A bathroom mirror size calculator has to do two different jobs at once. One is proportion: a mirror that is too wide for its vanity or too short for the wall looks wrong however good the glass is. The other is geometry with a hard constraint in it, because the height at which the bottom edge lands decides whether a person using a wheelchair can see themselves in it. This page does both, and it keeps them separate so you can see when one is fighting the other.
Arb Digital builds free calculators that state which of their numbers are conventions and which are published requirements. That distinction matters here more than on most pages: the width ratio is taste, the accessible mounting height is a figure in a federal standard, and a tool that presents them in the same tone is misleading.
What This Bathroom Mirror Size Calculator Does
Width comes from your vanity. The calculator multiplies the counter width by the percentage you choose and also shows the 70% to 100% band that most single-vanity installations fall inside, so you can see whether your chosen figure is unusual.
Height comes from the wall. The bottom edge is the counter height plus the backsplash plus whatever gap you want above it. The top edge is the ceiling height less the space you are reserving for a light, a shelf or moulding. The mirror height is simply the distance between those two, and the recommended size is that height against the calculated width.
Two checks follow. The first compares the top edge against the eye level of the tallest regular user, because a mirror that ends below someone's eyes is useless to them regardless of how well it suits the room. The second compares the bottom edge against the maximum mounting height published in the ADA Standards, and says plainly whether it clears.
How to Use It
- Measure the vanity width and choose a percentage. Seventy-five per cent is a common starting point for a single vanity; a wide double vanity is usually served better by two mirrors sized to each basin.
- Measure the counter height from the finished floor and the height of the backsplash, if there is one.
- Set the gap you want between the backsplash and the mirror. This is the field to adjust if the accessibility check fails.
- Measure the ceiling and decide how much space to reserve above the mirror for lighting or trim.
- Enter the tallest regular user's height and read both checks in the note beneath the results.
The Formula: How It's Calculated
The arithmetic is deliberately plain:
mirror width = vanity width × ratio
bottom edge above floor = counter height + backsplash + gap
top edge above floor = ceiling height − reserved space
mirror height = top edge − bottom edge
Work the defaults through in inches. A 60 inch vanity at 75% gives a mirror 45 inches wide, with a sensible range of 42 to 60 inches. A counter at 34 inches with a 4 inch backsplash and a 4 inch gap puts the bottom edge at 42 inches above the floor. A 96 inch ceiling with 12 inches reserved puts the top edge at 84 inches. The mirror is therefore 45 inches wide by 42 inches tall, a proportion of 1.07 to 1.
Now the checks. The tallest user at 74 inches has an eye level of about 69.5 inches, and the top edge at 84 inches clears that by 14.5 inches, which is comfortable. But the bottom edge at 42 inches is above the 40 inch accessible maximum, so this arrangement fails the accessibility check — and it fails purely because of the 4 inch decorative gap. Set the gap to 2 inches and the bottom edge drops to 40 inches, which complies exactly.
The Accessible Mounting Height, and Where It Comes From
Two figures matter, and both are in the 2010 ADA Standards for Accessible Design, published by the US Department of Justice as the 2010 ADA Standards. Section 603.3 covers mirrors. The US Access Board's own guide to toilet rooms states them directly: for a mirror above a lavatory or countertop, the bottom of the reflecting surface must be a maximum of 40 inches above the finished floor; for a mirror not above a lavatory or counter, the maximum is 35 inches.
Three things are worth understanding about that. First, it is a maximum, not a target — lower always complies. Second, it applies to the reflecting surface, not the frame, so a deep frame does not buy you height. Third, the standard governs facilities covered by the ADA; a private home is not subject to it. The calculator reports the check anyway, because it is a good proxy for whether a mirror is usable by a seated person, a child or a shorter adult, and because retrofitting a mirror is far more annoying than positioning it correctly the first time.
This is also why the gap field is called out. The habit of floating a mirror four to six inches above the backsplash is purely visual, and on a standard 34 inch counter with a backsplash it is exactly what pushes the bottom edge past 40 inches. If you want both, the answer is a taller mirror set lower, not a higher one.
When One Mirror Is the Wrong Answer
The percentage-of-vanity rule quietly assumes a single basin. On a double vanity it produces a very wide mirror that looks like a shop fitting and reflects the middle of the room rather than either person.
Two mirrors, each sized to its own basin, almost always read better and work better. Size each one against its half of the counter rather than against the whole: enter the width of one basin's section in the vanity field and the calculator will do the rest. The gap between them then becomes a design decision of its own, and running the sconce lighting in that gap is a common and effective arrangement.
The same logic applies to an off-centre basin. A mirror centred on the vanity when the basin is not centred on the vanity looks like a mistake, because it is one. Centre the mirror on the basin, and let the width be whatever leaves an even margin on the tighter side.
Lighting, Depth and the Things a Size Calculator Cannot See
Sizing the mirror is half the job. Three practical constraints sit outside the arithmetic and regularly override it.
Lighting is the first. A single fixture directly above a mirror throws downward shadows across the face, which is exactly wrong for shaving or makeup. Sconces at roughly eye level either side give flatter light, but they need wall space, and that space competes directly with mirror width. If lighting is going beside the mirror rather than above it, reduce the width ratio before you buy. Our room lighting lumens calculator handles the output side of that decision.
Depth is the second. A mirrored cabinet is not a mirror; it projects four to six inches from the wall, and over a shallow vanity it becomes something you hit your head on. Check the projection against the counter depth before treating a cabinet and a flat mirror as interchangeable.
The third is the wall itself. Tile layout, an outlet, a window reveal or a stud position can all rule out the calculated size. This is why measuring the tile field first is worth doing — a mirror that lands mid-tile on both sides looks considered, one that clips a cut tile does not. Our tile calculator and square footage calculator cover that side of the planning.
How This Differs From the Mirror Equation Calculator
Our mirror equation calculator is a physics tool. It solves the thin-mirror relation between object distance, image distance and focal length for concave and convex mirrors, and reports magnification. It has nothing to do with bathrooms and nothing to say about mounting heights.
This page is a fitting and layout tool: it never touches optics, and it works entirely in wall dimensions. The two share a word and nothing else. If you arrived looking for the optics, that page is the one you want.
Arb Digital designs and builds free interactive calculators that cite their sources, state which figures are conventions and which are requirements, and earn links because they are genuinely useful. Browse what we have published, or tell us what your audience keeps searching for.
Browse the Free Tools Hub Talk to Arb DigitalCommon Mistakes to Avoid
- Measuring the counter height from an unfinished floor. Tile and underlay can add an inch or more, and the accessible limit is measured from the finished floor.
- Buying a mirror the same width as the vanity by default. It works on a floating vanity with clear wall either side, and looks cramped almost everywhere else.
- Treating the frame as the reflecting surface. The 40 inch maximum applies to the glass. A deep bottom rail raises the reflecting edge above where you measured.
- Forgetting the light fixture. Reserving no space above the mirror and then adding a sconce is how mirrors end up cut down or returned.
- Using one wide mirror on a double vanity. Two mirrors sized to each basin are almost always the better result, and they leave room for lighting between them.
Related Free Tools From Arb Digital
Plan the surfaces around the mirror with the tile calculator, the paint calculator and the square footage calculator, and size the lighting with the room lighting lumens calculator. If your measurements are in mixed units, the feet and inches calculator and length converter will tidy them up. For the optics of curved mirrors rather than the fitting of flat ones, see the mirror equation calculator. Everything else is on the free online tools hub.
Frequently Asked Questions
Most single-vanity installations sit between 70 and 100 per cent of the counter width, and the calculator shows that whole band alongside your chosen percentage. It is a design convention rather than a rule, and a floating vanity with clear wall either side carries a full-width mirror far better than a vanity boxed in between walls.
For a mirror above a lavatory or counter in a facility covered by the ADA, the 2010 ADA Standards set a maximum of 40 inches above the finished floor for the bottom of the reflecting surface, and 35 inches where the mirror is not above a lavatory or counter. Lower always complies; those figures are ceilings, not targets.
No. The ADA Standards govern covered facilities, not private residences. The calculator reports the check anyway because it is a useful proxy for whether the mirror will work for a seated person, a child or a shorter adult, and because moving a fixed mirror later is a nuisance.
Anything from nothing to about six inches is used in practice, and it is purely visual. Be aware that on a standard counter with a backsplash, a four inch gap is usually what pushes the bottom edge past the 40 inch accessible maximum, so it is the first field to reduce if the check fails.
Two mirrors sized to each basin generally work better and look better than one very wide mirror, and they leave the space between for sconce lighting. To size them, enter the width of one basin's section of counter rather than the full vanity width.
It is taken as roughly 4.5 inches below standing height, a rounded convention used to check that the top edge is not below the tallest user's eyes. It is a rule of thumb for a sanity check, not a measurement of any particular person, and the top edge should sit comfortably above it rather than at it.
No. That tool solves the thin-mirror optics relation between object distance, image distance, focal length and magnification for concave and convex mirrors. This page is a fitting tool that works in wall dimensions and never touches optics.
This page is a planning aid. Width ratios and gaps described here are design conventions rather than requirements; the mounting heights are those published in the 2010 ADA Standards, which apply to covered facilities, and the authority having jurisdiction over your project governs what you actually have to build.
