Cash flow & ROI
Margin tells you whether a product is worth selling. How fast it can grow on its own cash depends on two other numbers: the return on every dollar you put into stock, and how long that dollar is gone before it comes back. This shows what reinvesting does to your monthly profit over time. Everything runs in your browser; nothing you enter leaves this page.
Monthly profit as you reinvest
steps up each time a batch sells through
Over time
Tap any in the calculator to jump to its explanation here.
What you enter
What the customer pays for one unit.
↑ back to the calculatorWhat one unit costs to make and land in stock: product, packaging, freight to the warehouse, duty. This is the money that sits in a box while the batch is made, shipped and sold, so it's what your ROI is measured against.
↑ back to the calculatorThe FBA fee per unit, or your postage and packing if you ship yourself. It's taken out when the unit sells, so it lowers profit without tying up cash up front.
Where to find it: Seller Central → Fee Preview, or the FBA revenue calculator.
↑ back to the calculatorThe percentage the marketplace keeps from each sale. Amazon's referral fee is 15% in most categories; Etsy and eBay charge their own.
↑ back to the calculatorAnything else that comes out of each sale: ad spend divided by units sold, storage fees, the cost of returns. Leave it at 0 to see the product on its own, but a real plan needs it. A $2 ad cost per unit can halve the growth rate.
↑ back to the calculatorDays from paying the supplier to the goods being ready to ship.
↑ back to the calculatorDays from the factory to sellable in the warehouse, including customs and check-in. Ocean freight from Asia to an Amazon warehouse often runs 45–70 days; air is closer to 10–15.
↑ back to the calculatorOnce stock arrives, how long until it has sold and the marketplace has paid you. Your money doesn't come back the day the box lands; it comes back as it sells. Set it to 0 to assume every batch sells the day it arrives.
↑ back to the calculatorWhat the product makes a month today. The calculator works out the stock that level needs and grows it from there.
↑ back to the calculatorWhen a batch sells, you get back what it cost plus its profit. This is the share of that total you spend on the next batch; the rest is yours to take out. At 80%, a batch that cost $1,000 and earned $1,500 returns $2,500, and $2,000 goes into the next order.
Below a certain level the business shrinks instead of growing; the calculator tells you where that is.
↑ back to the calculatorHow far out to project. Growth only happens when a batch completes, so a horizon shorter than one cycle shows no growth.
↑ back to the calculatorReinvesting can't grow sales past what the market will buy. Set this to your best guess of the most this product could make a month, for example the profit the top sellers on your keyword are making. Use 0 for no limit.
↑ back to the calculatorTry a trade-off side by side: a cheaper supplier with longer lead times, air freight that costs more but arrives in two weeks, or a price change. A higher margin doesn't always win; a faster cycle often does.
↑ back to the calculatorWhat you get
What the product makes a month at the end of your look-ahead period if you keep putting cash back into stock. Every completed cycle multiplies it by the growth factor, until it hits your ceiling.
↑ back to the calculatorThe inventory, at cost, that you'd need to hold to run at that later profit level. It grows at the same rate as the profit, because more sales need more stock in the pipeline.
↑ back to the calculatorThe stock needed to keep today's profit going without running out: one full cycle of sales (manufacturing + shipping + selling through), at product cost.
↑ back to the calculatorProfit per unit ÷ product cost: what each dollar in stock earns per cycle. Margin is measured against price; ROI is measured against the cash you actually put in, which is why it drives growth. A 36% margin on a $4.30 unit is a 156% ROI.
↑ back to the calculator