This page links to three different software programs to calculate credit math functions, including calculating an Annual Percentage Rate (APR), a Military Annual Percentage Rate (MAPR), and Rule of 78 rebates. Each program has different limitations, such as whether it is compatible with MacOS or iOS. Some of the programs deal with irregular payments better than others or offer features the others do not. 

One program is available from the federal government and two were developed for NCLC.  Both NCLC programs are quite old and NCLC no longer supports them but they have been informally tested and should be usable for most versions of Microsoft Windows.  The federal government program was updated in December of 2022 and should work with any operating system.These programs use only one method each of estimating APRs, but the Federal Reserve Board sanctions several methods for computing APRs and allows creditors to disregard certain irregularities and to disclose APRs with certain tolerances. See the discussion in Truth in Lending § 5.5.


Federal Financial Institutions Examination Council (FFIEC) software is an online tool for verifying annual percentage rates and reimbursement adjustments. This program includes relevant finance charge and APR tolerances for verifying the accuracy of annual percentage rates and finance charges on loans secured by real estate or a dwelling. The online APR program also is a tool for verifying Military Annual Percentage Rates (MAPR) for loans subject to the Military Lending Act. It handles irregular payments. As an online tool it has the distinct advantage over the two NCLC programs in that it is compatible with any operating system—Windows, Android, MacOS, iOS, etc.

The FFIEC is a council with representation by the major federal agencies with authority to regulate banking and credit. The FFIEC updated this program on December 15, 2022, and the linked page will indicate any more recent updates. This makes this tool more recently updated by decades compared to the two NCLC calculation tools. The Office of the Comptroller of the Currency (OCC) has discontinued use of the OCC’s Annual Percentage Rate Calculation Program for Windows (APRWIN) in favor of this FFIEC computational tools.

The FFIEC program does not retain any entered information, so results should be printed out or otherwise saved after use. Although the FFIEC states that great care has been taken in the preparation of these programs, the FFIEC makes no warranty of complete accuracy. Further, the FFIEC does not recommend this program over other methods to calculate annual percentage rate or yields.

Consumer Law Math Program
(Developed for NCLC)

Consumer Law Math, for Windows, provides a user-friendly way of calculating the Annual Percentage Rate and Rule of 78 rebates, and making amortization charts, for regular transactions and transactions in which only the first and last payment are irregular. Of the three programs, it is the simplest and easiest to use.  It is not compatible with MacOS.

Consumer Law Math must first be installed on your hard drive before you can use it. Before installing, close other active applications. Unzip the file downloaded above and double-click Setup.exe. When finished with the setup process, the Consumer Law Math icon (a pencil) will be on the Start > Programs menu. (Users can create a desktop shortcut by right clicking the Consumer Law Math icon on the Programs menu and then clicking Send to > Desktop shortcut.). 

Usage pointers:

  • “Settings” allows for changes in screen colors and print fonts and how to enter dates.
  • There is no need to type “0” for zero in various entries.
  • Tab, Enter or Down Arrows accepts or skips over any entry.  Shift, Tab” or “Up Arrow” backs up to the prior entry.  The Left and Right Arrow keys are active only when editing text, not for moving from box to box.

Consumer Law Math is copyright (c) 2002 by Custom Legal Software, all rights reserved. NCLC thanks Custom Legal Software for providing this software.

NCLC APR Program

The NCLC APR Program calculates the Annual Percentage Rate, Rule of 78 rebates, and makes amortization charts. This program handles not only regular, but also irregular transactions — for example, where monthly payment amounts change over the course of the loan. It also prepares amortization charts for split rate and variable rate transactions. For regular transactions it also calculates the payment if one knows the principal, the term, and the contract rate of interest. For more information about amortization charts, split rates, and other interest rate calculation issues, see NCLC’s Consumer Credit Regulation ch. 5

Click the button above to download a zip file containing this program. After you unzip this file, simply double-click the icon to run the program. It is not compatible with MacOS.

The NCLC APR Program is copyright (c) 2002 by the National Consumer Law Center, all rights reserved. NCLC thanks Mark Leymaster, Renaissance Software, for developing and programming NCLC’s venerable (1980’s) DOS APR program, and the NCLC APR Program APR2002.exe, its successor for Windows 95 and later.