# Mass units

The basic units for all CSI Software include force (lbs, kip, N, kN, etc.), length (ft, in, m, mm, etc.), time (second), and temperature (F, C). Mass is then derived from these values.

Some notes which concern force and mass units are as follows:

• lbs and kips are only used as force units, not mass units
• Consistent mass units are force divided by unit acceleration, given as F/(L/T2), in which L is length and T is time. Weight is a force.
• Given SI units, in which force is in Newtons (N), the consistent mass unit is N*sec2/m = kg.
• Given US Customary, in which force is in pounds (lbs), the consistent mass unit is lbs*sec2/ft = slug. Under a unit acceleration, a mass of one slug generates a one pound force. When accelerated by gravity (g), this same mass generates a force of 32.2 lbs.
• The pound-mass unit is not a consistent unit, where 1 slug = 32.2 pound-mass. Additional information is available in the Wikipedia Pound (force) article.

• Weight density has units of F/L3
• Consistent mass density has units of (F/(L/T2))/L3 = F*T2/L4
• The relationship between weight and mass, and weight density and mass density, is W = M * g, all in consistent units.
• To convert weight density W = 150 lbs/ft3 into mass density (kip-in), the calculation proceeds as follows:

M = W / g = 150 lbs/ft3 / (32.2 ft/sec2) = 4.658 lbs*sec2/ft4 * (kip / 1000 lbs) * (ft / 12 in)4 = 2.246E-7 kip*sec2/in4