Soap the mirror before you shower
The fix is already in the bathroom. A smear of soap polished into the glass stops fog forming at all — not wiped away after, prevented.
One trick, argued over by a lot of people who had tried the variants on mirrors, goggles and glasses. There is no count here — the honest units for this material are not the ones this badge was built for. Where those conversations disagreed about durability, the page says nobody settled it.
The fix has been sitting in the bathroom the whole time, and it is the soap.
Rub a little soap over the dry mirror, buff it off with a towel until the glass looks clean again, and the mirror stops fogging. Not clears faster — stops. Bar soap, dish soap, baby shampoo, shaving cream: they all do it, and people arrived at it separately, in conversation after conversation.
The reason it works is worth thirty seconds, because it also tells you when it won’t. Fog is water beading up into thousands of tiny domes, and it is the domes that scatter light. Soap kills the surface tension that lets water bead, so the same amount of moisture lies down flat and you see through it. You are not keeping the water off. You are flattening it.
Which means the trick belongs to a family. Divers spit on their masks for exactly this reason and have done for generations. The same smear works on swimming goggles, safety glasses, a motorcycle visor, ski goggles — anywhere warm damp air meets cold glass. One person mentioned that spacewalk helmets get the same treatment with liquid soap, which is a pleasing thing to know while standing in your own bathroom.
The other half of the problem
Soap treats the glass. It does nothing about the reason the room is full of steam, and the people who care about that go after the air instead: run the extractor fan, open a window, leave the door ajar. It is the more thorough fix, since damp air is also what grows mildew on the ceiling, and a mirror is only the most visible symptom of it.
The two aren’t rivals. Soap is what you do when the fan is inadequate, absent, or not yours to install — which covers a lot of bathrooms. The fan is what you do when the mirror was never the real complaint.
Neither is much help in a genuinely cold bathroom, where the glass sits far enough below the dew point that it fogs regardless and the room is too cold to want ventilated. That one has no clean answer here.
What nobody worked out
How long a single application really lasts. The reports run from a few showers to several months, and the difference is probably how much soap and how much polishing, but no one pinned it down. Try it, and find out what your own mirror does.
Either way the cost of finding out is a smear of something you already own and about a minute of buffing.
Common questions
Why does soap work?
Fog isn't a film of dirt, and it isn't really water sitting on the mirror. It's water forming thousands of tiny separate beads, and it's the beads that scatter the light. Soap breaks the surface tension that lets water bead up, so the same moisture spreads into a sheet thin enough to see straight through. Nothing is being kept off the glass — it's being made flat.
How long does it last, and is there something better?
Soap holds for a while and then washes away, and the honest answer is that people disagree about how long — some report weeks from one application, some reapply often. Shaving cream is the same trick with more staying power. If reapplying annoys you, the hydrophobic coatings sold for car windscreens do outlast soap considerably on a mirror, and that is the real tradeoff here: free and occasional, or bought and rare. Some people argued the coating genuinely outperforms soap rather than merely lasting longer, which is worth knowing before you decide soap is enough.
The mirror's already fogged. Now what?
Point a hairdryer at it on the hot setting. This gets argued about, and the argument has an answer: warming the glass lets the droplets evaporate, whereas cold air makes the air above a cold pane worse rather than better. It also beats a towel, which clears the fog and leaves streaks. A cheap squeegee does the same job silently, and if you run it over the walls and tub while you're there you'll slow the mildew down too.
Full tip: https://findangel.org/tips/soap-the-mirror-before-the-shower · FindAngel.org — free, always.