Small, but perfectly formed: an interest calculation book from 1700

How was interest calculated in the 18th century? Learn about the smallest item in the Bank of England library’s special collection in our latest blog.
Published on 21 August 2023


Dorothy Fouracre, Collections Management Librarian

Deep within the Bank of England is an in-house library which not only supports the work of its staff, but also has a special collection of around 2,000 old or unusual books, gathered throughout the Bank’s history. Searching through this collection recently, I noticed a very small, unassuming book. Intrigued, I did some research, and discovered that not only is it (probably) the smallest book we have, but also extremely rare, and closely linked to the development of the City as a financial centre.

An interest calculation book title page

Title page from John Castaing’s Interest-book, 1700.

John Castaing’s 'An interest-book at 4, 5, 6, 7, 8 per c. from 1000 l. to 1 l., for 1 day to 92 days, and for 3, 6, 9, 12 months' was published in 1700. It sets out tables allowing the user to look up the interest gained on different sums of money over various time periods, rather than having to calculate it. Because it was used as a practical item in daily life, and therefore handled roughly and regularly, few copies have survived. There are three others in the UK, and one in the US.

Interest is the amount a lender charges for lending out money, usually expressed as a percentage of the loan. Determining interest was crucial to many of the financial and economic innovations of the Financial Revolution, which occurred during the late 1600s. Although earlier textbooks explained how to calculate it, handy reference books that eliminated the need for any sums, such as this one, grew in popularity from the 1690s onwards. The practical nature of this work was emphasised in advertisements for it, which described it as ‘very exact, and convenient, not much bigger than a spectacle case’. Its success is evident in the eight further editions that were published up until 1753. It was also heavily pirated, and the numerous errors in counterfeit copies led later editors to sign the genuine books in an attempt to authenticate them.


An interest calculation book table

Table from the book showing the interest gained on sums of money from £1 to £1,000 at 7%.

The Interest-book’s author, John Castaing, played a key role in the history of the London Stock Exchange. A French Huguenot, he came to England in the 1680s, escaping religious persecution in his home country. He became a broker in the emerging stock market and, in 1698, began posting the prices of stocks and commodities on the walls of Jonathan’s Coffee House in Change Alley, close to where the Bank now stands, twice a week. This is the first evidence of an organised exchange of this type in London, and the beginning of the modern stock exchange. Castaing’s Course of the Exchange became the de facto ‘official’ stock price publication. It is still published today as the Stock Exchange Daily Official List and is the longest continuously running financial periodical in the world.

Our copy of the Interest-book is signed by a John Ware with a stamp of a coat of arms, most likely his. Someone with this name worked at the Bank between 1746 and 1766, initially as an ‘out teller’ – collecting money due on bills and notes outside the building – before moving to the Cashier’s Office. Although the role of out teller might have involved calculating interest, this copy is in excellent condition and seems quite unused. It is also a little early for Ware’s job at the Bank. Perhaps, then, Ware just collected or enjoyed owning this little pocket calculator of its day.


An interest calculation book signature

Signature of John Ware and a coat of arms stamp.

An interest calculation book house list

‘House list’ of staff of the Bank in 1746. John Ware’s name appears at number 109, at the top right of the image. At the time, the Bank employed 137 people in total (Bank of England Archive reference E20/2).

The Bank’s library holds 11 other examples of such interest books, dating from 1700 to 1853. Chance encounters such as this remind me that much of our special collection remains relatively unexplored, and there must be many more (little) gems waiting to be rediscovered.