Inventory growth calculator
Why your first 500 books might barely make you money
Sell-through matters. But inventory scale matters too. Enter four numbers and watch your business build month by month.
Every month in detail
Starting books + books added, minus what sold, carries into the next month.
| Month | Starting | Added | Sold | Ending | Profit |
|---|
The important number isn’t just how many books you source. It’s how many profitable books you can consistently add faster than inventory sells out.
Your growth, month by month
Milestones
Why adding 300 books doesn’t mean 300 more books
Books keep selling while you source. Part of every month’s haul just replaces what sold. Only the rest is growth.
Example: start with 1,000 books, add 300, and sell about 130 (10% of 1,300). You end the month with about 1,170 books, not 1,300.
Why the first 500 books can feel so slow
Same 10% sell-through. Same $20 profit per book. Completely different business.
Simplified example. Real results vary with inventory quality, pricing, demand, sourcing cost, Amazon fees, returns, seasonality and more. It shows the effect of scale, not a promise of income.
What would it take?
Pick a monthly profit goal
Uses your sell-through rate and profit per book from above.
Simplified planning estimate, not a forecast.
Where the 10% default comes from. One active used-book seller’s recent numbers: 119 books sold in the first 25 days of a month, on roughly 1,090 active books. That’s about 10.9% so far, or about 13.1% on a full 30-day pace. 10% is used as a conservative default. It is not an Amazon-wide benchmark, so change it to match your own business.
How the math works. Each month: starting inventory + books added = available inventory. Books sold = available inventory × sell-through. Ending inventory = available − sold, and it carries into next month. Profit = books sold × profit per book. Sales are rounded for display only.
This is a planning tool, not a guaranteed earnings forecast. Results depend on actual sell-through, inventory quality, pricing, seasonality, returns and refunds, Amazon fees, sourcing costs, competition, and how consistently you add inventory.