Does Catholicism teach that there is a just interest rate? Is there a 'just interest rate'?  We see banks having interest rates when loaning money.  Can this be done justly? I am interested in the Catholic viewpoint.
Is there a 'just interest rate'?

The “just rate of interest” is the fair rate of interest which should corresponds to the average gain that those engaged in business may generally expect in a determined centre.

According to the Catholic Church, the Catholic Encyclopedia explains it as such:


  Just rate of interest
  
  Even today one can still sin against justice by demanding too high an interest, or usury, as it is called. What interest then is just and moderate? Theoretically, and in an abstract way, the fair rate of interest nearly corresponds to the average gain that those engaged in business may generally expect in a determined centre. It nearly corresponds, for the interest being guaranteed, whilst the profit is uncertain, we must discount the value of an insurance premium from the average profit. Accordingly, in a determined centre, if those who sink their money in buildings, land, or industrial undertakings generally look for a profit of 6 percent, the just rate of interest will be about 4 or 5 percent. This rate covers the risks and ordinary inconveniences of lending. But if one had to run special risks or had to give up an extraordinary premium, one might in all justice exact a higher rate of interest. Such, therefore, is the theoretical rule. In practice, however, as even the answer of the Sacred Penitentiary shows (18 April, 1889), the best course is to conform to the usages established amongst men, precisely as one does with regard to other prices, and, as happens in the case of such prices, particular circumstances influence the rate of interest, either by increasing or lowering it. In this way, the security offered by advances to the governments of wealthy countries and those that cover mortgages diminish the rate for public loans and loans on mortgage. On the contrary, the interest on shipping, and mercantile business is higher than that in civil business, on account of the greater uncertainty in sea voyages and in commercial enterprise.