Ask most small businesses what's actually in stock right now, and the honest answer is usually "let me go check." Someone counts the shelf, cross-references a spreadsheet that was last updated three days ago, and hopes nothing sold in between. Stock counts that live in a spreadsheet are only ever as accurate as the last time someone remembered to update them.

In this tutorial, we'll build an Inventory Stock Monitor — a system where each product's stock level updates itself automatically every time a sale or restock happens. Open the table, and the number you see is the number that's actually true right now.

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Step 1: Create the Product Table

Start with a table called Product. Add the fields Name, SKU (Text), and Unit Price (Currency, USD).

Product table with Name, SKU, and Unit Price columns

Add a few products to work with:

• Wireless Mouse — SKU WM-100 — $25.00
• Mechanical Keyboard — SKU MK-200 — $65.00
• USB-C Hub — SKU UC-300 — $40.00

Product table showing three records for Wireless Mouse, Mechanical Keyboard, and USB-C Hub with their prices

We'll add the stock level once we have real sales and restocks to calculate it from.

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Step 2: Create the Sale Table

Create a new table called Sale. Add a One-to-Many field linking each sale to a Product, a Quantity field (Number), and a Date field.

Sale table with Product link, Quantity, and Date columns

Add a few sample sales:

• Wireless Mouse — Quantity 5 — 2026-08-20
• Mechanical Keyboard — Quantity 2 — 2026-08-21
• Wireless Mouse — Quantity 3 — 2026-08-22

Sale table listing three sale records for Wireless Mouse and Mechanical Keyboard with quantities sold

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Step 3: Create the Restock Table

Create another table called Restock, also linked to Product via a One-to-Many field. Add a Quantity field (Number) and a Date field.

Restock table with Product link, Quantity, and Date columns

Add a sample restock:

• Wireless Mouse — Quantity 20 — 2026-08-15
• Mechanical Keyboard — Quantity 10 — 2026-08-15
• USB-C Hub — Quantity 15 — 2026-08-15

Restock table showing initial stock records for all three products

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Step 4: Add the Stock Level Field

Now go back to the Product table. Add a new column and set its data type to Custom Logic — this needs to be chosen from the start, since a plain column can't be converted into one later.

Adding a new column on the Product table and selecting Custom Logic as the data type

Describe the rule: for each product, take the sum of all linked Restock quantities and subtract the sum of all linked Sale quantities, to get the current Stock Level.

Describing the stock level logic as total restocked quantity minus total sold quantity

Confirm the clarification screen once the system shows its interpretation, then click Save.

Clarification screen confirming the aggregation logic across Sale and Restock tables to compute Stock Level

Product table showing the Stock Level column filled in automatically for all three products

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Step 5: Watch a Sale Update Stock Instantly

Let's test it. Add a new sale: USB-C Hub — Quantity 4 — 2026-08-23.

Adding a new sale record for USB-C Hub with a quantity of four units

Go back to the Product table. USB-C Hub's Stock Level has already dropped from 15 to 11 — no refresh, no manual recalculation.

Product table showing USB-C Hub's Stock Level updated instantly after the new sale was recorded

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Step 6: Watch a Restock Update Stock Too

Now add a restock: Mechanical Keyboard — Quantity 15 — 2026-08-24.

Adding a new restock record for Mechanical Keyboard with a quantity of fifteen units

Mechanical Keyboard's Stock Level jumps from 8 to 23 immediately, reflecting the restock without any extra step.

Product table showing Mechanical Keyboard's Stock Level increased after the restock was recorded

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Recap

What you built:

1. A Product table holding basic item details
2. A Sale table linked to Product, tracking units sold
3. A Restock table linked to Product, tracking units received
4. A Custom Logic field on Product that continuously derives Stock Level from both linked tables

This same pattern — a running total derived from linked activity — works well beyond inventory too. Think available seats after bookings and cancellations, remaining budget after expenses, or available leave days after requests are approved.