Bathroom Mirror Size Calculator

Background.

A mirror that runs wider than the vanity below it tends to look as if it was bought for a different room. Most bathrooms look settled when the glass stops a little short of the countertop's ends on both sides, so the vanity frames the mirror rather than the other way round. No building code or industry standard fixes that margin; it is a judgement about proportion and about what else has to share the wall.

The arithmetic fits on the back of a receipt, which is exactly why it gets fumbled in the shop: take the vanity width, remove the margin you want on the left and the same margin on the right, and what remains is the widest mirror that keeps the gaps even. A 48-inch vanity with 2 inches a side leaves a 44-inch mirror; a 60-inch vanity with 3 inches a side leaves 54.

Height is a separate decision, set by the space between the top of the backsplash or faucet and the light above. The page takes the height you have chosen and works out the face area in square feet. That is the unit glass shops quote custom-cut mirrors in, and it also tells you roughly what the piece will weigh: ordinary quarter-inch mirror glass comes to a little over 3 pounds per square foot, so a generous vanity mirror can call for a mounting cleat or proper clips rather than a single picture hook.

Wall lights mounted beside the mirror, a window reveal or a towel bar all eat into the side margins. Measure those before you settle on the inset, and choose one that clears them.

What is bathroom mirror size calculator?

Mirror width, as used here, is the horizontal span left over the vanity once an equal margin is reserved at each end: vanity width minus twice the side inset, in inches. It is a fitting dimension rather than a style rule. The page does not decide what margin looks right, how high the glass should hang or whether a framed or frameless piece suits the room. Its second figure, mirror area, multiplies that width by the height you entered and converts the result to square feet.

How to use this calculator.

1. 01Measure the vanity top from end to end in inches. If the countertop overhangs the cabinet box, use the countertop, because that is the edge your eye lines the mirror up against.
2. 02Enter the gap you want between each end of the vanity and the mirror's edge. The same figure is taken off both sides; enter 0 if you want the mirror exactly as wide as the vanity.
3. 03Enter the mirror height you have settled on after measuring the clear wall from the backsplash or faucet up to the underside of the light above.
4. 04Read the width as the largest outside size that fits, frame included, and take the square footage with you when asking for a glass quote or choosing wall anchors.

The formula.

W = V − 2s, where V is the vanity width and s is the inset you want at each end, both in inches. The inset is doubled because it comes off both ends; the mirror is assumed to sit centred over the vanity. The area is A = W × H ÷ 144, where H is the mirror height in inches and 144 is the number of square inches in a square foot (12 × 12). A change in vanity width passes straight through to the mirror inch for inch, but the inset works twice as hard: widening the margin by 1 inch on each side takes 2 inches off the glass. Height affects only the area, never the fit. If twice the inset equals or exceeds the vanity width there is no mirror left to size, and the page asks for a smaller inset instead of returning zero or a negative width.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Kitchen and Bath Association. Bathroom planning context current 2026. Bathroom design and fixture planning resources. Retrieved 2026-08-06.
2. [2]National Institute of Standards and Technology. NIST SI Guide, 2008 edition. Appendix B unit relationships. Retrieved 2026-08-06.
3. [3]OpenStax, Rice University. University Physics, 2016. Volume 1 mechanics and Volume 2 thermodynamics. Retrieved 2026-08-06.