LOG Function in Excel: Logarithm to Any Base

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MathLevel: ExpertAvailable in: Excel 2007+

In this article

1. Syntax
2. Examples
3. Example 1
4. Example 2
5. LOG in practice
6. Where you will use it
7. Worked example
8. Mistakes people make
9. Related functions
10. Continue learning

LOG answers “what power gives this number?” — how many doublings, how many digits.

Syntax

=LOG(number, [base])

Argument	What it means
base	Default 10.

Examples

Example 1

=LOG(A2/B2, 2)

How many times a value doubled between B2 and A2.

Example 2

=INT(LOG(A2))+1

Number of digits in a positive whole number.

LOG in practice

Where you will use it

• Logs in any base
• Doubling periods: LOG(2,1+r)
• Information and decibels

Worked example

LOG(2,1.08) = years to double at 8%.

Number	Base	Formula	Result
8	2	=ROUND(LOG(A2,B2),4)	3
1000	10	=ROUND(LOG(A3,B3),4)	3
2	1.08	=ROUND(LOG(A4,B4),4)	9.0065

Results calculated in Excel.

Mistakes people make

Mistake	What to do instead
Missing base	Default is 10
Base 1	#DIV/0!

Related functions

LN · EXP · POWER

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📎 Practice files for this article

• 📗
  Practice workbook: Math & rounding24 tasks on realistic sales data — type your formula, the Check column turns green when it is right. Answers sheet included.
  ⬇ XLSX · 35 KB

Free to use for learning. Files with macros (.bas) are plain text — import them with Alt+F11 → File → Import File, and always test on a copy.