How to Use the Logarithm Function on Your Calculator

Finding and pressing the log button

Most scientific calculators have a LOG button that calculates base-10 logarithms. On standard calculators, it is usually near the top or middle of the keypad, often grouped with other math functions like sine, cosine, and square root. On graphing calculators like the TI-84, LOG is typically on the left side of the screen.

To find a logarithm, type the number first, then press LOG. For example, to find the log of 100, you would type 100, then press LOG. The answer appears on your screen — in this case, 2, because 10 to the power of 2 equals 100. The order matters: number first, then the function.

If your calculator does not have a dedicated LOG button, check the manual or look for a button labeled with a small number or symbol that might hide the function. Some calculators require you to press a SHIFT or 2ND key first to access LOG from a different button.

Key Takeaways

Press the number first, then the LOG button — the order is important and affects your result.
The LOG button on most calculators finds base-10 logarithms, which is what you need for most everyday math.
If you need a logarithm with a different base, use the change-of-base formula: divide the log of your number by the log of the base you want.
Graphing calculators often hide LOG behind a SHIFT or 2ND key, so check your manual if you cannot find it on the main keypad.

Understanding what the log button actually does

A logarithm answers the question: "What power do I raise 10 to in order to get this number?" When you press LOG on 1,000, the calculator returns 3, because 10 to the power of 3 equals 1,000. When you press LOG on 10, it returns 1. When you press LOG on 1, it returns 0.

This is why LOG is called a base-10 logarithm — it always uses 10 as the starting number. If your math problem asks for a different base, you will need to use a different method, which we cover in the next section.

One common mistake is pressing LOG before typing your number. If you do this, the calculator either shows an error or waits for you to type the number. Always type the number on your screen first, then press LOG.

Using natural logarithm (LN) when you need it

Many calculators also have an LN button, which calculates the natural logarithm. This uses a special number called e (approximately 2.718) instead of 10. In science, statistics, and higher math, natural logarithms come up often. On most calculators, LN is right next to LOG.

Use LN the same way you use LOG: type your number, then press LN. For example, typing 2.718 and pressing LN gives you approximately 1, because e to the power of 1 equals e. The process is identical to LOG — only the base changes.

If your problem specifically asks for "log base e" or "natural log" or uses the notation "ln(x)", use the LN button. If it just says "log" without specifying a base, use LOG. When in doubt, check your textbook or problem statement to see which one is expected.

Finding logarithms with bases other than 10 or e

Sometimes you need a logarithm with a base that is not 10 or e — for example, log base 2 or log base 5. Your calculator probably does not have a button for these, but you can find them using the change-of-base formula. This formula says: log base b of x equals (log of x) divided by (log of b).

Here is how to do it step by step. Say you need to find log base 2 of 8. Type 8, press LOG, and write down the answer. Then type 2, press LOG, and write down that answer. Finally, divide the first answer by the second answer. On most calculators, you can do this in one chain: type 8, press LOG, press divide, type 2, press LOG, press equals.

The same method works with LN instead of LOG if your problem uses natural logarithms. The base formula does not change — only which button you press. Many graphing calculators also let you type the formula directly into the screen, which is faster once you get used to it.

Fixing common mistakes with calculator logarithms

The most frequent error is pressing LOG and then typing the number. This does not work on most calculators — you must type the number first. If your screen shows an error or seems to be waiting for input after you press LOG, you pressed the buttons in the wrong order. Clear the screen and start over.

Another mistake is using LOG when you need LN, or vice versa. Check your problem statement carefully. If it says "natural log" or "ln", use LN. If it just says "log" with no other information, use LOG. Some textbooks and fields prefer one over the other, so context matters.

A third mistake is forgetting that logarithms of negative numbers do not exist on a standard calculator. If you type a negative number and press LOG or LN, you will see an error. Logarithms only work on positive numbers. If your problem involves a negative number, check whether you set up the problem correctly.

Using logarithms on phone calculators and computer programs

If you are using the calculator app on your phone, look for a scientific mode. On most phones, you can rotate your device to landscape orientation, and the calculator will show more buttons, including LOG and LN. The process is the same as on a physical calculator: type the number, then press LOG.

On a computer, programs like Excel and Google Sheets have LOG and LN functions built in. In Excel, you would type =LOG(100) to find the log of 100, or =LN(2.718) to find the natural log of e. The syntax is slightly different from a calculator, but the math is identical.

If you are using a graphing calculator like Desmos or GeoGebra online, you can usually type log(x) or ln(x) directly into the equation. These programs often accept both notations, so check the documentation if you are unsure which one to use.

Frequently Asked Questions

What is the difference between LOG and LN on my calculator?

LOG uses base 10, while LN uses base e (approximately 2.718). LOG is more common in everyday math and science, while LN appears in calculus, statistics, and natural sciences. Use whichever one your problem asks for.

Why do I get an error when I press LOG on a negative number?

Logarithms of negative numbers do not exist in standard math. If your problem involves a negative number, check whether you need to take the absolute value first or whether you set up the problem correctly.

Can I find log base 2 on my calculator?

Your calculator probably does not have a base-2 button, but you can use the change-of-base formula: log base 2 of x equals (log of x) divided by (log of 2). Type your number, press LOG, divide, type 2, press LOG, then press equals.

Do I type the number before or after pressing LOG?

Always type the number first, then press LOG. If you press LOG first, the calculator will either show an error or wait for you to type the number, depending on your calculator model.

What if my calculator does not have a LOG button?

Check whether LOG is hidden behind a SHIFT or 2ND key — many calculators group functions this way. If your calculator truly has no logarithm function, you can use the change-of-base formula with a different base, or switch to a scientific calculator app on your phone.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.