How these work, and what they assume
These are the tools that turn a hygrometer reading into a decision. Each one runs entirely in your browser: nothing you type is sent anywhere, there is no account, and they work offline once the page has loaded.
They all rest on the same piece of physics. The amount of water air can hold rises steeply with temperature — roughly doubling every 20 °F — so a fixed amount of water in the air produces a completely different relative humidity at 60 °F than at 75 °F. Saturation vapor pressure is calculated here with the Magnus–Tetens approximation, which is accurate to within about 0.7 °F across every temperature a house sees.
Where a calculator has to assume something, it says so on the page rather than burying it. The mold estimates assume a cold surface running about 5 °F below room air, which is typical of an uninsulated exterior wall or a window reveal; if you can measure the real surface temperature with an infrared thermometer, use that instead and the answer gets considerably better.
Which one you need
If you want to know whether a surface will be wet
The dew point calculator. Every surface in the house colder than the number it gives you is condensing right now. Pair it with an infrared thermometer and there is no guesswork left.
If you want to know whether you have a mold risk
The mold risk calculator works out the humidity at the surface rather than in the room, which is the figure mold actually responds to, and compares it with the 80% germination threshold.
If you are about to spend money
The dehumidifier size calculator gives a capacity in DOE 2019 pints from your floor area and how damp the space is, so you buy for the room rather than for the headline number on the box.
If you are deciding what to do right now
The window calculator compares indoor and outdoor dew points and tells you whether opening up will dry the house or wet it. The winter setting calculator does the same job for the thermostat season, giving the highest humidity your windows can take at today’s outdoor temperature.
The tools
Dew point
The temperature at which your air starts turning into water. Every condensation problem in your house is decided by this one number.
Dehumidifier size
Square footage and dampness set the capacity. Temperature decides the type — and getting that wrong wastes the whole purchase.
Mold risk
Explains the thing that confuses everyone: why one corner grows mold while the room reads a perfectly healthy 50%.
Should I open the window?
The advice "open a window" is right in winter and wrong on a muggy summer day. This tells you which one today is.
Winter setting
The winter target is not one number — it drops as it gets colder outside. This works out yours for today.
Moisture converter
Turns a percentage into the quantity it stands for, and shows what the same air reads at a different temperature.
Which one do you need?
If you have a hygrometer reading and want to know whether it is a problem, start on the home page checker — it gives you a ranked plan rather than just a verdict.
If mold keeps returning to one spot while the room reads fine, you need the mold risk calculator. That mismatch is the most misdiagnosed problem in a damp house, and it is a surface temperature issue rather than a room humidity one.
If your windows fog up, the dew point calculator tells you how much margin you have before it happens. And before you open a window to fix any of it, check the outdoor air — in muggy weather that move backfires.
Common questions
Is anything I type into these calculators stored or sent anywhere?
No. Every calculation happens in your browser in JavaScript; there is no server call, no analytics event carrying your numbers and no cookie recording them. Close the tab and the values are gone. The privacy policy says the same thing in more formal language.
Why do two calculators give slightly different answers for the same air?
Because they answer different questions. One reports the humidity of the room, another the humidity at a cold surface, another the total water content. They are consistent with each other — a wall five degrees below room air genuinely does read higher than the room — but they are not interchangeable, and the surface figure is the one that decides whether mold grows.
Can I use these with a Celsius thermometer?
Yes. Every calculator that takes a temperature has a °F/°C switch, and it converts the outputs as well as the inputs. The moisture-content figures are given in both grains per pound, which is the unit US HVAC work uses, and grams per cubic meter, which is the metric equivalent.