How to Measure a Room for Paint
Accurate measurement is the difference between the right amount of paint and multiple store trips. Follow these steps to get the paintable wall area, then enter your numbers in the calculator below.
Step 1: Measure length and width
Measure the floor plan of the room — the length and width of the floor, not the wall diagonal. Measure to the nearest half-foot. For a room with a bump-out or alcove, use the full outer dimensions and treat the alcove walls as additional wall sections.
Step 2: Measure ceiling height
Measure from floor to ceiling at one corner of the room. Most US residential rooms are 8 ft, 9 ft, or 10 ft. Vaulted or cathedral ceilings need to be measured at multiple points — use the highest point as a conservative estimate, which will give you slightly more paint than needed (better than running short).
Step 3: Count doors and windows
Count every door (including closet doors) that opens into the room and every window. The calculator deducts 20 sq ft per door (standard 36×80 in = 20.0 sq ft exactly) and 15 sq ft per window (industry-standard 3×5 ft average). If your windows are significantly smaller or larger, you can adjust by switching to the paint-by-sq-ft calculator and entering your own deduction.
Step 4: Enter dimensions and calculate
Enter your measurements in the calculator below. The formula is:
gross wall area = 2 × (length + width) × ceiling height paintable area = gross wall − (doors × 20) − (windows × 15) gallons to buy = ⌈ (paintable × coats) ÷ 350 ⌉ Tips for specific scenarios
- L-shaped rooms: measure as two rectangles; add their wall areas (not their floor areas)
- Vaulted/cathedral walls: measure the rectangular lower section separately from the triangular upper section: triangle area = 0.5 × base × height
- Stairwells: measure each wall segment individually; stairwell walls often extend over two stories
- Wainscoting: if painting only above the wainscoting rail, measure from the rail top to ceiling, not from the floor
Room Dimensions
Openings & Coats
How the math works
Step 1 — gross wall area
gross_wall = 2 × (length + width) × ceiling_height Step 2 — subtract openings
paintable = gross_wall − (doors × 20) − (windows × 15) Each standard 36×80 in door = 20.0 sq ft. Average window = 15 sq ft (industry convention). Paintable area is clamped to ≥0.
Step 3 — gallons
gallons_to_buy = ⌈ (paintable × coats) ÷ coverage ⌉ Coverage defaults to 350 sq ft/gal — the conservative figure used by Sherwin-Williams, Benjamin Moore and Behr field guides. Always round up to whole gallons; running out mid-job risks a dye-lot mismatch.
Frequently Asked Questions
Use a measuring app on your smartphone (iOS Measure app or Google Measure) — they use augmented reality to estimate distances. For a rough figure, use your pace: one normal adult step is approximately 2.5 feet. A 5-pace room is about 12–13 feet. These methods give ±10% accuracy, which is enough for a first gallon estimate.
Divide the room into rectangles on paper. Measure each section separately (length, width, height). Calculate wall area for each rectangle: 2 × (L + W) × H. Add the totals. Alternatively, enter the total paintable sq ft into the paint-by-square-footage calculator on this site.
Treat built-ins and fireplaces like large windows if you are not painting behind them. Measure the width × height of the feature and subtract it from the wall area. If you are painting the sides and interior of built-in shelves, those surfaces add to your total — measure them separately and add.
Yes, always. Ceiling paint is usually a different product, and the ceiling area is calculated differently (length × width, no deductions). The interior calculator here is for walls only. Use the ceiling paint calculator on this site for the ceiling.
Add 10% as a standard buffer — this covers tray loss, roller end-caps, overlap, and small mistakes. For heavily textured walls or very complex rooms (many angles), add 15%. The calculator already rounds up to whole gallons, which provides a small natural buffer, but a deliberate 10% gives you a match-pot safety net.