Jaap's Mechanical Calculators Page

Monroe Calculating Machines

Monroe Model K

My Model KA
Books, Manuals
Advertisements and Articles

The Monroe Calculating Machine Company was founded by Jay R. Monroe in 1912 in order to make calculators based on the latest design by Frank Baldwin. The calculators had a hand crank and a moving carriage, similar to the Odhner pinwheel calculators, but to enter a number it had a keyboard similar to a Comptometer instead of dials or sliders. This allowed anyone familiar with the pinwheel calculators to use it with little practise and to perform calculations much faster than before. Although the Comptometer was still faster in operation, it needed a lot of training to reach that speed on a Comptometer compared to the Monroe.


A Monroe machine typically a keyboard that has 8 columns of keys. Each column has 10 keys, numbered from 0 to 9. When you press a key to select a digit, the key stays pushed in. If you push a second key in the same column, the first key pops back up so that you can only select (at most) one digit in a column. After entering a number in this way, you can turn the hand crank on the right of the machine. Turning the crank clockwise once adds the selected number to the register, and turning it the other way subtracts it. On an electrically driven model you have to press the Plus (+) button or the Minus (-) button to activate the motor instead of turning a crank.

At the right of the keyboard are three buttons. The largest one is labelled Clear, and it clears the selected number from the keyboard, causing all pushed-in buttons to pop up. The other two buttons are marked Repeat, and Non-Repeat, and when one of them is pressed, the other pops up. When Non-Repeat is selected, the keyboard will be cleared automatically when the turn of the hand crank is completed. When Repeat is selected this does not happen, so that you can turn the crank repeatedly to add the selected number to the register several times.

The register, typically 16 digits long, is located in a carriage at the back of the machine, like a standard typewriter. The carriage can be moved along a digit to the left or right by turning a handle at the front of the machine. This allows you to add 10 times the selected number to the register with each turn of the hand crank, and by shifting the carriage again you can add 100 times the selected number, and so on. In this way it becomes easy to do multiplications. For example, to multiply the number you entered by 123, you would make sure that Repeat was pressed down, and then turn the crank three times, shift the carriage to the right, turn the crank twice, shift the carriage again, and turn the crank once more.

It should be noted that the register does not have a carry mechanism inside the carriage. Carries are handled by rotary mechanism in the body of the machine that drives the register. This means that if the carriage is shifted to the left and sticks out, the digits of the register in that part of the carriage will not be updated by a turn of the hand crank, and only the digits aligned with the keyboard will update together with one or two further digits on the left. In practice this rarely becomes a problem.

The carriage also has a second smaller register which acts as a counter. Every time you turn the crank to add a number, the counter increments the digit that is lined up with the right column of the keyboard. If you subtract a number, the digit will be decremented. The digits shown can be the values 0 to 9 displayed in black, but when a digit gets decremented below zero, it will show as a red digit 1 to 9 to represent -1 to -9. In a multiplication, the counter keeps track of what number you have multiplied the selected number by so far, and similarly in a division it shows the number of times you have subtracted so far. Note however that the counter does not perform any carries, so if you turn the crank more than 9 times without moving the carriage, the counter's digit will roll over from +9 to -9 or vice versa without adjusting the next digit in the counter.

A clockwise turn of the handle on the right of the carriage will clear the counter, and an anti-clockwise turn clears the register.

If you need a counter that does perform carries, then you can use some columns in the main register. To this end there is a knob to the left of the keyboard labelled Item Count. If you turn that knob clockwise, and press down the 1 key in the left column, that key will become locked down. It will not be cleared with the rest of the selected number. This gives you a 3-digit counter in columns 8 to 10 of the main register which will count the number of turns of the crank. When adding a long list of numbers, it gives you how many numbers were in the list.


The first calculator that Monroe marketed was the model D in 1915. This was followed by the models E, F, and G over the next four years. These were not very successful, and not very many of them survive today. It seems that models A, B, and C were the names attached to prototypes that never made it to production. The same may be true of models H, I, and J, as there are no known machines with that model letter.

The model K was released in 1921, and produced through most of the 1920s. It was commercially very successful, even though it is actually not very different to the previous models. What may have helped its success is that an electrically driven version was made, the model KA where the A stands for automatic.

In 1929 Monroe made the model L which is virtually the same as the model K except that it is minituarised to about half the size. Whereas the model K is large and very heavy, and the model KA even more so with its heavy electric motor, the model L is much more portable. Several further models followed, many of them electrically driven. They also produced many listing calculating machines.

My Model KA

I own a Monroe KA which has the serial number KA160-129883. The 3-digit number specifies the exact variant of the model, where the first two of those digits show the length of the register. I do not know the difference between the 160 and 161 variants, but the 163 variant has an extra counter register at the top left of the carriage.

The calculator has some problems apart from the trivial cosmetic issues. The electrical wiring of this calculator has deteriorated badly, and needs to be replaced. The Clear button should have a large red key top but it has been replaced by a black key labelled Error. There are two small parts missing: There should be a mount at the top left of the keyboard that holds the linkage between the locking bar of the keyboard and the rotary mechanism (this prevents keys being pressed while the calculator is performing an operation). Also a ring is missing on the axle of the carriage shifting knob, which can cause the axle to shift forward and collide with the calculation mechanism. Overall, the mechanism seems to be in working order since it will add or subtract if you twist the knob on the right hand side where a hand crank would be on the manual version.

Monroe KA-160
Monroe KA-160, rear
Monroe KA-160, keyboard
Monroe KA-160, logo
Monroe KA-160, motor label
Monroe KA-160, opened, top
Monroe KA-160, opened, left
Monroe KA-160, opened, rear
Monroe KA-160, opened, right
Monroe KA-160, serial number
Monroe KA-160, opened, carriage
Monroe KA-160, opened, planetary gears

Books, Manuals

Instruction Book Monroe Calculating Machine
Monroe Calculating Machine Co., New York
48 page stapled book
177mm × 241mm

This is the manual for the Monroe calculator. It has a copyright year of 1919, which is when the model F was being produced. It describes addition, subtraction, multiplication, division, decimals, discounts, and interest, followed by oiling instructions and several tables. The main difference between handling the model F and later models is that to clear the register the carriage has to be manually lifted while turning the clearing handle. Later models lift the carriage automatically when the clearing handle is turned.

A complete scan of the book can be found at Cris vande Velde's site.

Monroe Instruction Book, front cover
Monroe Instruction Book, title page
Monroe Instruction Book, foreword
Monroe Instruction Book, part I
Monroe Instruction Book, index

The Monroe Calculating Machine (Office Machine Practice)    (PDF, 16.2 MB)
Chester H. Katenkamp
The Gregg Publishing Company
58 page stapled book
142mm × 204mm

Chester Katenkamp wrote a series of four Office Machine Practice booklets, of which this is the one for the Monroe. The others were for the Comptometer, the Burroughs calculator, and the Sundstrand machine. It is essentially a manual, explaining the four arithmetic operations, fractions, discounts, chain discounts, and decimal points.

This book is in good condition, partly because the centre pages have been strengthened with tape along the spine to prevent the staples from tearing through, and similarly there is tape along the spine on the outside of the cover. The name of the original owner is written on the title page, but the book is otherwise clean.

Office Machine Practice - The Monroe Calculating Machine, front cover
Office Machine Practice - The Monroe Calculating Machine, title page
Office Machine Practice - The Monroe Calculating Machine, index
Office Machine Practice - The Monroe Calculating Machine, pages 2 and 3
Office Machine Practice - The Monroe Calculating Machine, pages 26 and 27

Monroe Machine Methods For the Extraction of Square Root    (PDF, 5.07 MB)
Monroe Calculating Machine Company, Inc.
11 page stapled booklet
150mm × 229mm
Form 769-S

This booklet explains how to take square roots when using the Monroe calculator.

Monroe Machine Methods For the Extraction of Square Root, front cover
Monroe Machine Methods For the Extraction of Square Root, title page
Monroe Machine Methods For the Extraction of Square Root, first page
Monroe Machine Methods For the Extraction of Square Root, last page
Monroe Machine Methods For the Extraction of Square Root, back cover

Monroe Machine Methods For the Extraction of Cube and Other Roots    (PDF, 3.90 MB)
Monroe Calculating Machine Company, Inc.
8 page stapled booklet
149mm × 228mm
Form 720 S

This booklet explains how to take cube or higher roots when using the Monroe calculator. It uses Newton's Method to get successively better approximations.

It does not have a copyright year inside, but the similar booklet for the square root refers to this one, so it is presumably from the same year, 1933.

Monroe Machine Methods For the Extraction of Cube and Other Roots, front cover
Monroe Machine Methods For the Extraction of Cube and Other Roots, first page
Monroe Machine Methods For the Extraction of Cube and Other Roots, last page
Monroe Machine Methods For the Extraction of Cube and Other Roots, back cover

Advertisements and Articles


I found many advertisements in online archives of newspapers and magazines. They are too numerous to show here, so they can be found on a separate Monroe Advertisements page.


Here are a few articles taken from various online archives.

1920-04-18 Arizona republican
Sponsored article promoting the Allen car used by a Monroe salesman.
(1920-04-18 Arizona republican)
1922-03-09 The Michigan Alumnus
Jay R. Monroe going on a European tour.
(1922-03-09 The Michigan Alumnus)
1922-05-06 The Washington herald
Monroe donates series of calculators to the National Museum.
(1922-05-06 The Washington herald)
1926-09 PopularMechanics
Advert for LaSalle Extension University, focussing on a Monroe district manager.
(1926-09 PopularMechanics)

Section about Monroe calculators.
(1925 Die Rechenmaschinen by Ernst Martin)

1925 Ernst Martin
1925 Ernst Martin
1925 Ernst Martin
1925 Ernst Martin
1925 Ernst Martin
1925 Ernst Martin


The Monroe company filed a large number of patents on every detail of the calculators. I have therefore cut this list off at 1935, leaving off the 170 or so patents that followed in the next 35 years.

PatentFiling datePriority dateNameDescription
US 1,080,24515-06-191202-12-1913Baldwin, Frank S.Calculating machine
US 1,180,41004-06-191425-04-1916Monroe, Jay R.; Phinney, Edgar E.Carriage-shifting means for calculating-machines
US 1,197,53804-12-191405-09-1916Phinney, Edgar E.Carrying mechanism for calculating machines
US 1,207,70024-02-191512-12-1916Baldwin, Frank S.; Baldwin, George H.Calculating Machine
US 1,208,19025-09-191412-12-1916Monroe, Jay R.Carriage-lock for calculating-machines
US 1,208,36627-10-191412-12-1916Phinney, Edgar E.Spring brake for gears
US 1,215,18603-08-191406-02-1917Phinney, Edgar E.Carrying Mechanism for Calculating Machines
US 1,272,08327-08-191509-07-1918Phinney, Edgar E.Carrying mechanism for calculating machines
US 1,275,11924-09-191406-08-1918Baldwin, Frank S.Zero setting mechanism
US 1,335,34908-04-191930-03-1920Phinney, Edgar E.Automatic carriage raising means for calculating machines
US 1,354,83908-03-191705-10-1920Phinney, Edgar E.Keyboard clearing means for registers
US 1,358,51715-03-191909-11-1920Burne, James C.Attachment to adding-machines
US 1,384,59214-04-192012-07-1921Britten, Edwin F.Shock-absorber for calculating-machine carriages
US 1,384,63225-02-192012-07-1921Phinney, Edgar E.Key lock for calculating machines
US 1,384,63328-02-192012-07-1921Phinney, Edgar E.Reverse operation check for registers
US 1,384,63407-02-192012-07-1921Phinney, Edgar E.Carriage-lock for Calculating-machines
US 1,396,61223-10-192008-11-1921White, NelsonZero setting mechanism
US 1,396,66226-12-191808-11-1921Phinney, Edgar E.Calculating machine
US 1,396,66315-03-192008-11-1921Phinney, Edgar E.Crank-handle latch
US 1,432,25603-06-191917-10-1922Phinney, Edgar E.Calculating Machine
US 1,432,61608-03-192017-10-1922Phinney, Edgar E.Keyboard clearing means for calculating machines
US Re 16,20721-09-192310-11-1925Phinney, Edgar E.Keyboard-clearing means for calculating machines
US 1,462,05814-04-192017-07-1923Britten, Edwin F.Carry Mechanism for Registers
US 1,462,05903-08-192017-07-1923Britten, Edwin F.Automatic carriage raising means for calculating machines
US 1,462,06023-11-192017-07-1923Britten, Edwin F.Carry set mechanism for calculating machines
US 1,462,06123-11-192017-07-1923Britten, Edwin F.Overthrow check for calculating machines
US 1,544,80616-01-192207-07-1925Chase, George C.Full cycle stop for calculating machines
US 1,664,66107-11-192503-04-1928Chase, George C.Full cycle positioning mechanism
US Re 18,13018-03-193021-07-1931Chase, George C.Full cycle positioning mechanism
US 1,685,07415-03-192618-09-1928Chase, George C.Multiplier mechanism for calculating machines
US Re 17,46607-08-192922-10-1929Chase, George C.Multiplier mechanism for calculating machines
US 1,750,56522-09-192511-03-1930Britten, Edwin F.Zero key machanism for key set registers
US 1,773,02503-10-192712-08-1930Chase, George C.Carriage-shift control for calculating machines
US 1,773,02615-10-192712-08-1930Chase, George C.Carriage-shifting mechanism for calculators
US 1,773,02712-08-192912-08-1930Chase, George C.Calculating machine
US 1,781,32010-04-192911-11-1930Crosman, Loring P.Zero setting device
US 1,794,51406-04-192903-03-1931Chase, George C.Driving mechanism for calculating machines
US 1,801,48426-02-193021-04-1931Bricken, JohnKeyboard for calculating machines
US 1,805,06024-06-193012-05-1931Walter, Edward C.Calculating machine
US 1,805,06122-07-193012-05-1931Walter, Edward C.Register
US 1,829,21005-01-192727-10-1931Chase, George C.Automatic Carriage Shifting Mechanism for Calculators
US 1,849,44802-02-192915-03-1932Britten, Edwin F.Registering Mechanism
US 1,877,80129-05-193020-09-1932Britten, Edwin F.Calculating Machine
US 1,877,80213-02-193220-09-1932Britten, Edwin F.Calculating Machine
US 1,883,76022-04-193118-10-1932Bricken, JohnCalculating Machine
US 1,911,79623-07-193130-05-1933Brown, Lee R.Register
US 1,929,05313-06-193103-10-1933Brown, Lee R.Register
US 1,964,21122-09-193226-06-1934Overbury, Austin A.Calculating Machine
US 1,965,87913-03-193410-07-1934Britten, Edwin F.Calculating Machine
US 1,968,12328-02-193131-07-1934Bricken, JohnCalculating Machine
US 1,974,52920-04-193125-09-1934Brown, Lee R.Register


Monroe Systems has a page on the history of the company, including interviews with Frank Baldwin and Jay Monroe.
John Wolff's Web Museum's Monroe page.
rechenmaschinen-illustrated's Monroe page.
rechnerlexikon's Monroe page.
Cris vande Velde has pages for the Model F, Model G, Model K, and an unusual Model K for British currency.
History of Computers page about Frank Baldwin's pioneering work before the Monroe machines.

© Copyright 2016 Jaap Scherphuis, .