Paint Calculator: How Much Paint Do I Need?
Nothing is more frustrating than running out of paint with one wall left to finish — or dragging home two extra gallons you will never use. This free paint calculator estimates exactly how much paint you need for any rectangular room: it measures the wall area, subtracts your doors and windows, multiplies by the number of coats, and rounds up to whole gallons using the coverage rate on your paint can. Use it before you shop and you will never guess at the paint counter again.
Room Paint Calculator
How to Use This Paint Calculator
Start by measuring your room with a tape measure and entering the length, width and wall height in feet. The calculator finds the wall area using the formula 2 × (length + width) × height, then subtracts the openings you will not paint.
Next, count the doors and windows in the room. The tool assumes a standard interior door is 21 square feet and a standard window is 15 square feet — if yours are different (patio sliders, picture windows), adjust the door area and window area fields.
Choose the number of coats. Two coats is the right default for color changes and new drywall; one coat only works when repainting the same color. Then check the coverage rate printed on your paint can (most paints list 350 to 400 square feet per gallon) and adjust it if needed. Click Calculate — the tool shows the exact gallons to buy and, if you entered a price, the total paint cost.
The Paint Calculation Formula
The math behind the tool is simple:
Wall area = 2 × (length + width) × height, then paintable area = wall area − door and window area, then total area = paintable area × coats, and finally gallons = total area ÷ coverage per gallon, rounded up to the next whole gallon.
Worked example: a 12 × 12 room with 8-foot walls has a wall area of 2 × (12 + 12) × 8 = 384 square feet. Subtract one door (21 sq ft) and two windows (15 sq ft each = 30 sq ft): 384 − 51 = 333 square feet of paintable wall. For two coats that is 666 square feet. At 350 square feet per gallon, 666 ÷ 350 = 1.9 gallons — so you buy 2 gallons. You cannot buy 1.9 gallons, and the spare is exactly what you want for touch-ups after the furniture moves in.
Pro Tips for Buying the Right Amount of Paint
Coverage drops on rough surfaces. Fresh drywall that has not been primed can drink 20 to 30% more paint than the label promises, which is why primer always pays for itself on new walls and on dramatic color changes.
Check the sheen as well as the color. Flat and matte finishes hide wall imperfections but scuff easily in hallways and kids' rooms; eggshell is the most forgiving all-rounder; satin and semi-gloss stand up to moisture and scrubbing, so they belong in kitchens and bathrooms.
Finally, buy all your paint at once. Paint is mixed in batches, and even the same color code can vary slightly between cans mixed on different days. One rounded-up purchase in a single trip avoids visible stripes across the wall.
Frequently Asked Questions
How much paint do I need for a 12x12 room?
A 12x12 room with 8-foot walls has 384 square feet of wall area. Subtract one door (21 sq ft) and two windows (15 sq ft each) to get 333 square feet. For two coats that is 666 square feet, which needs 2 gallons of paint at the standard coverage of 350 sq ft per gallon.
How many square feet does a gallon of paint cover?
Most interior latex paints cover 350 to 400 square feet per gallon on smooth, primed walls. On rough or unpainted drywall, plan for 250 to 300 square feet per gallon. Check the label on your specific paint can — it always lists the expected coverage.
Do I need one coat or two coats of paint?
Plan for two coats whenever you change color, paint new drywall, or cover a darker shade. One coat is usually enough only when repainting the same or a very similar color over a surface in good condition.
Should I buy primer separately?
Yes, for bare drywall, stained walls, or when going from dark to light colors. Primer seals the surface so the finish coat covers evenly. Primer covers roughly 200 to 300 square feet per gallon.
Should I round up when buying paint?
Yes — you cannot buy 1.9 gallons, and a little extra is useful for touch-ups later. Buy the rounded-up number of gallons, and store the leftover can properly (lid sealed, kept from freezing) for future repairs.