Historical Interest Rate Calculator

How to use

This website provides two approaches to calculating interest rates. Both use the same methodology but the front-end data entry interfaces differ based on how the data are presented in the historical sources.

  1. Calculate by known period - this is a simple interface to be used in those cases where the source describes the length of the loan as a given period of time but does not necessarily give any precise dates. For example, an account might state that the borrower paid £10 interest on a one-year loan of £30.
  2. Calculate by date range - this is a more complicated interface designed for use in those cases where the dates on which the principle was loaned and repayments were made are known. This will automatically calculate the length of the term of the loan from the dates given. It also supports multiple repayments and date ranges.

Dates

Dates should be given in the format dd/mm/yyyy - i.e. August 4, 1265 should be entered as 04/08/1265.

There were a number of ways of expressing dates in the medieval period. Invaluable help in working out historical dates can be found in A Handbook of Dates for Students of British History, ed. C .R. Cheney and M. Jones (2nd ed., Cambridge, 2000). Particular care should be taken with the start of the year of grace, which was frequently taken as 25 March in medieval England. This should be modernised to a year starting on 1 January, or the length of loan will be incorrectly calculated (i.e. a one-month loan advanced on 20 March 1300 and repaid on 20 April 1301, by the medieval reckoning, would appear as a loan of one year and one month to the calculator).

Date ranges

Where the precise date of a payment is not known, it is often possible to work out a range of possible dates. Obviously, the more closely this range can be narrowed, the more accurate will be the final interest calculation.

For example, an order for payment might be entered into an account for Michaelmas term. The earliest possible date should be entered into the first box (date of loan) and the latest possible date should be entered into the second box (latest possible date of loan). The calculator will then automatically work out the mid-point, minimum and maximum date ranges and interest rates.

Monetary values

Monetary values are entered in old-style pounds (£), shillings (s) and pence (d), with an additional box for marks (a unit of account equal to 13s 4d). Halfpence (obuli) and farthings should be entered into the pence box as 0.5 and 0.25 respectively. Half-marks can be entered into the marks box as 0.5, or as 6 shillings 8 pence. These historical figures will then be automatically converted into a single integer representing the value in pounds.

This system of money was fairly standard in the Middle Ages, forming the basis for the English sterling and the French livre tournois. From the later Middle Ages, however, a number of different coins were introduced, such as florins, the noble in England or the Franc in France. These can be entered into the pounds box (using decimal places to express fractions of a unit). An alternative would be to convert such coins into their equivalents in pounds, shilling and pence and then enter those values normally. Where values are given in mixed units, it will be necessary to convert them all into the same monetary format. Information about the relationships between different types of coin can be found in the Handbook of Medieval Exchange, ed. P. Spufford (Woodbridge, 1986).

Currencies

The interest rate calculations are based on the assumption that all monetary values are in the same currency (given this, it does not matter what that currency is, subject to the caveats expressed above). Where payments are expressed in different currencies, it is necessary to express all of the payments in the same currency. Many accounts or contracts will specify an exchange rate, and some information about changing exchange rates over time can be found online or in the Handbook of Medieval Exchange. It should be noted that adjusting exchange rates was another method of disguising the charging of interest.