Room Size Calculator
Calculate room area in square feet and square meters, estimate paint gallons, and suggest an air conditioner size.
Measure a rectangular room
Enter length, width, and ceiling height in feet or meters. The calculator reports floor area in square feet and square meters, subtracts doors and windows from the wall and ceiling area, and estimates gallons from your coverage rate and number of coats. It also suggests a cooling size from the floor area and a short furniture note.
Formulas
Floor area is length times width:
$$A = L \times W$$Paintable area adds the four walls and the ceiling, then subtracts openings. Gallons divide that area, times coats, by coverage in square feet per gallon:
$$P = 2(L + W)H + LW - O$$ $$G = \frac{P \times C}{R}$$The BTU figure follows common room-size bands, starting at 5,000 BTU for up to 150 square feet and stepping up as the floor gets larger. It is a screening estimate, not a Manual J load calculation. High ceilings, sunny windows, and kitchens need more capacity than the band suggests.
Related measuring tools
For other shapes, use the Square Footage Calculator. For flooring waste, try the Flooring Calculator. For a coverage-only estimate, use the Paint Coverage Calculator.
Frequently Asked Questions
How do I measure an L-shaped room?
Split it into two rectangles, run each one, and add the floor areas and paint quantities.
What should I enter for doors and windows?
Enter their combined area in the same unit as the room. A typical interior door is about 21 square feet. The default starts there.
Why is paint coverage always in square feet per gallon?
Can labels in the US quote coverage in square feet. The calculator converts metric room sizes to square feet before dividing.
Is the BTU number exact?
No. It is a floor-area band for a typical room. Use it to narrow a product search, then confirm with the unit manufacturer.
Does switching units change the room?
No. Length, width, height, and openings are converted so the physical room stays the same.
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