Home Monroe Model LA5-160 Mechanical Calculator.

Monroe Model LA5-160 Mechanical Calculator.


Monroe Model LA5-160

These are two examples of the Monroe Model LA5-160 product line. This machine is the successor to the model K, with the biggest changes being electric drive as opposed to a hand crank and automatic division. The machine was produced starting in the 1930’s. Below is an ad for the small electric calculator.

Monroe Model LA5-160 Ad

The model LA5 line carried on for serval years. I have two LA5 machines. One being from the 1930’s (left side below) and the other from the 1940’s (right side below). The machines have slightly different stylings and logos but the internals are primarily the same. The 1930’s machine was the first calculator I ever acquired! It came from the Flea Market at Hillsville, VA.

Rear of the machines

The model number “LA5-160” does follow the Monroe model naming scheme. It is an “L”-series machine, “A” for Automatic since it doesn’t require a hand crank, version 5, with a 16-place accumulator.



The model LA5-160 has a 16 digit accumulator register (per the “16” in the model name) and a 8 digit counter register above it. The counter register does not have a carry mechanism. Both register include several decimal sliders below each register. On the rightmost side of the carriage is the clearing lever. Rotating the clearing lever forwards clears the counter register. Rotating the lever backwards clears the accumulator register.

The carriage can be moved using the knob on the right side of the carriage. The carriage must be lifted slightly to disengage the gears connecting the registers to the machine, than the carriage can be moved.

Keyboard and Controls


The model LA5-160 has an 8 column keyboard, each numbered 1 - 9 and an extra button for clearing that column. There are 7 decimal markers that can be spun via the wheel at the top of the keyboard. To the left of the keyboard is a red lever used to activate division. To the right of the keyboard are two black buttons. The - button subtracts the number on the keyboard from the accumulator register. Below that is the + button. This button adds the number on the keyboard to the accumulator.

Below those buttons are two small red buttons, the lower one having an R on it. These are part of the repeat switch. If the lower red button with the R is pushed down, the addition and subtraction buttons will continually add or subtract until the key is released. If the upper red button is pressed, holding the addition or subtraction key will only perform each operation once until the button is released.

Below the repeat buttons is the big keyboard clear button. This button clears all columns of the keyboard at once.

Multiply Lever

At the bottom left of the keyboard is a “T” handle that can be rotated. This lifts and then rotates the carriage one digit placement for each half turn of the handle. This handle is used for multiplication or division.


This model of calculator was designed to be small, easy to use, and able to quickly perform multiplication and division.

Addition and Subtraction

Simply enter the number on the keyboard you want to add or subtract, and press the appropriate + or - key. It will be added to or subtracted from the number in the accumulator register. If successive additions or subtractions are needed, make sure the red R key is pressed, then hold down the appropriate + or - button until the desired number of operations has been reached.


Multiplication works by successive additions. First, enter the multiplicand into the keyboard. Make sure the repeat key is pressed. Then hold the + key until the last digit of the multiplier is in the counter register. Then, rotate the “T” handle one half turn to the right shift the carriage one place to the right. The hold the + key until the next digit of the multiplier is in the counter register. Repeat until all digits of the multiplier are in the counter register. The product will be in the accumulator register.

For example, if you wish to multiply 25 times 25, first enter 25 into the keyboard. Then press the + button 5 times, or hold it until 5 is in the counter register. Then shift the carriage one place to the left. Then hold or press the + key until 25 is shown in the counter register. The result will be in the accumulator register.


First, move the carriage all the way to the right. Make sure both registers are clear. Then, enter the dividend starting at the left side of the keyboard (for maximum accuracy) and press the + button. Then, clear the counter register by rotating the clear lever forwards. Next, enter the divisor into the keyboard starting from the left side. Push the division switch upwards and the machine will automatically perform the division. The quotient will be in the counter register.

Division Video

Video of the earlier machine performing division in slow motion and with the outer panels removed.

Internal pictures

Right Side Internals

Rear Internals

Left Internals

More Pictures

30s Front Logo

1930’s Monroe logo.

Both Front

Both machine’s logos.


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