These are the definitions of mass units slug and lbm:

Use these ratios (or their inverses) when converting. All three ratios are dimensionless and equal to 1:

Some textbooks use the archaic g_c, which is really just the inverse of the middle ratio above:

Note: It is not proper to say that one lbm equals one lbf, but it is proper to say that one lbm weighs one lbf. Example of how to use these conversion ratios properly - What is the weight of one lbm? Solution:

Useful conversions (in terms of dimensionless ratios) for energy, power and volume flow rate:

or

Example: If N = 3600. rpm, the angular velocity can be calculated as

Ideal Gas Equations
Note: The ideal gas constant, R, here is the specific ideal gas constant, not the universal gas constant, R_u. To convert from one to the other,

where M is the molecular weight of the gas.

Thermodynamicists like to use specific volume,

whereas fluid mechanicians like to use density, which is simply the inverse of specific volume, i.e.
.
Here are some ideal gas relationships:

Ideal gas constants for air in both SI and BG units:

English (BG) system:

Metric (SI) system: