How this instrument works
A bathroom mirror that's sized purely by eye tends to look either cramped against a wide vanity or oddly small under a tall ceiling, because both the vanity below it and the ceiling above it set practical proportions the mirror should respect. Two simple margin rules cover most standard installations: leave a small gap on either side of the mirror relative to the vanity's width, and leave clearance both above the vanity cabinet and below the ceiling rather than running the mirror edge to edge in either direction.
This calculator applies those margins directly: a single rectangular mirror's width is set 4 inches narrower than the vanity (2 inches of breathing room on each side), and its height fills the space between 12 inches above the cabinet top and the ceiling. For a double vanity — two sinks sharing one wide counter — the same total width budget is instead split across two mirrors with an 8-inch total margin (4 inches per mirror side), rather than one oversized single mirror spanning the whole counter.
These are common, sensible proportions rather than a strict code requirement — nothing in a building code mandates a bathroom mirror's exact size, so treat the output as a solid, well-proportioned starting point you can nudge up or down slightly to match a specific mirror your retailer actually stocks, rather than a measurement you need to hit to the exact hundredth of an inch.
- Enter Vanity width — the full width of your countertop or cabinet, in inches.
- Enter Vanity cabinet height — how tall the cabinet or counter stands from the floor.
- Enter Ceiling height — the full height of the room, floor to ceiling.
- Read Rectangular mirror — width and height for a single-mirror installation.
- Read Double-vanity — width per mirror if you're splitting the counter between two sinks and two mirrors instead of one.
Worked example — a 60-inch vanity under an 8-foot ceiling
A 60-inch vanity with a 32-inch cabinet height, under a standard 8-foot (96-inch) ceiling: a single rectangular mirror comes out to 60 − 4 = 56 inches wide and 96 − (32 + 12) = 52 inches tall — a proportionate, well-margined mirror for that vanity.
Split across a double vanity instead, each of the two mirrors gets (60 − 8) ÷ 2 = 26 inches of width, leaving a visible gap between the pair rather than crowding two mirrors edge to edge above the same 60-inch counter.
Questions
Why should a mirror be narrower than the vanity instead of the exact same width?
A mirror cut to the exact same width as the vanity below it tends to look visually heavy and leaves no breathing room at the edges, especially once sconces or a medicine cabinet door are factored in nearby. A few inches of margin on each side — 2 inches per side in this calculator's convention — reads as intentional framing rather than an afterthought sized to fit.
Where does the 12-inch gap above the vanity cabinet come from?
It's a standard practical clearance that leaves room for a faucet, backsplash, or light fixture mounted at the back of the counter without the mirror's bottom edge crowding into them, and it matches the typical distance most vanity mirrors are hung above the counter surface in mainstream bathroom design guidance.
Is there a building code that requires a specific bathroom mirror size?
No — mirror size is an aesthetic and functional choice, not something building codes regulate the way they regulate things like ventilation or outlet placement near water. This calculator's margins reflect common, well-proportioned design practice, which you're free to adjust to fit a specific mirror model, a recessed medicine cabinet, or a decorative frame style.
What if my ceiling height calculation comes out too short or negative?
That happens when the vanity cabinet height plus the 12-inch clearance leaves less than the ceiling height to work with — typically on a very tall vanity under a low ceiling. In that situation, either accept a shorter mirror than the standard proportions suggest, reduce the top clearance margin manually, or consider a mirror mounted lower relative to the cabinet than this calculator's default assumption.
Should I use the double-vanity formula for two separate mirrors that aren't the same size?
The double-vanity formula assumes two identically-sized mirrors sharing the width budget evenly, which is the most common double-vanity layout. If you want two different-sized mirrors instead, use the single-mirror formula separately for each one against whatever sub-width of the counter you're allocating to it.