Updated by Margot Sandoval. A $18 hygrometer settles this in one night. This is how the score works.
Quick answer
Measure before you buy either, because the best dehumidifier for bedroom use and the best humidifier for bedroom use solve opposite problems. Under 30 percent, you want a humidifier. Over 60 percent, you want a dehumidifier, and a humidifier will make the problem worse. Between 30 and 50 you are where the EPA wants you and need neither. An air purifier is a third appliance that changes the humidity not at all.
What people worry about here
My bedroom feels damp. Do I need a dehumidifier instead?
Check before you buy either. If your hygrometer sits above 60 percent, you want moisture out, not in, and a humidifier will make the problem worse. The CDC's mold guidance is to keep household humidity as low as you can and no higher than 50 percent. Both appliances are cheap next to a mold problem in a wardrobe.
Above 60%: buy the other applianceCDC mold guidance; US EPA
Between 30 and 50 percent, and the top of that range is a hard ceiling rather than a target. The EPA's humidifier fact sheet says plainly: do not humidify above 50 percent, because higher levels encourage biological growth in the room. Consumer Reports uses the same 30 to 50 band. Everything else on this page is in service of hitting that number.
30 to 50%US EPA, Use and Care of Home Humidifiers; Consumer Reports
Match the machine to the room, which is the first thing Consumer Reports' humidifier testing lead says in their buying advice, and resist the temptation to oversize. A machine built for 600 square feet in a 120-square-foot bedroom with the door shut will push you past 50 percent and start condensing on the window. The calculator below turns your floor area into a tank size and an output band.
Match the unit to the roomSamantha Ostrander, Consumer Reports
Dry enough for cracked lips, static and a sore throat on waking
A humidifier, sized to the room
30 to 50%
The band the EPA and Consumer Reports both name
Nothing. Keep the hygrometer
50 to 60%
Above the EPA's ceiling; watch the window glass in the morning
Turn the humidifier down or off
Over 60%
Damp. Mold-friendly, especially in wardrobes and on outside walls
A dehumidifier, plus ventilation
The CDC's mold guidance is to keep household humidity as low as you can and no higher than 50 percent. Cold rooms read higher than warm ones at the same water content, so take the reading where you sleep.
What to add
A hygrometer that is not inside the humidifier
SMARTRO SC42 digital hygrometer
Every piece of advice on this site is a number between 30 and 50 percent. Without a second thermometer-hygrometer about three feet from the machine, you are trusting a sensor that sits in the wettest air in the room.
What to look for: A stated accuracy figure, and a display you can read from the bed.
People search for both in the same week, so it is worth saying plainly: a purifier removes particles, a humidifier adds water. Neither does the other's job.
What to look for: Match the CADR to the room; a purifier does not change humidity at all.
Measure first. Below 30 percent, humidify. Above 60 percent, dehumidify. The CDC's mold advice is to keep household humidity no higher than 50 percent, so a damp bedroom is a dehumidifier problem even though it feels like an air problem.
Do I need an air purifier or a humidifier?
They solve different problems. A purifier removes particles from the air and changes the humidity not at all. A humidifier adds water and filters nothing. If your complaint is dust or allergens, buy the purifier; if it is a dry throat and cracked lips, buy the humidifier.
What humidity should I keep my bedroom at?
Between 30 and 50 percent relative humidity. The EPA's fact sheet tells you not to humidify above 50 percent because higher levels encourage biological growth. Buy a separate hygrometer and put it across the room from the machine.
Where should I put a humidifier in a bedroom?
On a flat surface raised at least two feet off the floor, which is Consumer Reports' advice, away from walls and wooden furniture, and not pointed at the bed. Put a tray under it if the surface is wood.
How often should I clean a humidifier?
Every third day, and empty and dry the tank every day you use it. That is EPA guidance, not a maker's recommendation. Scrub off scale and film, wipe dry, and rinse thoroughly if you used anything other than water.