Browser-only humidity tool
Your weather app can say 70 percent humidity while your living room sits near 15 percent. Heating air does not add water to it, so cold outdoor air that leaks inside and warms up reads far drier than it did outside. Enter what is happening outdoors and what your thermostat says, and this works out the indoor relative humidity that produces, how far it sits below the range the EPA gives for homes, and what that means for how you moisturize.
Sources reviewed August 10, 2026
Run your numbers
Outdoor temperature and outdoor humidity are on the front screen of every weather app. If yours shows a dew point instead, switch the input and use that directly, as it is the figure the whole calculation runs on anyway.
Read the two outdoor figures off any weather app, then set the number on your thermostat.
30 to 40 percent is the usual heating-season target. Higher tends to condense on cold glass.
Indoor relative humidity, unhumidified
16%
Very dry
Shaded band: the 30 to 50 percent the EPA gives for homes. Scale runs 0 to 70 percent.
Outdoor dew point
21.4 F
Outdoor air reads 70% humidity at its own temperature.
Below EPA range by
14 points
Measured against the bottom of the 30 to 50 percent range.
Water to add per cubic metre
3.5 g
To reach 35 percent, before any air the room exchanges with outside.
What this reading means
This is the reading most heated homes produce during a cold snap. It sits well below the EPA range, and it is the usual reason a routine that worked in autumn stops feeling like enough in January.
Keep the cleansing step short and lukewarm, apply while skin is still damp, and finish dry patches with a heavier balm-style layer rather than more of a light cream. Expect to reapply to hands more often than to face.
This is a floor, not a forecast. It assumes outdoor air is heated to your indoor temperature with no moisture added. People, cooking, laundry, plants, and showers all push the real number up, and a humidifier is designed to. A cheap hygrometer tells you where your rooms actually sit.
The mechanism
Relative, not absolute
Relative humidity is a ratio: how much water vapour the air holds against the most it could hold at that temperature. The bottom of that fraction climbs steeply with warmth. Saturation vapour pressure roughly doubles for every 10 C of warming, so the same water in warmer air reads as a much smaller percentage.
Dew point stays put
Dew point is the temperature at which air would become saturated, and it moves only when water is actually added or removed. A furnace does neither. That is why a cold, damp-sounding day outdoors becomes a very dry indoor day, and why dew point is the honest number to track in winter.
Skin sits in the gap
Water leaves skin toward air that is not saturated, and drier air widens that gradient. Egawa and colleagues measured a significant fall in stratum corneum water content and a rise in surface roughness after only three to six hours in a dry environment. Weeks of it is the ordinary winter experience.
Worked example
Thermostat at 68 F
It is 25 F outside with 80 percent humidity, which sounds damp. That air has a dew point near 20 F. Bring it inside and warm it to 68 F and it reads about 16 percent. The forecast said 80 and your skin gets 16.
Thermostat at 74 F
Nothing outside changed. Turning the thermostat up six degrees pushes the same air down to roughly 13 percent, because you raised how much vapour the air could hold without adding any. Running warmer is a humidity decision as much as a comfort one.
Both figures are floors, and a lived-in house sits somewhere above them because cooking, showers, laundry, and people all add water. The useful part is the direction and the size of the move. If your rooms feel dry in January and fine in May, the reason is arithmetic rather than anything that changed about your skin, and the two things you can act on are the moisture in the room and the layer you leave on the skin that faces it.
What the tool cannot do
This models one thing exactly: outdoor air heated to your indoor temperature with no moisture added and none removed. Every real building departs from that. A tight, well-sealed home exchanges its air slowly, so the moisture people generate inside accumulates and the true reading can sit well above the floor. A draughty house tracks the floor closely. Nothing in a temperature figure tells anyone which of those you live in.
The water-to-add figure is per cubic metre of air in the room, before accounting for how fast that air is replaced. A house that swaps its full volume of air every hour needs that amount every hour, not once. It is there to give the size of the gap, not to specify a humidifier.
It is also not medical guidance. It reports an air measurement and a general comfort range, and the routine notes are cosmetic suggestions about moisturizing technique. Skin that is broken, painful, swollen, or not settling is a reason to get individual advice rather than to consult a humidity number.
Acting on the number
The room is not the constraint. If skin still feels tight after washing, look at water temperature, how long the shower runs, whether the cleanser strips, and whether the moisturizing step happens consistently rather than occasionally. A simple routine done every day beats a complicated one done sometimes.
The usual autumn and shoulder-season reading in a heated house. One thorough moisturizing step applied while skin is still damp handles most of it, with a denser layer kept for hands, elbows, knees, and feet, which lose ground first. Raising the room a few points closes much of the remaining gap.
The deep-winter reading in most heated homes, and the level at which technique starts to matter more than product choice. Apply within a minute of towelling off while skin still holds water, keep showers shorter and cooler, and switch dry zones to a heavier occlusive layer instead of applying more of a light one. Hands need reapplication tied to washing rather than to a schedule.
Extreme, and usually a cold snap in a dry climate or a hard-running heating system. At this level a humidifier does more than any change of product, because it works on every square inch of skin all day including overnight. Pair it with an occlusive layer on the areas that get wet and re-dried most, and expect lips and hands to need attention several times a day.
Common questions
Because heating air does not add any water to it. A parcel of outdoor air that leaks into your house keeps the same amount of water vapour it had outside, but warming it raises how much vapour it could hold, and relative humidity is the ratio of those two things. Outdoor air at 30 F and 70 percent humidity is carrying very little water in absolute terms. Heat that same air to 70 F and it reads near 16 percent. Nothing was removed. The air simply got roomier.
EPA guidance for homes is 30 to 50 percent, and NDSU Extension puts the heating-season target at 30 to 40 percent and notes that below about 30 percent is where dry skin and dry nasal passages start being reported. Those are comfort and building-health figures rather than a medical threshold, and people differ. Treat anything under 30 percent as a reason to expect skin to feel tighter after washing, and anything under 20 percent as the level where most routines need a change of technique rather than only a change of product.
No, and that is the point. A weather app reports outdoor relative humidity at the outdoor temperature. It can read 70 percent outside on a freezing day while your living room sits near 15 percent. The number that matters for how your skin feels indoors is the one after that air has been heated, which is what this calculates.
The physics is exact for the case it models: outdoor air heated to your indoor temperature with nothing added. Real rooms are damper than that floor, because people, cooking, laundry, plants, and showers all release water, and a humidifier is designed to. How much damper depends on how airtight the building is and how fast it exchanges air with outside, which no calculator can guess from a temperature. A hygrometer costs a few dollars and tells you the truth about your own rooms. Use this to understand why the reading moves, and how far it can fall on a cold day.
They do different jobs. Raising room humidity reduces how fast water leaves skin everywhere in the house, all day, including while you sleep. An occlusive moisturizer slows water loss from the skin you put it on. In a room reading in the teens, most people notice the humidifier more on face and lips and the moisturizer more on hands, which get wet and re-dried many times a day. Neither one substitutes for the other.
No. Everything is computed in your browser as you type. Nothing is submitted, stored, or saved, and there is no signup.
The layer you leave on
A humidifier changes the room. A moisturizer changes what happens at the skin surface in the meantime. Misun makes two, both built on 100% grass-fed New Zealand tallow: the whipped cream for everyday coverage and the balm for the dry zones that need a denser layer. Amazon shows the current package, price, availability, delivery options, and customer reviews.
$19.97
100% Natural Whipped Beef Tallow Cream
$17.97
Dense Beef Tallow Balm
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