10 × 10 ft bedroom
A compact room with an eight-foot ceiling can still require more than one gallon when two coats, a deep color change, or a ceiling are included.
Paint Quantity Guide
Calculate the painted area before choosing the number of gallons.
A useful paint estimate combines the room perimeter, wall height, openings, coats, ceiling, primer, and the coverage printed on the actual can.
Written and edited by Christina HinesPublished August 23, 2026Last reviewed August 24, 2026Planning guidance, not product testingEditorial standards
Open the paint calculator →Direct answer
Turn wall area, coats, and listed coverage into a transparent estimate.
For four walls, multiply the room perimeter by wall height, subtract measured openings, multiply by the number of coats, and divide by the product's listed square-foot coverage. Round the purchase quantity up to whole containers, then decide whether a small touch-up reserve is appropriate.
Paintable square feet × coats÷listed coverage per gallon=gallons before rounding
Write these down first
- 01Room length and width
- 02Wall height
- 03Door and window area
- 04Number of coats
- 05Coverage listed on the paint
- 06Ceiling and primer plan
A practical sequence
Calculate each paint category separately before you shop.
- 01
Measure the full wall perimeter
Add the room length and width, double that total, and multiply by wall height. Measure separate wall sections when the room is not a simple rectangle.
- 02
Subtract openings carefully
Use real dimensions for large doors, windows, built-ins, and tile. Standard deductions are convenient estimates, not substitutes for unusually large openings.
- 03
Account for coats and coverage
Multiply by the planned coats and divide by the coverage printed on the selected product. Texture, porous surfaces, dramatic color changes, and application method can reduce real coverage.
- 04
Separate primer and ceiling quantities
Primer and ceiling paint often use different products and coverage rates. Calculate them independently instead of combining every surface into one number.
Real-room checks
Work the numbers before relying on the final gallon count.
12 × 14 ft living room
With eight-foot walls, one 21-square-foot door, two 15-square-foot windows, two coats, and 375 square feet of listed coverage: 2 × (12 + 14) × 8 = 416; 416 − 51 = 365; 365 × 2 ÷ 375 = 1.95. Round up to 2 gallons of wall paint.
Open plan with alcoves
Measure each wall run and subtract only surfaces that will not be painted. Keep the ceiling, trim, doors, and primer as separate calculations.
Common planning errors
A correct area can still produce the wrong purchase quantity.
- Using floor areaThe room's floor square footage does not describe the wall perimeter or height.
- Assuming one coatColor change, finish, surface condition, and product instructions may require another coat.
- Ignoring listed coverageCoverage varies. Replace a generic estimate with the number on the chosen product.
- Over-deducting openingsSmall openings may not reduce the number of containers after whole-gallon rounding.
- Mixing batches lateBuying the planned quantity together and boxing cans can reduce visible color variation.
Quick answers
Should I subtract doors and windows?
Yes, especially when openings are large. For a quick estimate, standard deductions can help, but measured opening dimensions are more reliable.
How much extra paint should I buy?
That depends on product size, surface texture, color change, touch-up needs, and return policy. Round up to full containers and compare the remaining amount with the project's real risk.
Do I calculate primer separately?
Yes. Primer may have a different coverage rate and is usually planned as its own coat rather than folded into the finish-paint total.
When the standard formula changes
Adjust the estimate when the room or product changes the assumptions.
Textured or porous walls
Fresh drywall, masonry, heavy texture, and other absorbent surfaces can reduce actual spread rate. Plan primer and use the selected product's surface guidance.
Major color or sheen changes
A dramatic light-to-dark, dark-to-light, or finish change may require additional coats even when the area calculation is correct.
Oversized openings or built-ins
Measure unusually large doors, windows, cabinetry, tile, and other unpainted surfaces instead of relying on standard deductions.
Spraying or complex rooms
Overspray, alcoves, stairwells, vaulted walls, and many separate wall runs can change both waste and the practical container plan.
Methodology and limitations
Use the formula as a transparent planning estimate, then verify the product.
This guide applies standard room geometry to the dimensions entered by the reader, subtracts openings, multiplies by the planned finish coats, and divides by the coverage printed on the chosen product. The worked example uses transparent assumptions so it can be checked by hand. It does not claim hands-on paint testing and cannot predict absorption, application loss, installer technique, or a manufacturer's surface-specific instructions.
Read our editorial standardsSources and verification
Compare the estimate with current manufacturer guidance.
These references support the planning method. The label and instructions for the exact product you buy remain authoritative.
- Sherwin-Williams Paint Calculator FAQManufacturer guidance describing typical gallon coverage and why actual surface conditions matter.
- Benjamin Moore Paint CalculatorA second manufacturer calculator for comparing room measurements, openings, and estimated quantity.
Use your exact measurements