{"id":"misun-indoor-humidity-calculator","version":"1.0.0","reviewedAt":"2026-08-10","purpose":"Convert outdoor temperature and humidity, plus an indoor temperature, into the indoor relative humidity that heating that air produces when no moisture is added, and place the result against published indoor humidity guidance.","constants":{"magnusReferencePressureHpa":6.112,"magnusCoefficientA":17.62,"magnusCoefficientBCelsius":243.12,"absoluteHumidityConstant":216.679,"epaIndoorRangeMinPercent":30,"epaIndoorRangeMaxPercent":50,"heatingSeasonTargetMinPercent":30,"heatingSeasonTargetMaxPercent":40,"defaultTargetPercent":35},"formulas":{"saturationVapourPressureHpa":"6.112 * exp(17.62 * t / (243.12 + t)), t in Celsius","dewPointCelsius":"alpha = ln(rh / 100) + 17.62 * t / (243.12 + t); dewPoint = 243.12 * alpha / (17.62 - alpha)","relativeHumidityPercent":"100 * es(dewPoint) / es(airTemperature)","indoorRelativeHumidityFloor":"100 * es(outdoorDewPoint) / es(indoorTemperature), capped at 100","absoluteHumidityGramsPerCubicMetre":"216.679 * es(dewPoint) / (airTemperature + 273.15)","moistureDeficitGramsPerCubicMetre":"absoluteHumidity(indoorTemperature, dewPointAtTarget) - absoluteHumidity(indoorTemperature, outdoorDewPoint), floored at 0"},"modelAssumptions":["Outdoor air enters the building and is heated to the indoor temperature","No moisture is added indoors by occupants, cooking, laundry, plants, showers, or a humidifier","No moisture is removed indoors by air conditioning, dehumidification, or condensation","The result is therefore a lower bound on indoor relative humidity, not a prediction of a specific room"],"bands":[{"id":"extremely-dry","label":"Extremely dry","minPercentInclusive":0,"maxPercentExclusive":10,"provenance":"Boundaries below 30 percent are Misun Health's own subdivision of the sub-guidance region, not a published threshold"},{"id":"very-dry","label":"Very dry","minPercentInclusive":10,"maxPercentExclusive":20,"provenance":"Boundaries below 30 percent are Misun Health's own subdivision of the sub-guidance region, not a published threshold"},{"id":"below-range","label":"Below the recommended range","minPercentInclusive":20,"maxPercentExclusive":30,"provenance":"Boundaries below 30 percent are Misun Health's own subdivision of the sub-guidance region, not a published threshold"},{"id":"in-range","label":"Inside the recommended range","minPercentInclusive":30,"maxPercentExclusive":50,"provenance":"Boundaries taken from EPA guidance for homes"},{"id":"above-range","label":"Above the recommended range","minPercentInclusive":50,"maxPercentExclusive":null,"provenance":"Boundaries taken from EPA guidance for homes"}],"processing":{"location":"Browser","submittedToServer":false,"storedByCalculator":false},"excludedByDesign":["No safety, efficacy, suitability, diagnostic, or medical result","No claim that any product treats, heals, prevents, or soothes a skin condition","No local climate dataset, forecast, or city lookup","No estimate of a building's air exchange rate or humidifier capacity","No claim that the output matches a hygrometer reading in a specific room"],"knownLimits":["The Magnus coefficients used are the over-water set, valid roughly -45 C to 60 C; readings over ice at very low temperatures carry a small additional error","Real indoor humidity is higher than this floor whenever people are present, and how much higher depends on the building's air exchange rate, which the calculator does not model","The 30 to 50 percent EPA range is comfort and building-health guidance, not a dermatological threshold","The band descriptions below 30 percent are Misun Health's own division of that region for routine guidance, chosen for readability rather than measured","The water-to-add figure is per cubic metre of indoor air and must be supplied continuously in proportion to the air exchange rate, so it is not a humidifier sizing figure"],"sourceLedger":[{"title":"WMO Guide to Meteorological Instruments and Methods of Observation (CIMO Guide, WMO-No. 8), Annex 4.B","url":"https://library.wmo.int/idurl/4/68695","supports":"The Magnus form for saturation vapour pressure over water and its coefficients, and therefore the dew point inversion"},{"title":"US EPA: Care for Your Air, a Guide to Indoor Air Quality","url":"https://www.epa.gov/indoor-air-quality-iaq/care-your-air-guide-indoor-air-quality","supports":"Keeping indoor humidity between 30 and 50 percent, the shaded band on the gauge"},{"title":"NDSU Extension: Measure Humidity Level to Avoid Dry Skin, Respiratory Irritations and Excess Moisture Problems","url":"https://www.ndsu.edu/agriculture/extension/measure-humidity-level-avoid-dry-skin-respiratory-irritations-and-excess-moisture","supports":"The 30 to 40 percent heating-season target and the observation that levels below about 30 percent are associated with dry skin and dry nasal passages"},{"title":"Egawa M, Oguri M, Kuwahara T, Takahashi M. Effect of exposure of human skin to a dry environment. Skin Res Technol. 2002;8(4):212-218.","url":"https://pubmed.ncbi.nlm.nih.gov/12423539/","identifiers":{"pmid":"12423539"},"supports":"Measured decrease in stratum corneum water content and increase in surface roughness after three to six hours of dry-environment exposure"},{"title":"Goad N, Gawkrodger DJ. Ambient humidity and the skin: the impact of air humidity in healthy and diseased states. J Eur Acad Dermatol Venereol. 2016;30(8):1285-1294.","url":"https://pubmed.ncbi.nlm.nih.gov/27306376/","identifiers":{"pmid":"27306376","doi":"10.1111/jdv.13707"},"supports":"Review-level summary of dry-air effects on the stratum corneum, and the authors' own statement that the literature overall is inconclusive"}],"correctionPolicy":"Review sources quarterly and correct confirmed constant, formula, or citation errors in place with a new version and reviewedAt date."}