How to calculate the square footage of an apartment
An apartment's square footage is the total floor area of every room added together, measured in square feet. The math is the same as any room measurement — length times width — but an apartment is several rooms, so the job is to measure each one and sum them. Sketch a quick floor plan from above, label each room, and work through them one at a time so nothing is missed and nothing is counted twice.
Measure the length and width of each rectangular room in feet, multiply the two numbers for each room, and add every room's area together. Include the bedrooms, living and dining areas, kitchen, bathrooms, closets, and hallways. If a room is L-shaped, split it into two rectangles at the corner, calculate each, and add them. The calculator above does the multiplication and keeps a running total as you enter each room.
Interior area vs. the number on the listing
The figure you measure inside the walls is the interior or carpet area — the space you actually live in. Many listings quote a larger number, because a gross or rentable figure can include the thickness of the walls and a share of common areas like hallways and lobbies. That's why a listing sometimes reads bigger than a tape measure suggests. Neither number is wrong, but only interior area describes usable space, so measure it yourself when the difference matters.
This is also why apartment figures and house figures aren't always measured the same way. When you compare units, make sure you're comparing interior to interior. If a listing gives only a gross number, measuring the rooms yourself gives you the honest, usable total to weigh against the rent.
Using square footage to compare rentals
Once you have an interior total, rent per square foot turns two very different apartments into a fair comparison. Divide the monthly rent by the interior square footage: a $1,800 unit at 600 sq ft is $3.00 per sq ft, while a $2,100 unit at 800 sq ft is $2.63 — the larger apartment is the better deal per foot even though it costs more each month. Do the same math for every unit on your shortlist.
Keep balconies and patios out of the interior figure so the comparison stays clean; outdoor space is nice, but it isn't heated, walled living area. With a consistent interior number for each apartment, you can weigh price, space, and value side by side and pick the unit that genuinely fits your budget and your life.