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Units_in_MC.xmcd 9/25/2007 Using Units (SI) Units of Pressure - Pascal (Pa), Pounds per square inch (psi) which is really lb-force per square inch 1 lbf 2 3 = 6.895 × 10 Pa 6895Pa = 1 ⋅ psi in 1 lbf 2 3 = 1 ⋅ psi 1psi = 6.895 × 10 Pa in Now, multiplying psi * in^2 yields a force: 2 1psi⋅ 1 in = 4.448 N Remember, there is a difference between lb (pound-mass) and lbf (pound force) So: 4.448N = 1 ⋅ lbf (substitute lbf for units) if you use lb, then you get strange results such as: 4.448N = 9.806 m 2 ⋅ lb s You might recognize the 9.806 m/sec^2 as the acceleration of gravity. Unfortunately, if this was used in a complex function, you would have no idea why you had strange values and units. The lb is pound-mass as far as Mathcad is concerned. Units_in_MC.xmcd 9/25/2007 To set the units for an entire system, use Tools - Worksheet Options