Specific humidity vs humidity ratio

Specific humidity (q) and humidity ratio (W) are both absolute moisture measures, but they use different bases (moist air vs dry air). Learn when each is used and how to convert.

If you work with psychrometrics, you will see several absolute moisture variables. Two of the most common are specific humidity (often q) and humidity ratio (often W, sometimes called mixing ratio). They are closely related, but they are not the same.

Definitions (what the denominator is)

Humidity ratio (W)

Mass of water vapor per mass of dry air.
Formula: W = mv / m_da
Common in HVAC and psychrometric charts because many energy and moisture balances are written on a dry-air basis.

Specific humidity (q)

Mass of water vapor per mass of moist air (dry air + water vapor).
Formula: q = mv / (m_da + mv)
Common in meteorology and atmospheric science because it is bounded between 0 and 1.

Conversion (the key relationship)

Since m_moist = m_da + mv, you can convert between them:

q = W / (1 + W)

W = q / (1 - q)

In typical comfort/HVAC conditions, W is small (a few g/kg), so q ~= W numerically, but the distinction still matters when comparing formulas, units, or software outputs.

Humidity ratio vs relative humidity (do not mix them)

Relative humidity (RH) is temperature-dependent and describes how close to saturation the air is. W/q are closer to absolute moisture content.

Using FluidTool

FluidTool's humid air calculator uses humidity ratio W as an input option (dry-air basis), which is standard for psychrometric work. If you have specific humidity q from another source, convert it to W first using the formulas above, then enter Tdb + W (or W + h) depending on what you know.

Quick links: Open humid air tool and Psychrometric chart guide.

Common pitfalls

Units mismatch: W is often shown as g/kg or kg/kg; q may be kg/kg. Always confirm units before comparing numbers.
Naming differences: "mixing ratio" and "humidity ratio" are sometimes used interchangeably, but the denominator matters. Check the definition in the source you are using.
Pressure/altitude: derived moisture properties depend on pressure. Use the correct site pressure (or altitude) for best accuracy.

Definitions (what the denominator is)

Humidity ratio (W)

Mass of water vapor per mass of dry air.
Formula: W = mv / m_da
Common in HVAC and psychrometric charts because many energy and moisture balances are written on a dry-air basis.

Specific humidity (q)

Mass of water vapor per mass of moist air (dry air + water vapor).
Formula: q = mv / (m_da + mv)
Common in meteorology and atmospheric science because it is bounded between 0 and 1.

Conversion (the key relationship)

Since m_moist = m_da + mv, you can convert between them:

q = W / (1 + W)

W = q / (1 - q)

In typical comfort/HVAC conditions, W is small (a few g/kg), so q ~= W numerically, but the distinction still matters when comparing formulas, units, or software outputs.

Units mismatch: W is often shown as g/kg or kg/kg; q may be kg/kg. Always confirm units before comparing numbers.
Naming differences: "mixing ratio" and "humidity ratio" are sometimes used interchangeably, but the denominator matters. Check the definition in the source you are using.
Pressure/altitude: derived moisture properties depend on pressure. Use the correct site pressure (or altitude) for best accuracy.

Definitions (what the denominator is)

Conversion (the key relationship)

Humidity ratio vs relative humidity (do not mix them)

Using FluidTool

Common pitfalls

Table of Contents

Specific humidity vs humidity ratio

Definitions (what the denominator is)

Conversion (the key relationship)

Humidity ratio vs relative humidity (do not mix them)

Using FluidTool

Common pitfalls

Table of Contents