# AI Assistant (Pro feature)
The **Cost Calculator Builder** plugin includes a powerful **AI Assistant** integrated directly into the Formula field. This feature leverages **OpenAI’s GPT model**, allowing users to create complex formulas by simply writing prompts in plain English.
**The AI Assistant** takes the hassle out of writing formulas by translating your instructions into a fully functional calculation. Whether you’re creating a simple arithmetic formula or a more complex conditional expression, the AI **handles it for you.**
## How to obtain OpenAI API
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Please note that using OpenAI’s API **is NOT free**. Users need to have an active subscription that suits their usage requirements.
* [**How to Set up Billing?**](#how-to-set-up-billing)
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1. Go to the and login to the system:
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If you have not done it before, you might be asked to **verify your phone number.**
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2. Click on the **Create new secret key** button:
3. Set the parameters **as above**, **name** your project according to your needs, and click on **Create Secret Key:**
4. Copy the generated **Secret Key:**
5. Follow Cost Calculator **Global Settings > AI Formula** and paste the key to the designated area. Save the changes:
If this is your first time using the AI Assistant within the calculator, you can paste the API key directly into the calculator as well, and it will be saved for future uses:
6. And you are **done, happy using:**
## How to Set up Billing
OpenAI charges for its API based on usage, so if you haven’t already set up a payment method for billing, you’ll need to do so. **Otherwise, the API key you created won’t function.**
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1. To begin, click the settings (cog) icon in the top right corner of the API keys screen:
2. On the opened page move to the **Billing tab a**nd click on **Add Payment Details:**
3. Enter your card information and billing:
4. Set up the payment amount and/or automatic recharge settings according to your needs, and initiate the payment:
5. Now that you’ve set up billing, it’s a good idea to set usage limits to control your monthly API spending. Simply click **Limits** in the left menu, set your desired monthly budget, and choose an email notification threshold according to your needs. Finally, click **Save** to apply your settings.
