Home Monroe IQ-213 - The Calculator with a Memory

Monroe IQ-213 - The Calculator with a Memory


Monroe IQ-213

The Monroe IQ-213 is a rather large fully automatic, full keyboard mechanical calculator. Even compared to something like a Marchant ACR-10M, this calculator takes up more desk space and weighs in at about 39 pounds! The outer casing even has wheels on the back making it easier to move the calculator around a desk. This is a very special calculator though. The Monroe IQ-213 can be considered the cumulation of Monroe full-keyboard calculator technology. This calculator was the top of the line Monroe machine near the end of the mechanical calculator era in the 1960’s. Advertisements certainly tried to convey the technical achievement that this calculator was.

IQ-213 AD

Today, this is very funny due to how far computer technology has progressed. But at the time, this was very true. This calculator does have special party trick up it’s sleeve that helps it compare to computers! I’ll let the sticker on the machine reveal it:

Memory Sticker

Yep, you read that right - “The calculator with a memory”. The Monroe IQ-213 has a 10 digit read-write memory. It’s useful, but it is more of a party trick. The “memory” is just another register that is connected to the keyboard. The memory can be “written to” using only digits entered on the keyboard. Likewise, when the memory is “read” it can only be read to the keyboard. The extra storage register and other buttons designed to use the memory for a function are still useful though. This function is explained later in the Controls section.



The carriage is very similar to the the Monroe 8N-213’s carriage. The carriage has a total of 4 registers. The 10 digit counter register on the top left with the white dials is the counter register without a carry. This register counts at the same time as the other register on the left. During multiplication and division, this register remains the same as the other. However, if doing successive additions or subtractions that get higher than 9, the counter on the left will roll over to 0 again instead of displaying 10. This is useful in certain statistical calculations. The 11 digit register in the top right is a normal counter register with carry.

Above the 10 digit counter register are the tabulation keys. These allow the register to stop at a certain place when shifting the carriage or performing division. The rightmost tabulator key clears the others.

The levers to the left of the registers prevent clearing of the registers. The topmost lever prevents the 11 digit counter register on the right from being cleared by the Upper clear button. If the switch is up, both registers will clear. If the switch is down, only the left register will clear. The lower two levers prevent sides of the accumulator from clearing. If both are down, the entire register cannot be cleared by the Lower clear button. If the left one is up and the right one is down, the rightmost 11 digits will not be cleared and the left 10 digits will be cleared. The C lever on the right stands for Constant. If this lever is in the down position, any number in the multiply register will remain after a multiply operation. If it is up, the number will be cleared after each operation.

To the left of the Constant lever is the Half Cent Control button. This button allows for automatic half cent rounding in dials 2, 4, 5, 6. and 8. To setup half cent rounding, enter a 5 in a keyboard column that supports half cent rounding, and press the add key. Next, hold the Half Cent Control button and also press the lower register clear button at the same time. Now, every time the lower dials are cleared, that column will reset to 5. To set the column back to normal, enter 5 in the column and press the subtract key. Then hold the Half Cent Control button and press the lower register clear button at the same time again. Now the column will clear back to 0.

Finally, below the accumulator is the multiplier register. This will be discussed in depth with along with multiplication.

Keyboard, Buttons and Levers


The Monroe IQ-213 contains 10 columns of keys, with 9 rows for digits and another bottom row for 0 or clearing that column of an entry. To the right of the keyboard are the addition and subtraction keys. To the right of those keys are the carriage shift keys. These keys can be quickly pressed for a single placement shift ort held down until the desired shift is reached.

Below the two carriage shift keys is the Enter Multiplier key. This key places the number on the keyboard into the multiplier register. If pressed and released, the keyboard will be cleared for entry of the multiplicand. If held down, the keyboard will not be cleared, meaning the same digit is in the multiply register and left in the keyboard. This is done for quickly squaring a number.

Below the addition key is the Enter Divd button. This button shifts the carriage all the way to the right and enters the number on the keyboard into the accumulator register for division.

To the right is a small switch. The is the Repeat switch. If in the up position, the addition or subtraction key being held down will perform successive additions or subtractions until the key is released. If in the down position, the addition and subtraction keys being held down will still perform only one operation.

To the right of the Repeat key is the Counter Dials Control switch. This switch has three positions. If the switch is in the upper position, the counter register is incremented. If the switch is in the middle position, the counter is neither incremented or decremented. If the switch is in the lowest position, the counter is decremented.

Below the Enter Divd button are the the Upper and Lower clear buttons. The Upper button clears the two counter registers and the Lower button clears the accumulator register. Below those buttons is the Clear Keyboard button used to clear the keyboard. All three of these buttons can be pressed at the same time.


Beneath the Clear Keyboard button are the Trans Mult and Memory Mult buttons. The Memory Mult transfers a number set on the keyboard to the multiplier, clears the keyboard, and multiplies by the number stored in the memory. The Trans Mult transfers an amount in the accumulator register to the multiplier register, and multiplies it the number set on the keyboard.

To the left is the Multiply button. This two functions depending on the machines state. If the carriage is any place other than the leftmost position, it will clear the registers and return the carriage it to it’s leftmost position. If the carriage is in the leftmost position, the carriage is cleared, and a number in the multiply register is multiplied by the number on the keyboard.

To the left of the Multiply key are Neg Mult and Acc Mult keys. Neg Mult stands for negative multiply. This key multiplies the number in the multiplication register by the number on the keyboard and subtracts the result from the number in the accumulator register. The Acc Mult key beside it stands accumulative multiply. This key multiplies the number in the multiply register times the number on the keyboard and adds the result to the number in the accumulator register.


To the left of the Neg Mult button is the Recall Memory button. This button essentially sets a number stored in memory to the keyboard. The number is not shown on the keyboard via the buttons, but any function can be used as if the number was entered on the keyboard.

To the left of that is the Div button. This method starts division once the dividend is in the accumulator and the divisor is entered on the keyboard. And on the left is the Stop button used for stopping multiplication or division.

Further to the far left is the Divd Align lever. If this lever is up, the carriage shifts all the way to the right when the Enter Divd button is pressed. If the lever is down, the carriage is not shifted and left in it’s current place for the dividend to be placed into the accumulator.

Enter Memory

Finally, there is the Enter Memory button. This button places a number entered on the keyboard into the memory register. If the keyboard had no digits entered and the keyboard is cleared, the memory registered is cleared when this button is pressed.


Addition and Subtraction

Simply enter the number on the keyboard and then press the + or - buttons to add or subtract it from the accumulator.


Make sure the carriage is shifted all the way to the left. Enter the multiplier onto the keyboard. Then press and release the Enter Multiplier button. The multiplier should be visible in the multiplier register. Next enter the multiplicand onto the keyboard. Then press the Clear Mult button to clear the registers and multiply the numbers. The result will be in the accumulator. If you want to add the result to the digits in the accumulator press the Acc Mult key. Likewise, if you want to subtract the result from the accumulator press the Neg Mult key.


First, decide how many decimal places you want. If you want a set amount, move the carriage to the right the number of decimal places required and move the Div Align switch to the down position. If it doesn’t matter or the most decimal places are required you can ignore carriage position and leave the lever in the up position. Next, enter the dividend onto the keyboard and press the Enter Divd button. This will place the dividend into the accumulator. Finally, enter the divisor onto the keyboard and hit the Div key. The result is in the right counter register.


Enter the number you wish to square onto the keyboard. Then press and hold the Enter Multiplier. Then press Clear Mutl. The result will be in the accumulator register.

Accumulate multiply constant

The number you want to multiply by must be at least one place from the rightmost place in the accumulator. Shift the carriage to the rightmost digit of the number in the accumulator. Then, move the metal digit selector in front of the multiplier register to align with the same digit. Using the lever on the right side of the multiply register, move it until it stops against the metal selector. Then select the Clear Mult button. The number in the accumulator will be transferred to the multiply register. Then move the C switch to the down position. Now the number from the accumulator can be multiplied as many times as needed.

Multiply Acc. by Memory

First, place a number in memory with the Enter Memory button. Next, perform the calculations to get the multiplier in the accumulator. Move the Slider in front of the multiply register to where the rightmost side is lined up with the final digit of the number in the accumulator. Press Recall Memory and then press Trans Mult. The accumulator will be transferred to the multiplier and then multiplied by the number recalled from the memory.


Video of the machine performing division with the outer panels removed.

Internal Pictures












Monroe IQ-213 Users Manual

IQ-213 AD Booklet

Monroe 6N Technical Drawing and Parts Catalog

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