> ## Documentation Index
> Fetch the complete documentation index at: https://docs.learnterms.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Calculator

> Use the scientific calculator and optometry formulas without leaving the current question.

Open **Calculator** from the module **Tools** area. You can also open the standalone calculator at [learnterms.com/tools/calculator](https://learnterms.com/tools/calculator).

## Enter a calculation

* Type in the expression field or use the keypad.
* Use `sin`, `cos`, `tan`, `ln`, `log`, square root, powers, and `π` as needed.
* Switch **DEG** and **RAD** before using trigonometric functions.
* Read the result as you type.
* Click the result to copy it.
* Press `Enter` or click `=` to add the result to **History**.

**Ans** inserts the newest history result into the current expression.

## Use a formula

Select **Formulas**, then choose one of the built-in formulas:

| Formula               | Inputs                              | Result              |
| --------------------- | ----------------------------------- | ------------------- |
| Spherical equivalent  | Sphere and cylinder                 | Diopters            |
| Prentice's rule       | Decentration and lens power         | Prism diopters      |
| Vertex conversion     | Spectacle power and vertex distance | Corneal-plane power |
| Focal length to power | Signed focal length                 | Diopters            |
| Percentage            | Part and whole                      | Percent             |

Enter the labeled values. The displayed equation fills in and recalculates as the values change. Keep the prescription sign when a formula asks for a signed value.

## History and saved state

* History keeps up to 30 calculations.
* Click a history expression to reuse it.
* Click a history result to copy it.
* Select **Clear** under **History** to remove the list.
* The current expression, selected formula, formula inputs, angle mode, and history stay in local browser storage.

Calculator state is browser-specific. It does not sync through your LearnTerms account.

## Why use it

* Keep the question visible while calculating.
* Reduce copying between LearnTerms and another app.
* Reuse a result without retyping it.
* Use labeled optometry formulas to reduce unit and transcription mistakes.

The calculator supports the work. You still need to choose the correct formula, units, signs, and clinical interpretation.

## Related guides

* [Quiz dock and tools](/docs/students/quiz-dock-and-tools)
* [Question types and answering](/docs/students/question-types-and-answering)