| Parameter | Metric | Imperial |
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Moisture Content Reference: Common Canadian HVAC Conditions
| Condition | T (°C) | RH (%) | W (g/kg) | W (gr/lb) | Tdp (°C) | Pw (kPa) |
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All values at sea-level pressure (101.325 kPa). ASHRAE saturation pressure equations used throughout.
Moisture content of moist air: what it is and why it matters in Canadian HVAC
Moisture content is the mass of water vapour carried per unit mass of dry air. It goes by several names: humidity ratio, specific humidity, or simply W. In metric units it's expressed as g/kg (grams of water vapour per kilogram of dry air). In imperial, HVAC engineers typically use gr/lb (grains per pound), where one pound equals 7,000 grains. The two are related by a fixed factor: 1 g/kg = 7 gr/lb. Neither changes when you heat or cool the air without adding or removing moisture, which makes moisture content far more useful than relative humidity for latent load calculations, since you can't directly multiply a percentage by a mass flow rate, but you can multiply W (in kg/kg) by mass flow to get a moisture mass transfer rate in kg/s.
This calculator uses the ASHRAE Fundamentals saturation pressure equations, the same reference data NIST maintains for water's thermophysical properties, to convert between dry-bulb temperature, relative humidity, and moisture content. The same formula applies regardless of whether the input is RH, wet-bulb temperature, or dew point: first find the partial vapour pressure Pw, then compute W = 0.62198 × Pw / (Ptotal − Pw). The result is in kg/kg, which converts directly to g/kg by multiplying by 1000, or to gr/lb by multiplying by 7000. Use the vapor pressure calculator if you need Pw as a standalone output.
Moisture content and latent load calculations
Latent load in a cooling coil equals the air mass flow rate multiplied by the moisture content difference between entering and leaving air, multiplied by the latent heat of vaporisation: Qlatent = ṁ × (W1 − W2) × hfg, where hfg ≈ 2501 kJ/kg at 0°C. For practical HVAC work, the coil load calculator combines this automatically with the sensible component using full enthalpy values. But knowing moisture content independently is essential for specifying humidifier capacity (in kg/hr or lb/hr of steam), sizing dehumidification systems, and verifying that ventilation air doesn't overwhelm indoor moisture control. In Canadian winter, outdoor air at −20°C and 80% RH contains less than 0.6 g/kg of moisture. Heating it to 21°C without humidification drops the relative humidity to about 4%, far below the 25 to 30% minimum recommended for health and comfort. The moisture deficit that must be made up by humidification is the difference between the target indoor W and the outdoor W, multiplied by the outdoor air mass flow rate; the humidifier sizing calculator turns that deficit directly into a required humidifier capacity.
Absolute humidity vs. humidity ratio
Absolute humidity (ρv) is the mass of water vapour per unit volume of moist air, in g/m³ or lb/ft³. It differs from moisture content (W, which is per unit mass of dry air) because air density varies with temperature and pressure. Absolute humidity is useful for IAQ sensors and some ventilation calculations, but it's less commonly used in HVAC psychrometric analysis than W because absolute humidity changes with temperature even when no moisture is added, making it awkward to track across heating and cooling processes. This calculator outputs both W and absolute humidity so you can use whichever is appropriate for your application. For moisture removal rate calculations, always use W multiplied by dry-air mass flow, not absolute humidity multiplied by volumetric flow, to avoid density correction errors. See the absolute humidity calculator for the g/m³ calculation on its own.
Frequently Asked Questions
Humidity ratio (also called moisture content or specific humidity) is the mass of water vapour per unit mass of dry air, expressed as kg per kg or g per kg in metric, or grains per pound in imperial. It is an absolute measure of the actual amount of moisture in the air and does not change when the air is heated or cooled (unless moisture is added or removed). Relative humidity, by contrast, is the ratio of the actual vapour pressure to the saturation vapour pressure at the same temperature, expressed as a percentage. It changes with temperature even when the moisture content stays constant: heating air raises its saturation pressure while the vapour content stays fixed, so relative humidity drops. Moisture content (humidity ratio) is therefore the more useful parameter for HVAC latent load calculations, because it directly measures the moisture mass that must be added or removed regardless of temperature. The humidity ratio calculator computes W from any two psychrometric properties, and the relative humidity calculator handles RH conversions.
Grains per pound (gr/lb) is a direct measure of moisture mass per unit mass of dry air, making latent load calculations straightforward. One grain equals one seven-thousandth of a pound. To calculate the moisture removal rate in a cooling coil, multiply the air mass flow in lb per hour by the humidity ratio difference in lb per lb (or equivalently, divide the gr/lb difference by 7000 to get lb per lb, then multiply by mass flow). Relative humidity cannot be used directly in these calculations because it changes with temperature and does not represent a fixed amount of moisture. In Canadian HVAC practice, gr/lb appears frequently in imperial equipment specifications and psychrometric charts, while g per kg is used in metric work. Both express the same quantity: the mass of water vapour carried per unit mass of dry air. The grains of moisture calculator is useful when equipment specs list capacity in grains per hour of moisture removal.