Amazon FBA startup calculator: how far will your capital actually go?
Every other FBA calculator works out the fees on a single product. Useful, but it will not tell you the thing you actually want to know, which is whether the money you have is enough to build something.
This one models the whole business instead: how fast your cash comes back, how much of your stock actually sells, and what it costs you to keep the lights on. Drag the sliders and it will tell you honestly when the numbers do not work.
Run your own numbers
Start with what you have and adjust from there. Everything updates as you drag, and the assumptions are listed underneath so you can check our working.
Money you can put into stock, not counting living costs.
After Amazon's fees, before prep and subscriptions. This is the number SellerAmp shows you.
Buy to money back in your bank. Four weeks is about the floor, because Amazon pays out every fourteen days and stock has to arrive and sell first.
Free, but it is your evenings.
More assumptions
Share of stock that sells at your target price. The rest is assumed to clear at cost.
Cheap units mean more of them, which means more prep cost per pound invested.
What you take out of the business each month. Every pound taken out stops compounding.
Where your money sits, month by month
What one cycle really returns
Workable
Adjust the sliders to see how the numbers move.
What this model assumes, and where it will be wrong
- Fixed costs of £63.99 a month — Amazon's Professional plan at £30, a deal analyser at £14.99 and Keepa at £19. Choosing a prep centre adds a £24.99 account fee.
- Prep centre rate of 45p a unit, which is our tier one price. Higher volumes cost less per unit, so this errs on the expensive side.
- Unsold stock clears at cost. In reality some of it clears below cost, so treat the sell-through slider as generous.
- Your ROI stays flat all year. It almost certainly will not. Finding £2,000 of 30% deals is very different from finding £20,000 of them, and most sellers see ROI compress as they scale. If the twelve month figure looks large, lower the ROI slider and look again.
- No VAT. Once you pass the £90,000 threshold the maths changes materially. This model is aimed at sellers below it.
- No returns, no lost stock, no gated brands and no month where you simply do not find anything worth buying.
- This is a model, not a forecast. It is useful for comparing choices, not for predicting your bank balance.
The cycle time is the bit you can actually change
Prep is where most sellers lose two or three weeks. Ours turns stock around in 24 to 48 hours, from 35p a unit.
The number that decides it is not your ROI
It is how fast the money comes back. Two sellers, both starting with £2,000, both working hard.
Great margins, slow money. Finds brilliant deals but the cash sits in stock, in transit, or waiting on Amazon.
Half the margin, and comfortably ahead by December. Speed beats margin, and it is not close.
That is the whole game, and it is why most beginners optimise the wrong thing. Chasing a higher ROI is hard, competitive work. Cutting two weeks out of your cycle is mostly logistics.
Your cash cycle is the time from money leaving your account to money landing back in it. Here is where the weeks usually go.
| Stage | Typical | Can you compress it? |
|---|---|---|
| Stock delivered to you or your prep centre | 2 to 5 days | A little |
| Prep, labelling and boxing | 1 to 14 days | Yes, the most of any stage |
| Delivery to Amazon and check-in | 3 to 7 days | Sometimes |
| Sitting on the shelf until it sells | 1 to 4 weeks | Yes, at the buying decision |
| Amazon's payout cycle | Up to 14 days | No. This one is fixed |
Two of those are outside your control. The two that are not are prep speed and how fast the product sells, and the second one is decided the moment you buy. A low BSR product at 25% ROI will make you more money than a slow one at 45%.
Prep is where most people quietly lose two or three weeks, usually to their own evenings and weekends. It is also the easiest fortnight to get back.
What the numbers mean
The inputs that trip people up, and what to put in them.
What is a cash cycle in Amazon FBA?
The time between paying for stock and having that money back in your bank, profit included. It covers delivery, prep, Amazon check-in, time on the shelf and Amazon's payout schedule. For most UK sellers it runs six to ten weeks. It matters more than ROI because it decides how many times a year your money can work.
How many times a year can you turn your money?
At an eight week cycle, about six and a half times. At six weeks, closer to nine. Four weeks is roughly the floor, because Amazon pays out every fourteen days and stock still has to arrive and sell before that clock starts.
What ROI should I put in?
Use the net figure after Amazon's fees, which is the number SellerAmp shows you, not the gross margin off the retail price. Most UK arbitrage sellers work to 25 to 30%. Be careful assuming it holds as you grow: finding £2,000 of 30% deals is a very different job from finding £20,000 of them, and ROI usually compresses with volume.
Why does sell-through matter as much as ROI?
Because unsold stock earns nothing while still costing you storage. A 30% ROI at 80% sell-through is really 24%, before prep and subscriptions come out. The calculator shows that as a visible sum rather than hiding it, which is usually the moment the numbers stop looking quite so good.
Does a prep centre pay for itself?
It depends almost entirely on your average unit cost. At 45p a unit, prep on a £3 item is a 15% drag on every cycle and will eat most of your margin. On a £20 item it is closer to 2%, and the two or three weeks it saves you is worth far more than it costs. Switch the toggle in the calculator and change the unit cost slider to see where your own crossover sits.
How much money do I need to start?
Enough that your fixed costs are not eating the profit. At around £64 a month for Amazon's Professional plan, a deal analyser and Keepa, anything under about £500 tends to go backwards. We have written about this properly in our guide to starting Amazon FBA on a limited budget.
Is this calculator accurate?
It is a model, not a forecast. It assumes your ROI holds steady, ignores VAT, returns, lost stock and the months where you find nothing worth buying. It is built for comparing choices, such as whether to outsource prep or buy cheaper units, rather than predicting your bank balance. The full assumptions are listed under the results.