What this calculator does
The Wall Paint Surface Area Calculator estimates the total paintable wall area of a rectangular room and converts it into the number of gallons of paint you need. It starts from the room's perimeter and wall height, then subtracts the area taken up by doors and windows so you don't overbuy. Measurements use feet, the common unit on paint can coverage labels.
How to use it
Enter the room's length, width and wall height in feet. Add how many standard doors and windows the walls contain. Set the paint coverage rate printed on your can (typically 350–400 sq ft per gallon for one coat). The calculator returns the gross wall area, the area deducted for openings, the net paintable area, and the gallons needed for one coat.
The formula explained
The four walls form a band around the room. Its area equals the perimeter, \(2(l + w)\), times the wall height \(h\). From this gross figure we subtract the openings. This tool assumes each door is \(3\,\text{ft} \times 7\,\text{ft} = 21\,\text{sq ft}\) and each window is \(3\,\text{ft} \times 5\,\text{ft} = 15\,\text{sq ft}\). The net area divided by the coverage rate gives gallons.
$$\text{Gallons} = \frac{2\,(L + W)\,H - O}{C}$$$$\text{where}\quad \left\{ \begin{aligned} L &= \text{Length (ft)} \\ W &= \text{Width (ft)} \\ H &= \text{Height (ft)} \\ O &= 21\,\text{Doors} + 15\,\text{Windows} \\ C &= \text{Coverage (sq ft/gal)} \end{aligned} \right.$$
Worked example
A 12 ft × 10 ft room with 8 ft walls has a perimeter of \(2(12 + 10) = 44\,\text{ft}\), so the gross area is \(44 \times 8 = 352\,\text{sq ft}\). With 1 door (21) and 2 windows (30), openings total 51 sq ft, leaving a net 301 sq ft. At 350 sq ft per gallon that's about 0.86 gallons for one coat — round up to 1 gallon, or 2 for a second coat.
$$\frac{352 - 51}{350} = \frac{301}{350} \approx 0.86\ \text{gallons}$$FAQ
Should I round up the gallons? Yes. Always round up to the next whole can and double the figure if you plan two coats.
What if my openings are unusual sizes? The tool uses standard door/window sizes. For large picture windows or sliding doors, treat them as extra openings or subtract a custom area manually.
Does this include the ceiling? No — it covers only the four walls. Add the ceiling area (length × width) separately if you're painting it.
