Logarithm: log
The log template function returns the logarithm of a value. By default it computes the natural logarithm (base e). You can specify a different base, such as 10 for common logarithm or 2 for binary logarithm.
This is useful for scaling sensorSensors return information about a thing, for instance the level of water in a tank. [Learn more] data that spans a wide range into something more manageable. For example, sound level sensors often benefit from logarithmic scaling, or you might use a log scale to visualize energy consumption data that varies by orders of magnitude.
Usage
Here’s how to use this template function. Copy any example and adjust it to your setup.
{{ log(10, 10) }}1.0Function signature
The signature is a technical summary of this template function. It shows the name of the function, the values (called parameters) it accepts, and what type of data each parameter expects (for example, a piece of text or a number).
Function parameters that have a = with a value after them are optional. If you leave them out, the default value shown is used automatically.
Function parameters without a = are usually required. A few optional parameters have no default value, so the signature shows them without a = as well. To check whether a parameter is required, look at the parameter list below the signature. Each parameter is marked as Required or Optional.
The most common optional parameter without a default value is default. The default parameter sets the value a function returns when the function can’t produce a result, for example, when float gets text that isn’t a number. If you leave out default, there is no fallback value, so most functions raise an error and the template fails. The parameter list of each function says what happens. If you set default, the function returns your value instead.
How to read a signature
Here is the signature of the round filter as an example:
round(
value: Any,
precision: int = 0,
method: str = "common",
default: Any,
) -> float | int | Any
-
roundis the name of the function. - Between the parentheses are the parameters. Each parameter has a name, followed by a colon and the type of value the parameter expects. The type only tells you what kind of value to pass. For
default: Any, you pass a value of any type, such as0or"unknown", not the wordAny. -
= 0and= "common"are default values. You can leave outprecisionandmethod, androunduses these default values. -
defaulthas no=, but the parameter list marksdefaultas optional. If you leave outdefaultandroundgets a value that isn’t a number,roundraises an error. -
->shows the type of value the function returns. - Inside a signature,
|means “or”. This function returns a decimal number, a whole number, or the value you passed asdefault.
round is a filter, so the first parameter, value, is the value before the |. For example, {{ 3.14159 | round(2) }} returns 3.14.
The types you’ll see most often:
-
str: text, such as"hello" -
int: a whole number, such as5 -
float: a decimal number, such as3.14 -
bool:trueorfalse -
list: a list of values, such as[1, 2, 3].list[str]is a list of texts. -
dict: a set of keys and values, such as{"name": "Kitchen"} -
datetimeandtimedelta: a date and time, and a duration. For details, refer to Dates and times. -
None: no value -
Any: any type of value
Some signatures use special forms:
-
*argsmeans you can pass any number of values, separated by commas. For example,average(1, 2, 3). -
value | name(...)means the function is a filter. The value before the|is passed to the filter as its first parameter. Some filter signatures, like theroundexample, show this value as the first parameter between the parentheses instead.
log(
value: Any,
base: Any = e,
default: Any,
) -> float
Function parameters
The following parameters can be provided to this function.
The base of the logarithm. Defaults to e (Euler’s number, ~2.718) for the natural logarithm. Use 10 for common logarithm or 2 for binary logarithm.
Using a default value
If the input value might not be a positive number, provide a default to avoid errors. This keeps your templateA template is an automation definition that can include variables for the action or data from the trigger values. This allows automations to generate dynamic actions. [Learn more] from breaking when a sensor is temporarily unavailable.
{{ log(states("sensor.sound_level") | float(0), 10, default=0) }}0Good to know
- The default base is
e, which gives the natural logarithm. Pass10for common log or2for binary log. - Zero or negative inputs raise an error unless you supply a default.
Try it yourself
Ready to test this? Open Settings > Tools > Template, paste the example into the Template editor, and watch the result update on the right. Edit the values to see how the function adapts to your own entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more].
More examples
Real scenarios where this function comes up in automations and templates. Copy any example and adapt it to your setup.
Natural logarithm
When no base is specified, the natural logarithm (base e) is used.
{{ log(e) }}1.0Binary logarithm
Use base 2 for binary logarithm calculations.
{{ log(256, 2) }}8.0Still stuck?
The Home Assistant community is quick to help: join Discord for real-time chat, post on the community forum with your template and expected result, or share on our subreddit /r/homeassistant.
AI assistants like ChatGPT or Claude can also explain or fix templates when you describe what you want in plain language.
Related template functions
These functions work well alongside this one:
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Euler’s number: e - The mathematical constant e (Euler’s number), approximately 2.71828.
-
Square root: sqrt - Returns the square root of a value.