The single most common mirror mistake is buying the wrong width. Get the size right and a lighted mirror looks built-in; get it wrong and it either crowds the wall or floats awkwardly over the sink. Here's how to size by your specific bathroom.
How wide should the mirror be vs the vanity?
A bathroom mirror should be a few inches narrower than the vanity beneath it — never wider. For a 30" vanity choose a 24"–28" mirror; for a 36" vanity, 30"–34"; for a 48" vanity, a single 44"–48" mirror. The goal is a small, even margin of wall on each side.
What size mirror for a small or powder bathroom?
In a powder room or compact bath, stay between 20" and 28" wide and lean toward a round or oval to keep the wall feeling open. Leave at least 2–4" of wall on each side so the mirror doesn't visually fill the room.
What about a double vanity?
For a 48"–60" double vanity you have two good options: one wide 48"+ mirror centered on the whole counter, or two matching mirrors centered over each sink. One large mirror reads more modern and reflects more light; two mirrors define each person's space.
How high should I hang it?
Center the mirror at roughly 60–65" from the floor — eye level for most adults — with the bottom edge 5–10" above the backsplash or faucet. On a tall vanity, prioritize keeping the light source near eye level over a fixed measurement.