A bathroom mirror gets chosen the way most fixtures do: whatever size was already hanging there, or whatever looked right in a showroom, rather than a size actually measured against the vanity underneath it. That’s the difference between a vanity that reads as considered and one where a mirror floats in the middle of too much bare wall, or spans so wide it crowds the fixtures on either side. The fix is one proportion, checked before buying: the mirror’s width against the vanity’s own width.
Start With 70 to 80 Percent of the Vanity’s Width
A mirror at 70 to 80 percent of the vanity’s total width is the reliable starting proportion for most bathrooms. A 30-inch vanity pairs well with a mirror in the 21-to-24-inch range; a 36-inch vanity wants something closer to 25 to 29 inches. Going narrower than that starts to look like an afterthought rather than a fixture chosen for the space; going wider crowds whatever sits on either side, whether that’s a sconce, a towel bar, or the edge of the wall itself.
Leave a Real Margin on Each Side
The same math works from the other direction too. Leave 2 to 4 inches of margin between each edge of the mirror and the edge of the vanity or countertop below it. That margin is what reads as intentional from across the room, more than the exact inch count does. A mirror sized correctly but hung dead-center with uneven margins on each side will still look slightly off, so measure both sides once it’s up, not just the width before buying.
Hang It 5 to 10 Inches Above the Sink
Height matters as much as width. Hang the mirror so its bottom edge sits 5 to 10 inches above the top of the sink or backsplash, close enough to be genuinely useful at the counter without crowding the faucet or splash zone. Federal accessibility guidance caps a mirror’s bottom edge at 40 inches above the floor, worth checking against on a taller vanity so the mirror still works for shorter household members. Most bathroom mirrors, once correctly proportioned to their vanity, end up somewhere between 28 and 36 inches tall, though the exact number depends on the vanity’s own height and the ceiling above it.
Decide Deliberately for a Double Vanity
A double vanity has two honest options, not one default. Two individually-sized mirrors, each centered over its own sink using the same 70-to-80-percent rule per sink, read as considered and give each side its own symmetry. One continuous mirror spanning the full vanity also works, provided it’s centered on the vanity as a whole rather than skewed toward one sink. What doesn’t work is picking a size that’s correct for neither approach, a too-narrow single mirror stranded in the middle of a wide double vanity.
Getting a bathroom mirror right comes down to one number worth checking before buying anything: 70 to 80 percent of the vanity’s width, with the height and margin following from there. A mirror sized to that ratio rarely looks wrong, whatever frame style it happens to be.
Frequently Asked Questions
What if my vanity is an unusual width that doesn’t match standard mirror sizes?
Round down to the nearest standard mirror size that still falls within the 70-to-80-percent range rather than up, since a slightly narrower mirror with more margin reads better than one that crowds the vanity’s edges. A custom-cut mirror is also a reasonable option for an unusually wide or narrow vanity, and worth the cost if the standard sizes land noticeably outside the ideal range.
Should a mirror be exactly as wide as the sink basin below it?
No, the mirror should relate to the vanity’s total width, not the basin alone, since a mirror sized only to the basin usually ends up too narrow for the counter it’s actually hanging above. The basin’s position matters for centering the mirror, not for determining its size.
