How to Calculate Gallons of Paint for Any Room
The formula is: (room perimeter × ceiling height) minus (20 sq ft per door + 15 sq ft per window) = paintable wall area. Divide by 400 for gallons per coat. Multiply by 2 for a two-coat job. Always round up to the nearest half gallon.
For example: a 12x14 room, perimeter = 52ft, ceiling 8ft = 416 sq ft gross. Deduct one door (20 sq ft) and two windows (30 sq ft) = 366 sq ft paintable. Divide by 400 = 0.92 gallons per coat. For two coats = 1.83 gallons — buy 2 gallons.
Ceiling height significantly affects the total. A room with 9ft ceilings needs 12.5% more paint than the same room with 8ft ceilings. Vaulted ceilings can increase paint needs by 50% or more.
Gallons of Paint by Room Size — Reference Table
| Room Size | Wall Area | 1 Coat | 2 Coats |
|---|---|---|---|
| 10x8 | ~253 sq ft | 0.65 gal | 1.25 gal |
| 10x10 | ~285 sq ft | 0.75 gal | 1.4 gal |
| 12x12 | ~349 sq ft | 0.87 gal | 1.75 gal |
| 12x14 | ~366 sq ft | 0.92 gal | 1.83 gal |
| 14x14 | ~393 sq ft | 0.98 gal | 1.97 gal |
| 15x15 | ~425 sq ft | 1.06 gal | 2.1 gal |
| 15x20 | ~575 sq ft | 1.44 gal | 2.9 gal |
| 20x20 | ~740 sq ft | 1.85 gal | 3.7 gal |
Tips for Buying the Right Amount of Paint
Always round up to the nearest gallon — running short mid-wall is far worse than having a small amount of leftover paint.
Buy all cans of the same colour at the same time — paint is batch-matched at the store and different batches can have subtle colour variations.
Buy 5-gallon buckets when you need 5 or more gallons of the same colour — they are cheaper per gallon and guaranteed colour consistent.
Keep all leftover paint labelled by room — seal tightly and store in a temperature-stable location. Latex paint stays usable for 2 to 5 years.
Box multiple cans together before painting — pour all cans of the same colour into a large bucket and mix before applying to ensure uniform colour.
Related Paint Calculators
- Paint Coverage Calculator →
- How Much Paint to Cover 500 Sq Ft? →
- How Much Paint for a 12x12 Room? →
- How Much Paint for a Bedroom? →
- Two Coat Paint Calculator →
- How Much Paint for a Living Room? →
- Free Paint Calculator →
Frequently Asked Questions
How do I calculate gallons of paint for a room?
Multiply room perimeter by ceiling height, subtract 20 sq ft per door and 15 sq ft per window, divide by 400 for gallons per coat, then multiply by number of coats.
How many gallons for an average bedroom?
An average 12x12 bedroom needs about 1.75 gallons for two coats — buy 2 gallons. A larger 14x14 room needs about 2 gallons — buy 2.5 gallons.
Does ceiling height change how much paint I need?
Yes significantly. A room with 9ft ceilings needs 12.5% more wall paint than the same room with 8ft ceilings. Always use your actual ceiling height in calculations.
Should I buy paint in gallons or quarts?
Buy quarts for rooms under 150 sq ft of paintable wall area. For larger rooms, gallons are more economical. Buy 5-gallon buckets when you need 5+ gallons of the same colour.
What happens if I run out of paint mid-room?
Buy the same colour from the same store and request the same batch number if possible. Mix the new can with remaining old paint in a bucket before continuing to blend any slight colour variation.
How much extra paint should I buy for touch-ups?
Add 10% to your calculated quantity. Keep it sealed and labelled with the room name — it will stay usable for 2 to 5 years and is invaluable for inevitable future touch-ups.