How many gallons does a room really take for two solid coats? Guess low and you run dry with one wall left. The Paint Calculator turns your wall size, openings, and coat count into the gallons and liters you need.
It is built for homeowners and painters coating interior walls. You enter the total wall width, the height, a count of doors and windows, and the number of coats. The tool reports gallons, liters, and the net surface area.
Table of Contents
- What the Calculator Does
- How to Use It, Step by Step
- The Paint Coverage Formula
- Worked Example
- How to Read the Results
- Coats, Coverage, and Openings
- Tips for a Clean Estimate
- Limitations and Assumptions
- FAQs About the Paint Calculator
What the Calculator Does
The tool sizes the wall surface, then divides by a standard coverage rate. Four inputs feed three result cards.
- Gallons Needed (US): the whole gallons to buy, with the exact amount in parentheses.
- Liters Needed: the same quantity in liters for metric products.
- Net Surface Area: the wall area after openings are removed, in square feet.
How to Use It, Step by Step
- Enter Total Wall Width (feet). The sample is 40 feet. Add up each wall you will paint.
- Enter Wall Height (feet). The sample is 9 feet.
- Enter Doors and Windows (count). The sample is 3 openings.
- Enter the Number of Coats. The sample is 2 coats.
- Press Calculate Paint Needed to read the results.
The Paint Coverage Formula
The tool finds the wall area, subtracts 20 square feet per opening, multiplies by coats, then divides by 350 square feet per gallon.
Width x Height- Openingsx Coats /350=Gallons
- Gross area = width x height.
- Deductions = door and window count x 20 square feet.
- Net area = gross area minus deductions, with a floor of 10 square feet.
- Gallons = net area x coats / 350.
Worked Example
Noah paints a room with 40 feet of wall and a 9 foot ceiling. It has 3 openings, and he plans 2 coats.
| Step | Calculation | Result |
|---|
| Gross Area | 40 x 9 | 360 sq ft |
| Deductions | 3 x 20 | 60 sq ft |
| Net Area | 360 - 60 | 300 sq ft |
| Paint Area | 300 x 2 coats | 600 sq ft |
| Gallons | 600 / 350 | 2 Gallons (1.71) |
The tool rounds 1.71 gallons up to 2 full gallons. That equals about 6.5 liters for metric paint.
How to Read the Results
Gallons Needed (US) is the whole cans to buy. The number in parentheses is the exact amount before rounding.
Liters Needed is the same paint in liters. It helps when the product is labeled in metric.
Net Surface Area is the wall area after openings. It does not multiply by coats.
Coats, Coverage, and Openings
The tool assumes each gallon covers 350 square feet for one coat. Smooth walls may reach that easily. Rough or porous walls can need more paint per gallon.
Each door or window is counted as 20 square feet. Large picture windows remove more than that, so adjust the count if an opening is big. To measure the bare wall first, the wall area tool helps. For a ceiling, the ceiling paint tool sizes that surface.
Tips for a Clean Estimate
A primer coat counts as a coat, so add it to the number. Dark colors over light walls often need an extra coat. Buy one spare quart for touch ups after the job.
Enter each room on its own for a tidy order. Add up the wall widths in one room, not the whole home at once. This keeps the gallon count close to how you will buy and store the cans.
To price the paint, the paint cost tool adds a price per gallon. For a full room quote with trim and labor, the room paint cost tool is handy. More options sit in the home and construction calculators hub.
Limitations and Assumptions
- Coverage is fixed at 350 square feet per gallon. You cannot change the rate.
- Each opening is a flat 20 square feet, which may not match your doors or windows.
- The width and height labels use feet. The 350 rate assumes square feet, so enter feet.
- Net area never drops below 10 square feet, even with many openings entered.
- A blank or invalid entry shows a warning and sets the results to N/A.
- The tool does not paint ceilings or estimate cost.
FAQs About the Paint Calculator
How much paint covers a 300 square foot room in two coats?
The tool shows 2 gallons, or about 1.71 gallons exact. That covers 600 square feet of painting.
Does a primer count as a coat?
Yes. Add the primer to the coat count. Each pass uses roughly the same amount of product.
How large is each opening deduction?
The tool removes 20 square feet per door or window. Raise the count for a large picture window.
Does the tool accept meters?
No. Both the width and height use feet. A metric entry would undercount the gallons.
Why did it round up to a full gallon?
Paint is sold in whole cans. The tool rounds the gallons up so you do not run short.
Does this include the ceiling?
No. It covers walls only. Use a ceiling paint tool for the overhead surface.
This calculator gives planning estimates only and is not professional engineering, electrical, or building code advice. Reviewed for general accuracy by Prof. Dr. Khalil Mudassar, PhD.