Humidifier Size Calculator

A room can need little water to raise its humidity once, yet need much more over a day as drier outdoor air keeps entering. A humidifier must cover that continuing moisture demand as well as any initial shortage. Floor area alone cannot describe the load because ceiling height, air replacement, indoor temperature and outdoor moisture all affect it.

Relative humidity (RH) compares water-vapor pressure with the saturation pressure at the same temperature. It is not a direct measure of how much water the air contains. Cold outdoor air at 70% RH can contain less moisture per kilogram of dry air than a warm room at 40% RH. Warming that outdoor air without adding water lowers its relative humidity, which is why both temperature and humidity matter when estimating a winter humidification load.

Air changes per hour (ACH) describe the outdoor-air flow relative to the served room volume. At 0.5 ACH, an amount of outdoor air equal to half that volume enters each hour. Air merely circulating through a room fan or returning through a heating system does not count as new outdoor air. Closed doors also matter because a single humidifier may not distribute moisture throughout otherwise separate rooms.

Delivered moisture output

The water added to the air while the humidifier operates, expressed as a rate such as liters per hour. This is the quantity needed for a capacity comparison.

Usable tank volume

The water available before the tank needs refilling. A larger tank can extend operation between fills without increasing moisture output.

The initial room-air fill is a one-time addition to move from the current humidity to the target. Maintenance demand replaces moisture lost through continuing outdoor-air replacement. A first-day allowance combines both, but the initial fill should not be counted again as a recurring daily requirement. Fewer operating hours require a higher output while the humidifier runs, even when the daily water budget stays unchanged.

An air-only estimate does not describe how walls, furniture and other materials absorb or release moisture, so it cannot predict the time needed to humidify a real building. Capacity also does not establish a safe humidity setting. Indoor RH should not exceed 50%, and condensation on cold windows or walls can require a lower target.

How to Use This Tool:

Use measurements for the connected space and outdoor conditions you want to plan for. Results update as valid inputs change.

1. Enter Floor area and Average ceiling height for the space actually served. Choose the units before entering new measurements. Changing a unit converts an existing numeric value rather than simply relabeling it.
2. Enter Indoor temperature and measured Current indoor humidity, then set Target indoor humidity. The accepted target is 10% to 50%; that input range is not a recommendation for every room.
3. Set Outdoor temperature, Outdoor humidity and Outdoor air replacement. Below freezing, confirm that the outdoor RH reading is referenced to ice. The default 0.5 air changes per hour is a starting assumption, not a measured leakage rate; replace it with suitable leakage and ventilation information when available, excluding recirculation.
4. Set Available operation per day to the hours the humidifier can actually run. Air replacement is still budgeted for the full 24-hour day.
5. Open Advanced only to add a Capacity reserve, compare a Candidate delivered output, or enter a Usable water tank. Zero reserve adds no allowance; zero candidate or tank skips that check.
   An invalid reserve leaves an explicitly unreserved estimate. An invalid candidate or tank disables only its own check. Enter a supported value and unit, or use zero to skip, rather than leaving an optional number blank.
6. Review First-day output and Maintenance output in the Sizing ledger, then check the candidate decision when one is entered. If the summary says Check inputs, correct the highlighted required field or unit to restore the estimate.

Interpreting Results:

The Delivered output estimate runs from maintenance output to first-day output. These are two planning scenarios, not the lower and upper limits of a confidence interval. The L/h figures apply while the unit runs; the L/day figures describe the corresponding daily moisture demand. Initial room-air moisture is shown before reserve, while the daily demands include any valid reserve.

Meets first-day requirement. means the entered delivered rate can cover the modeled first-day water budget within the available hours. It does not verify that an advertised rating will be achieved in your room. Single-unit duty is the percentage of those available hours needed by one unit: 100% uses them all, while more than 100% means one unit cannot cover the budget.

An initial fill of zero can coexist with a positive maintenance demand. A room already at or above its target needs no immediate moisture addition, but dry replacement air can create a later maintenance need. A zero total means no humidification load is modeled, not that a dehumidification requirement has been calculated.

First-day tankfuls describes total water handling across the modeled space. First-day refills assumes one tank starts full; it is not a refill schedule for several separate units. Tank at full output is the operating time of one entered tank at one candidate's full delivered rate, not elapsed time under humidistat cycling.

Technical Details:

Moisture balance uses humidity ratio, the mass of water vapor per kilogram of dry air. Comparing humidity ratios accounts for the different indoor and outdoor temperatures; subtracting their RH percentages would not. The balance holds indoor temperature, outdoor conditions and air replacement constant at a standard pressure of 101325 Pa.

Moisture balance and delivered output

Saturation pressure follows the 2017 ASHRAE Handbook—Fundamentals equations documented by PsychroLib 2.5.0. For each air condition, let r be the RH percentage and p_s the saturation vapor pressure at its temperature. Actual vapor pressure p_v and humidity ratio W follow these equations, with pressures in pascals and W in kg water/kg dry air.

The subscripts t, c and o below mean target, current and outdoor. Target and current humidity ratios use the same indoor temperature; the outdoor ratio uses outdoor temperature. An entered RH of zero gives a humidity ratio of exactly zero.

Floor area A in m² and average ceiling height h in meters determine volume V. Dry-air mass m_da is evaluated at the target indoor condition, using the dry-air gas constant R_da of 287.042 J/(kg·K) and indoor absolute temperature T_i in kelvin.

Here p_t is target indoor vapor pressure. This target-condition dry-air mass is used for both initial fill and replacement demand; it is not recalculated at the current humidity.

Let a be outdoor air changes per hour, H the available operating hours within one day, and s the reserve percentage. Added water is approximated at a fixed density of 1 kg/L, denoted ρ_w. Initial fill I, maintenance water M and first-day water F are in liters. M and F cover one 24-hour period and are displayed as L/day.

The rates q_m and q₁ correspond to Maintenance output and First-day output in L/h. Reserve multiplies both the replacement load and the initial fill, although the separate initial-moisture figure remains unreserved. Shortening H increases the rates without reducing the 24-hour water demand.

The positive-part terms prevent negative humidification. Current RH at or above target makes initial fill zero. Zero air replacement, or an outdoor humidity ratio at least as high as the target ratio, makes maintenance zero. A moisture surplus outdoors is not subtracted from a positive initial fill, and neither surplus is converted into a dehumidifier load.

Checking a 72 m³ room

For 30 m² with a 2.4 m ceiling, use 21°C indoors, 25% current RH, 40% target RH, and 0°C outdoors at 70% RH. With 0.5 ACH, 24 available hours and no reserve, target, current and outdoor humidity ratios are approximately 0.006168416837, 0.003840975055 and 0.002637067779 kg/kg. The target-condition dry-air mass is about 85.555655 kg.

Using the unrounded values gives 0.199126 L of initial fill and 3.625523 L of maintenance water for the day. Their sum is 3.824648 L for the first-day allowance. Dividing by the available hours gives the output comparison.

The summary therefore displays 0.151 to 0.159 L/h. Reducing operation to 12 hours keeps the same daily water allowance but doubles the rates, displayed as 0.302 to 0.319 L/h.

Saturation pressure and temperature convention

Saturation pressure uses an ice branch at temperatures of 0.01°C or below, including the boundary, and a liquid-water branch above 0.01°C. The outdoor RH input must use the corresponding reference, particularly ice below freezing. There is no separate conversion of a subfreezing water-referenced RH reading.

For the equation below, T is the numerical temperature in kelvin and p_s is the numerical pressure in pascals. Convert °F to °C by subtracting 32 and multiplying by 5/9, then add 273.15 to the Celsius value to obtain kelvin. The logarithm is natural.

Exponentiating the right-hand side gives saturation pressure. These coefficients specify both temperature branches.

Equipment units and whole-unit counts

Candidate output is compared in L/h. Convert mL/h by dividing by 1000, L/day by dividing by 24, and US gal/day by multiplying by 3.785411784 and dividing by 24. A daily equipment rating always uses 24 hours for this conversion, not the selected available hours. For example, 500 mL/h and 12 L/day both represent 0.5 L/h; operating that rate for 12 hours supplies 6 L.

For a valid positive candidate rate C in L/h, single-unit duty D is a percentage of available hours and N is the number of identical units needed for the first-day total. Ceiling symbols mean upward whole-number rounding, subject to the numerical-equality allowance described below.

Zero candidate output skips the comparison. For valid positive candidates, the following decisions use the unrounded first-day rate, not maintenance output alone.

For a usable tank volume B in liters, tankfuls K and refills J use the whole space's first-day water allowance. Full-output tank time t_tank is available only when both tank volume and candidate output are valid and positive.

A zero tank volume skips refill estimates. One US gallon is exactly 3.785411784 L; the first-day US-gallon demand divides F by that factor. Geometry uses exactly 0.3048 m per foot and 0.09290304 m² per square foot.

Supported ranges and numerical precision

The limits below apply after unit conversion. They define accepted inputs, not recommended room sizes, operating conditions or humidity settings.

Calculations retain full floating-point precision. The sizing ledger generally displays up to three decimal places, or six for humidity ratios, with scientific notation for very small nonzero values. Matching rounded figures need not mean matching capacity: a candidate of 0.159 L/h is below the example's unrounded requirement of about 0.159360349 L/h, even though both can display as 0.159 L/h.

Capacity equality, the twice-capacity boundary and equipment maximums allow differences of at most eight relative binary64 rounding units, approximately 1.776 × 10⁻¹⁵ times the larger absolute value, with no absolute minimum tolerance. Whole-unit and refill counts round upward except when within both that relative tolerance and 0.0000001 of a positive integer. Positive demand never rounds down to zero units. Geometry and temperature range checks allow 10⁻¹⁰ in the converted base unit at their boundaries.

These allowances address numerical conversion roundoff, not uncertainty in a building or equipment rating. Extremely small candidate or tank entries that would produce non-finite results or unreliable whole-number counts are rejected for that optional check while the room's valid moisture estimate remains available.

Limitations and Humidity Control:

The result is an air-only planning estimate, not a complete building moisture calculation or an approved whole-home installation design.

• Wall and furnishing absorption, occupants, cooking and other internal moisture sources, heat or moisture recovery, and distribution between closed rooms are excluded. Standard pressure is fixed; altitude is not an input. A reserve adds a chosen percentage but does not calculate these missing effects.
• The first-day allowance adds an initial air fill to 24 hours of replacement demand evaluated at target conditions. It does not simulate the rise in RH or its fall between operating periods. Meeting the daily total with intermittent operation does not guarantee that the target is maintained throughout the day.
• Delivered output must be checked under the actual temperature and operating conditions. Tank size and advertised floor coverage are not substitutes for that rating. Furnace-mounted output also depends on furnace or blower availability, supply water and installation; the whole-home design needs a separate check.

Use a hygrometer away from the mist outlet and a humidistat to limit overshoot. The accepted 50% ceiling does not certify condensation safety. Lower the target or stop humidification if windows, walls or nearby materials become damp or show condensation.

For portable units, unplug before water handling or cleaning, empty and dry the tank, and refill it daily. Follow the manufacturer's cleaning and maintenance directions. A refill count of zero does not remove the need for daily water care; a directly plumbed unit simply has no portable tank burden to estimate.

Worked Examples:

References:

• Use and Care of Home Humidifiers, U.S. Environmental Protection Agency, April 29, 2026.
• Psychrometric equations and constants from the 2017 ASHRAE Handbook—Fundamentals, PsychroLib documentation, version 2.5.0.