Paint Calculator
Estimate gallons of interior paint from room dimensions, ceiling height, coats, doors and windows. Includes a cost estimate.
Paint needed
2 gal
- Wall area
- 416 sq ft
- Doors and windows deducted
- 87 sq ft
- Paintable area
- 329 sq ft
- Total area (2 coats)
- 658 sq ft
- Estimated paint cost
- $90.00
Uses 350 sq ft per gallon, 21 sq ft per door and 15 sq ft per window. Check the label on your paint for actual coverage.
Paint estimates go wrong for two reasons: forgetting the second coat and forgetting that doors and windows take up wall space. This calculator handles both, and can include the ceiling if you are painting the whole room.
Coverage varies by product and surface. The 350 square feet per gallon default is a safe planning number for typical interior latex on primed drywall; check the label before you buy the final gallon.
How the paint calculator works
Wall area = 2 × (length + width) × height. Subtract 21 sq ft per door and 15 sq ft per window, add the ceiling area if selected, then multiply by the number of coats. Divide by the paint coverage per gallon and round up.
Worked example
A 12 × 14 ft room with an 8 ft ceiling has 416 sq ft of wall. Two doors and three windows remove 87 sq ft, leaving 329 sq ft. Two coats is 658 sq ft, so at 350 sq ft per gallon you need 2 gallons.
How to measure walls accurately
- Measure the perimeter of the room at floor level, then multiply by the wall height. Do not rely on the floor area alone.
- Measure each wall separately if the room is not rectangular; angles and alcoves change the total.
- Subtract about 21 sq ft per door and 15 sq ft per window, but keep a little extra paint for touch-ups.
- Measure the ceiling separately if you are painting it; the wall calculation does not include it.
- Check the paint label for coverage. Rough, porous or deeply tinted surfaces can need much more than 350 sq ft per gallon.
Sheen and primer guide
| Sheen | Best for | Notes |
|---|---|---|
| Matte | Ceilings, low-traffic walls | Hides imperfections; harder to clean |
| Eggshell | Living rooms, bedrooms | Balanced washability and finish |
| Satin | Kitchens, hallways, trim | Easy to clean; shows flaws |
| Semi-gloss | Trim, doors, cabinets | Durable; strong sheen |
| Gloss | Accent trim | Most durable; least forgiving |
Common painting mistakes
- Skipping primer over bare drywall, patched areas or a dramatic color change.
- Trying to cover a dark wall with one coat of light paint.
- Over-thinning paint to stretch it, which reduces coverage and durability.
- Not boxing the paint, so gallons from different batches dry slightly different.
- Painting over damp drywall or active stains without fixing the source.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, two doors and two windows, a 12 × 12 room has about 312 sq ft of paintable wall. Two coats is 624 sq ft, which is 2 gallons at 350 sq ft per gallon. Add the ceiling and you will likely need a third gallon.
How many square feet does a gallon of paint cover?
Most interior latex covers 350–400 sq ft per gallon on a smooth, primed surface. Rough or porous surfaces, deep colors and spraying can reduce coverage significantly.
Should I subtract doors and windows?
Yes, for a realistic estimate. A standard door is about 21 sq ft and a window about 15 sq ft. It is still wise to keep a little extra paint for touch-ups.
Do I need primer?
Prime bare drywall, patched areas, stains and surfaces making a dramatic color change. A primer can also reduce the number of finish coats needed, which often offsets its cost.
How many coats of paint do I need?
Two coats is standard for a uniform finish. One coat can work when repainting the same color over a similar sheen, while dark colors over light walls may need three.
Does this calculator include the ceiling?
Only if you switch “Paint the ceiling?” to Yes. The default is walls only, which is the most common interior repaint.
Assumptions and accuracy
- Dimensions are converted to feet before the calculation; 1 ft = 12 in = 0.3048 m.
- Waste allowances are estimates. Add more for irregular shapes, many cuts and uneven subgrade.
- Bag yields, coverage rates and material densities vary by product. Check the label or ask your supplier.
- Local building codes and site conditions always take precedence over a general-purpose calculator.
- Read our full methodology