HP 12C Frequently Asked Questions

Over the years, I have received many questions about financial calculators. I will compile a list of the most frequently asked questions here.

Q: My 12C is showing a comma instead of a decimal point. How do I fix this problem?

A: This is easily the most commonly asked question. I wish I knew how people got themselves into this situation, because it seems unlikely that it could happen by accident. Here is how to solve the problem:

1. Turn off the calculator.
2. Hold down the . key while pressing ON.

A little background on this. In the U.S. and many other countries, we use a decimal point as the radix point (or decimal separator) and a comma as the thousands separator. Many other countries do exactly the opposite. For a complete list, see the decimal separator article on Wikipedia. Interestingly, as far as I can tell this has nothing to do with the side of the road on which you drive. Obviously, the 12C was created with worldwide users in mind.

Q: How do I reset the 12C to its factory defaults?

A: This is similar to the solution to the radix point solution above, but you need to hold down the − key instead. Here are the steps:

1. Turn off the calculator.
2. Hold down the − key while pressing ON. You will need to hold down both keys for several seconds before releasing them.
3. If you have successfully reset the calculator, then it will turn on and display Pr Error. Pressing any key will clear that message.

Note that after a reset, you will need to reset your customizations (such as the number of decimal places displayed).

Q: How do I change the number of decimal places that are displayed?

A: Simply press the f key and then a number key. For example, to display five decimal places, press f 5.

Note: I had a question from a reader who couldn’t get his 12C to change the number of decimal places. I suggested that he briefly remove the batteries and try again. It worked. So, try that if your 12C seems to be stuck. Also, see the previous question.

Q: What batteries does the HP 12C use, and how do I change them?

A: The 12C uses one coin-size battery numbered CR 2032. Change it by removing the sliding lid on the back of the calculator behind the screen. Make sure that you put it in correctly. There is a diagram on the case next to the batteries that shows the appropriate orientation. Do yourself a favor and look for the multi-pack of these batteries. That is exactly the number that you need, and you should get a significant discount compared to buying them individually. You can also get them a lot (!) cheaper online at Amazon.com than from your local store.

Q: When I solve for N (number of periods), I don’t get the exact answer. Why is the answer always an integer?

A: Good question! Virtually every other financial calculator (and Excel’s NPER function as well) will return the exact mathematical solution, including fractional periods. The 12C doesn’t work this way. I can’t claim to know the mind of the original designers, but I think that I can posit a good guess. If you think about it, there is really no such thing as a fractional payment (e.g., 3.66 payments). So, the fractional part indicates that you will make a final payment, but it will be less than the other payments. For example, it will take 3.65497 deposits of \$225 to reach a future value of $1,000 at 15% interest. However, the 12C will tell you it will take 4 payments. In actuality, you would make 3 annual \$225 deposits, followed by a fourth deposit of \$101.49. So, it really does take 4 deposits, but they aren’t all the same.

If you need to find the exact answer, please see my time value of money formula pages. Also, it has been pointed out to me that HP provides programming instructions (link goes to the Internet Archive because the original HP page no longer exists) to solve this problem, though you probably don’t enter that program because it is tedious. Just use a spreadsheet or other calculator instead.

Q: How do I enter a negative number into one of the TVM keys on the 12C?

A: Well, the proper way to do it is by using the CHS (change sign) key. For example, to enter -1,000 into FV, press 1000 CHS FV. However, if you’ve got 0 in the x register (the stack), then you could also type 1000 and then -. That could lead to problems, though, if there is anything else in the stack so you should never do that.

Q: Why do I get ERROR 5 instead of an answer when I try to solve time value of money problems on my 12C?

A: This is because you are ignoring the cash flow sign convention. At least one of your cash flows must be entered as a negative number (a cash outflow).

Q: What is that X><Y key on the 12C?

A: That is a very useful key! I’m not sure what HP calls it, but I call it the “swap key” because it swaps the X and Y memory registers in the stack. The X register is what appears on the screen, and the Y register holds whatever was previously entered or calculated. Suppose that I want to do a very simple calculation: $10\div2$, which obviously equals 5. Now, let’s further say that I accidentally pressed 2 ENTER 10 (don’t press the division key yet). At this point I realize that I made a mistake and should have entered the 10 first. I can fix this by pressing X><Y, which will “swap” the 2 and the 10. Now, if I press the division key, I will get the expected answer. Note that this key is not only useful for fixing data entry errors, but it often comes in handy during normal calculations as well.

Q: How do I calculate the number of days between two dates using the 12C?

A: The HP 12C supports two date formats, the default being M.DY (month.dayyear). The other is D.MY (day.monthyear). You either need to go with the default, or change to D.MY if that is what you prefer. You can change the date format by pressing g D.MY or g M.DY. Note that if you are using the D.MY format, it will be displayed on the screen. If you are using M.DY, then nothing will be displayed since this is the default format.

To enter a date, you merely type it in using whatever format you have chosen. For example: 30 July 2007 would be entered as 7.302007 if you are in M.DY mode, or 30.072007 if you are in D.MY mode.

To calculate the number of days between two dates, simply type the first date, press Enter, type the second date, and then press ΔDYS (change in days, this is g EEX).

Note that the 12C does not allow you to specify a day count basis (e.g., 30/360 or actual/360). The day count basis is always actual/actual.

Q: How do I find roots other than square roots using the HP 12C?

A: In finance it seems that we are forever calculating various roots (cube root, fourth root, 365th root, etc). Fortunately, this is pretty simple to do if you can remember a simple mathematical rule:

$$\sqrt[N]{X} = {X^{\frac{1}{N}}}$$

So, to calculate the 5th root of 100, we simply raise 100 to the 1/5th power. To do this on the 12C (using RPN data entry): 100 ENTER 5 1/x y^x. In this example, the 5th root of 100 equals 2.51189. Using this technique you can calculate any root.

Q: My calculator only has a key to calculate natural logarithms. How do I calculate logarithms to other bases (say, base 10)?

A: Most often, in finance, we use natural logarithms (base e), usually abbreviated as Ln(x). However, sometimes we need to use other bases. Converting from base e to any other base can be done with the following formula (I’m converting to base 10):

$$Lo{g_{10}}\left( X \right) = \frac{{Ln\left( X \right)}}{{Ln\left( {10} \right)}}$$

So, just calculate the natural log of your number, and then divide it by the natural log of the new base. For example:

$$Lo{g_{10}}\left( 3 \right) = \frac{{Ln\left( 3 \right)}}{{Ln\left( {10} \right)}}=0.478$$

Q: What does that asterisk (*) on the screen of my 12C mean?

A: That means that the batteries are running out. You need to replace them soon. I recommend that you use any decent brand of batteries in the 357 size (that number will be on the back of the battery as well as the package at the store). Note that this is the same as the 76A, LR44, and MR44 sizes. You will need three batteries. Be sure to replace them all at once to avoid problems.

Q: My 12C is showing a C on the screen and I don’t know how it got there. How do I get rid of it?

A: This is another of those that requires an odd keystroke combination, but people seem to do it anyway. To remove the C from the screen, press STO EEX. If you ever want to turn it back on, just press those same keys again.

A little background on this. Many loans have an odd first payment period. So, while payments are normally due monthly, the first payment may not be due for 45 or some other number of days after the agreement is signed. In these cases, you need to know how to calculate the interest during the odd portion – 15 days in this case. The HP 12C will use simple interest by default, which is the norm, but you can make it use compound interest. So, the C on the screen means that the calculator is set to assume compound interest in the odd period. Ordinarily, this setting won’t have any effect on the results of your calculations. It is only used when you are using exact dates and have an odd period.