create color or sheen differences. The tool shows the calculated volume and a whole-gallon purchase estimate.