Usage: Enter an expression with units, for example "3*ft" or "5e-2*gal/min" in the expression and then click Compute to convert to SI. Or enter an equivelant unit such as "L/hr" and click compute. For a list of units, consult the list on the left.
You can add and subtract similar units, for example "10*psi-2*N/ft^2"
There is also an excel version of the unit converter. It has similar functionality.