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Arc sine (inverse sine): asin

The asin template function returns the arc sine (inverse sine) of a value. The result is in radians, in the range [-pi/2, pi/2]. The input must be between -1 and 1.

This is useful when you need to convert a ratio back into an angle. For example, given a normalized sensorSensors return information about a thing, for instance the level of water in a tank. [Learn more] reading you could recover the original angle used to produce it.

Usage

Here’s how to use this template function. Copy any example and adjust it to your setup.

As a function

As a filter

Template

{{ asin(1) }}

Result (float)

1.5707963267948966

Function 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

round is 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 as 0 or "unknown", not the word Any.
= 0 and = "common" are default values. You can leave out precision and method, and round uses these default values.
default has no =, but the parameter list marks default as optional. If you leave out default and round gets a value that isn’t a number, round raises 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 as default.

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 as 5
float: a decimal number, such as 3.14
bool: true or false
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"}
datetime and timedelta: 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:

*args means 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 the round example, show this value as the first parameter between the parentheses instead.

asin(
    value: Any,
    default: Any,
) -> float

Function parameters

The following parameters can be provided to this function.

value float Required

The value to calculate the arc sine of. Must be numeric and between -1 and 1.

default any (Optional)

The value to return if the calculation fails, such as 0. For example, the calculation fails if the input is out of range or not numeric. If you leave out default and the calculation fails, asin raises an error and the template fails.

Good to know

Inputs outside the range -1 to 1 raise an error unless you supply a default.
The result is in radians. Multiply by 180 / pi to convert to degrees.

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.

Using a default value

If the input might be out of range or non-numeric, 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 sensorSensors return information about a thing, for instance the level of water in a tank. [Learn more] is temporarily unavailable.

Template

{{ asin(states("sensor.ratio"), default=0) }}

Result (float)

Convert arc sine to degrees

Convert the result from radians to degrees to make it more readable.

Template

{{ (asin(0.5) * 180 / pi) | round(1) }}

Result (float)

30.0

Still 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.

Tip

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:

Sine: sin - Returns the sine of a value given in radians.
Arc cosine (inverse cosine): acos - Returns the arc cosine (inverse cosine) of a value, in radians.
Arc tangent (inverse tangent): atan - Returns the arc tangent (inverse tangent) of a value, in radians.
Four-quadrant arc tangent: atan2 - Returns the four-quadrant arc tangent of y/x, in radians.

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