One of the most common — and most avoidable — DIY painting mistakes is buying too little paint and finding yourself one gallon short halfway through the second coat, or buying far too much and leaving three half-used cans in the garage. Smekys' paint calculator removes the guesswork: you measure the room, subtract the doors and windows, and it tells you exactly how many gallons you need for two coats and what the paint will cost.
The math behind it is simple and it is the same method contractors use: find the total wall area around the room, knock out the openings you will not paint, multiply by the number of coats, and divide by the coverage of a single gallon. Because paint is sold in whole gallons, the result is always rounded up — buying slightly extra is far better than running out.
How to Use
- Measure the Room Perimeter. Walk around the room with a tape measure and add the length of every wall together. For a square 10 ft × 10 ft room, that is 10 + 10 + 10 + 10 = 40 ft.
- Enter the Wall Height — the distance from the floor to the ceiling. Standard homes use 8 ft; vaulted or older homes may be 9 ft or 10 ft.
- Set the Number of Doors. Each standard interior door (about 3 ft × 6 ft 8 in) removes roughly 20 sq ft of wall from your total.
- Set the Number of Windows. A typical window (about 3 ft × 5 ft) removes roughly 15 sq ft. If you have oversized bay windows or floor-to-ceiling glass, add a note and measure them by hand.
- Choose the Coats of Paint. Two coats is the default and the industry standard for an even, durable finish — especially when changing colors.
- Adjust Paint Cost and Coverage per Gallon if you know the exact product. Most interior latex paints cover about 350 sq ft per gallon and run $20–$60 per gallon.
The result card shows your gross wall area, the net area after openings, the total square footage to cover across all coats, the whole gallons you should buy, and the estimated material cost.
The Paint Estimating Formula
Professional estimators work from a straightforward geometric model:
Net wall area = (perimeter × height) − (doors × 20 sq ft) − (windows × 15 sq ft)
Then:
Gallons needed = ceil(net wall area × coats ÷ coverage per gallon)
Where:
- Perimeter × height gives the total square footage of wall before any deductions.
- 20 sq ft per door and 15 sq ft per window are accepted rule-of-thumb deductions that avoid measuring every opening individually.
- Coats accounts for the fact that two coats cover roughly twice the wall area of one.
- 350 sq ft per gallon is the typical spread rate advertised on paint cans for a smooth, previously primed surface.
- The result is rounded up because paint is sold in whole gallons and overbuying is cheaper than a second trip to the store mid-project.
Worked example: a 10 ft × 10 ft room with 8 ft ceilings has a perimeter of 40 ft and a gross wall area of 320 sq ft. Subtract one door (20 sq ft) and two windows (30 sq ft) to get 270 sq ft of net wall. At two coats, you must cover 540 sq ft. Divided by 350 sq ft per gallon, that is 1.54 gallons — rounded up to 2 gallons, which at $35 per gallon costs about $70.
Coverage Rules of Thumb
Not every wall behaves the same way. The 350 sq ft per gallon figure assumes a smooth, lightly colored, already-primed surface. Real-world conditions change the spread rate:
- Unpainted drywall or fresh plaster is porous and drinks paint; plan for only 250–300 sq ft per gallon and always prime first.
- Dark color over light (or red over white) often needs three coats, not two.
- Textured or popcorn walls hold more paint and cover less — budget an extra 10–20%.
- Ceilings are not included in this wall-only number; paint a ceiling separately at roughly one gallon per 400 sq ft.
- Trim and baseboards need a separate, smaller quantity of trim paint.
When in doubt, add an extra quart. A single quart covers about 90 sq ft and solves most "just a little more" problems without buying a full gallon.
Examples
Example 1 — A standard bedroom. The room measures 12 ft × 11 ft with 8 ft ceilings, one door, and two windows. Perimeter is 46 ft, gross wall area is 368 sq ft. Deduct 20 sq ft for the door and 30 sq ft for the windows, leaving 318 sq ft. At two coats you cover 636 sq ft, which needs 2 gallons — about $70 at $35 per gallon.
Example 2 — A large living room. A 15 ft × 20 ft room with 9 ft ceilings, two doors, and three windows has a perimeter of 70 ft and a gross wall area of 630 sq ft. Deduct 40 sq ft for the doors and 45 sq ft for the windows, leaving 545 sq ft. Two coats cover 1,090 sq ft, requiring 4 gallons at roughly $140.
Example 3 — A whole-home refresh. When you paint several rooms at once, add the net wall areas together before dividing by coverage — you will usually save a fraction of a gallon per room because rounding up each room separately overstates the total. For a house with 1,500 sq ft of combined net wall across all rooms, two coats cover 3,000 sq ft, which is about 9 gallons — around $315 at $35 per gallon.
FAQ
How much paint do I need for a 12×12 room?
A 12 ft × 12 ft room with 8 ft ceilings has a 48 ft perimeter and 384 sq ft of gross wall. Subtract roughly 50 sq ft for one door and two windows, leaving about 334 sq ft. Two coats cover 668 sq ft, which rounds up to 2 gallons of paint.
Does one gallon really cover 350 square feet?
On a smooth, primed, previously painted wall, yes — most major brands advertise 350–400 sq ft per gallon. On fresh drywall, dark colors, or heavily textured walls, plan closer to 250–300 sq ft per gallon and add an extra coat or an extra quart.
Should I plan for one coat or two coats of paint?
Two coats is the standard for a durable, even finish, especially when changing colors or going from dark to light. A single coat may be enough if you are repainting the same color on a well-prepared wall, but professionals almost always apply two.
Do I subtract doors and windows when calculating paint?
Yes. Each standard door removes about 20 sq ft and each window about 15 sq ft. The calculator does this automatically. Skipping the deduction will overestimate your paint by 10–20%, which is harmless but can leave you with leftover cans.
How much does it cost to paint a room?
Interior latex paint typically runs $20–$60 per gallon. A typical bedroom needing 2 gallons costs about $40–$120 in material alone. Labor is separate and usually runs $25–$75 per hour or $200–$500 per room, depending on your market.
How many gallons does it take to paint a whole house?
A 1,500–2,000 sq ft home usually needs about 12–16 gallons of interior paint for two coats across the living areas, plus 2–3 gallons for ceilings. Run the room-by-room net wall area through this calculator and sum the gallons — remember to add 10% for touch-ups and waste.
Disclaimer
This paint calculator is provided for informational purposes only and does not constitute a professional painting or contracting estimate. Gallon and cost figures are based on standard assumptions (350 sq ft per gallon, ~20 sq ft per door, ~15 sq ft per window, $35 per gallon) and may vary by wall texture, surface condition, paint brand, color change, and local labor rates. Always buy slightly more paint than the estimate, and consult a licensed painter for any project involving lead paint removal, multi-story exteriors, or structural surfaces.