If you've ever built an invoice in a spreadsheet, you know the routine: type the product, type the price, type the quantity, then manually multiply them together. Do that for every line, then sum it all up at the bottom — and hope you didn't make a typo in the formula.

In this tutorial, we'll build a Simple Invoice Builder where adding a product line instantly calculates its subtotal, and the invoice total updates itself automatically — no formulas required.

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Step 1: Create the Product Table

Before we can build an invoice, we need a list of products to invoice for.

First create new table here

Create new table called Order

Add two fields: Product Name and Unit Price.

Click here to add fields

Create new field product name and unit price

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Step 2: Create the Invoice Table

Next, create an Invoice table — this represents one invoice sent to one customer.

Sidebar showing a new table being created and named Invoice

Add basic fields: Invoice Number and Invoice Date.

Invoice table with Invoice Number and Invoice Date fields added

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Step 3: Add Line Items to the Invoice Table

Here's the key part. An invoice usually has more than one product on it — but instead of manually building a separate table to hold each line, Ambisius handles this for you.

In the Invoice table, add a new column and choose Line Items as the data type.

Adding a new column in the Invoice table and selecting Line Items as the field type

Select Product as the table to pull items from, and add Quantity as a column to track alongside it.

Click here to add field to tracks

Select One to Many to pull data from Product

Pull data from product

Select Product for the related table

This is the final result

Click Save to confirm the your Line Items

Final result of table Invoice

The final structure the Invoice table

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Step 4: Calculate the Subtotal per Item

Now let's make each line item calculate its own subtotal — unit price multiplied by quantity.

In the Invoice table, add another new column and choose Custom Logic as the data type. Describe what you want: multiply the Product's Unit Price by the Quantity from each line item.

Add Custom Logic data type here

Describe the logic with plain sentence

There will be clarification that you need to confirm, making sure your intention matches system intention.

Clarification by system about the calculation

Click Save to confirm.

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Step 5: Add Your Data

Let's put this to work. Add a few products to the Product table.

Add Product table data

Add few products

Now let's put everything together. Create one Invoice record, open its Line Items field, and add a few products with their quantities.

Add Invoice data here

Fill in the data, notice that in Invoice Items section, there is a button, click it to add items inside it.

Button to add more line items

As you can see, you can insert more product and its quantity

Add Invoice items modal

Click in Product picker, and you can pick list of Products that you have entered, no need to manual input.

Product picker

add one record: Laptop Stand (Unit Price $45) and its Quantity 2

Fill in Laptop Stand

Also add one more record: Noise Cancelling (Unit Price $150) and its quantity 1

Fill in Noise Cancelling

This is the final invoice

Final invoice

Watch the Invoice's Total amount update immediately to reflect it.
Calculation (45$ x 2) + ($150 x 1) = $240 in total

Total amount updated

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Recap: What You Built

Across these steps, you created:

1. A Product table to store items and their prices
2. An Invoice table to represent each invoice
3. A Line Items field connecting Invoice and Product, letting you add multiple products per invoice without building anything extra
4. A Custom Logic field calculating total amount automatically
5. Real invoice data, calculating itself the moment you added it