Inventory growth calculator: why your first 500 books barely pay
You list new books every month, but the ones that sell leave your inventory. Enter how many you list and how fast they sell: the calculator shows your active inventory, sales and profit month by month — and the ceiling it all levels off at.
Your current active inventory. Enter 0 if you’re just starting.
How many new listings you create in an average month.
The share of your active inventory that sells in a month.
After fees and cost of goods. An example value, not a market average.
Your ceiling
At this pace your inventory levels off around 2,700 books. From there, every book you add just replaces one that sold: you sell 300 books a month, about €6,000 in monthly profit. To go higher, list more books or pick better ones.
You don’t get there within 12 months — extend the horizon to see the rest.
Your month 1
- You add
- +300
- Replace books that sold
- −80
- Real inventory growth
- +220
The bigger your inventory gets, the larger the share of new listings that only replaces sales.
Your growth, month by month
Milestones
- Month 1€500 profit in a month
- Month 1€1,000 profit in a month
- Month 31,000 books listed
- Month 4€2,500 profit in a month
Every month in detail
| Month | Start | Added | Sold | End | Profit |
|---|---|---|---|---|---|
| 1 | 500 | +300 | −80 | 720 | €1,600 |
| 2 | 720 | +300 | −102 | 918 | €2,040 |
| 3 | 918 | +300 | −122 | 1,096 | €2,436 |
| 4 | 1,096 | +300 | −140 | 1,257 | €2,792 |
| 5 | 1,257 | +300 | −156 | 1,401 | €3,113 |
| 6 | 1,401 | +300 | −170 | 1,531 | €3,402 |
| 7 | 1,531 | +300 | −183 | 1,648 | €3,662 |
| 8 | 1,648 | +300 | −195 | 1,753 | €3,895 |
| 9 | 1,753 | +300 | −205 | 1,848 | €4,106 |
| 10 | 1,848 | +300 | −215 | 1,933 | €4,295 |
| 11 | 1,933 | +300 | −223 | 2,010 | €4,466 |
| 12 | 2,010 | +300 | −231 | 2,079 | €4,619 |
How many books does your goal take?
Uses your sell-through and profit per book from above.
A simplified planning estimate, not a forecast.
A planning tool, not an earnings promise. Real results depend on fees, returns, seasonality, cost of goods, price drops and how well you source. Figures are rounded for display.
Raise your ceiling: list more books, better picked
Your ceiling is set by how many books you add. With BiblioScan you photograph a whole shelf: the AI recognizes every book by its spine and shows you which ones are worth buying. Less time per book, more books listed.
Create a free accountWhy adding 300 books doesn’t mean 300 more books
Two flows cross every month: the books you list come in, the books that sell go out. While your inventory is small, few books sell and almost everything you add stays, so inventory climbs fast.
Example: 1,000 books listed, 300 added this month, 10% sell-through. Out of 1,300 available, about 130 sell. You end the month at 1,170 — up 170, not 300. The bigger the inventory, the wider that gap, until the day sales match what you add.
Why the first 500 books feel so slow
Sales scale with active inventory. At 10% sell-through and $6 profit per book, 500 listed books bring roughly 50 sales and $300 a month. At 3,000 books it’s 300 sales and $1,800 — same sell-through, six times the result.
So the start is a build-up phase: each book you add earns a little this month, then keeps working in the months after. People who quit after three months usually judge the business at the worst point on the curve.
Where the 10% default comes from
10% is a deliberately cautious assumption, not an Amazon-wide benchmark. Your real rate mostly depends on what you list: well-ranked books with steady sales move far faster than random backlist.
To measure yours: divide the books sold over the last 30 days by your active listings at the start of that period. Repeat it every month, then replace the 10% with your own number.
How the math works
- Available inventory = starting inventory + books added.
- Books sold = available inventory × sell-through.
- Ending inventory = available − sold, carried into the next month.
- Profit = books sold × average profit per book.
- Ceiling = monthly additions × (1 − rate) ÷ rate: the level where sales equal additions.
Frequently asked questions
What is sell-through rate?
The share of your active inventory that sells over a period — here, a month. 10% means that out of 1,000 listed books, about 100 sell that month.
Why does my inventory stop growing?
Because sales grow with inventory. At a certain level — the ceiling — you sell as many books each month as you add. Inventory flattens out, and so does monthly profit.
How do I get past the ceiling?
There are only two levers: add more books each month, or add books that sell faster and for more. The first is about sourcing and listing time, the second about selection.
Does the calculator include Amazon fees?
Indirectly: the profit per book you enter should already be net of fees and cost of goods. To work out that net on a specific book, use BiblioScan’s free FBA calculator. Open the FBA calculator
Is this an earnings forecast?
No. It’s a simple model to understand how your inventory behaves. Seasonality, price drops, books that never sell and returns all move the real numbers.