I spend a lot of time talking to new and scaling ecommerce sellers, and one mistake keeps popping up over and over. They lock in a selling price, load up PPC budget, and start sending inventory, without answering one simple question: how many units do I actually need to sell just to break even?
Here’s the conversation I hear constantly:
"I can sell this for $39.99, my variable cost per unit is $15. This product should print money."
Then monthly fixed costs hit, ad spend burns, and they find themselves moving plenty of units but still losing money overall.
The two things that mess sellers up
Most profit calculators ignore proper fixed‑cost handling.
They show you per‑unit contribution margin, but do not account for recurring monthly fixed costs — base ad budgets, software subscriptions, brand marketing, overhead. A product with healthy per‑unit margin can still lose money if you cannot sell enough volume to cover those fixed expenses.Sellers misclassify ad spend.
Performance sales‑driven ads are variable costs. Brand‑focused monthly marketing budgets are fixed costs. Mixing these two categories breaks your whole break‑even forecast. Many sellers get this wrong and set unsafe ad scaling targets.
Why I couldn't find a tool that fixed this
I tested existing spreadsheets and online break‑even calculators, and they all shared frustrating limitations:
- Many are built for only one platform, either Amazon‑only or Shopify‑only.
- Useful fixed‑cost inputs sit behind paywalls.
- No built‑in way to separate fixed vs variable marketing spend.
- Very few calculate advanced ecommerce metrics: break‑even ROAS, break‑even MER.
So I built the thing I couldn’t find, as part of the MerchCalc toolset.
MerchCalc Break‑Even Calculator
It’s a browser‑based break‑even calculator built for cross‑channel sellers: Shopify, Amazon, TikTok Shop and more. All the math runs locally in your browser — none of your product research numbers ever get uploaded, and there’s no account to create.
Here's what it handles:
✅ Clear separation for per‑unit variable costs and monthly fixed costs
✅ Works for Shopify, Amazon FBA/FBM, TikTok Shop and other marketplaces
✅ Real‑time updates: adjust selling price or costs and watch outputs change instantly
✅ Outputs break‑even units, break‑even revenue, contribution margin & ratio
✅ Computes ecommerce‑critical metrics: break‑even ROAS and break‑even MER
✅ Built-in scenario simulation: test price increases and variable‑cost shifts side‑by‑side
✅ Runs offline in‑browser — no data ever leaves your device, no account, no paywall, 100% free.
You can try it here: https://merchcalc.com/tools/break-even-calc
The math that changed my mind about a "good" product
This example uses the exact sample numbers shown in the screenshot above:
Selling price: $39.99, Variable Cost Per Unit: $15.00, Monthly Fixed Costs: $2000, Monthly Ad Budget: $500
How most sellers quickly evaluate this product (only looking at per‑unit contribution):
| Line | Amount |
|---|---|
| Selling price | $39.99 |
| Variable Cost Per Unit | −$15.00 |
| Contribution Margin Per Unit | $24.99 |
| Contribution Margin Ratio | 62.49% |
On a per‑unit basis this looks fantastic. Many sellers will immediately ramp ads based only on this number.
Reality after adding your monthly fixed overhead and ad budget into break‑even math:
| Line | Amount |
|---|---|
| Monthly Fixed Costs | $2000.00 |
| Break‑Even Units | 80 units per month |
| Break‑Even Monthly Revenue | $3,200.48 |
| Break‑Even ROAS | 2.67x |
| Break‑Even MER | 8.00x |
| Est. Months To Profitability | 3 |
If your monthly sales volume only hits 50 units, every individual sale has positive contribution margin, yet your business is losing money for the month. That is the trap this calculator is designed to catch.
We also added scenario simulation to test what happens when pricing or sourcing costs shift.
For this test case:
- Price Change: +10%
- Variable Cost Change: -10%
- New break-even: 66 units / month ($2,885.57)
- That’s 14 fewer units you need to sell to hit break-even.
You can plug these exact numbers into our live tool and reproduce these results yourself.
What's next
This is a side project and I'm still iterating. I plan to add more sensitivity testing options and export functions for sellers who want to save their scenarios.
If you're an ecommerce seller (or an indie builder poking around), I'd genuinely love feedback — what's missing, what's wrong, what you'd want to see next. Bug reports welcome.
What fixed‑cost line item do you most often forget during forecasting? For me it was recurring SaaS tool subscriptions.
Also, if you sell on Amazon FBA, I built a standalone Amazon FBA profit calculator too. You can read about it here: I built a free Amazon FBA profit calculator because Amazon's own one kept lying to me.
Built as part of MerchCalc — free tools for cross‑border sellers, no login, no paywall.
MerchCalc is built for global ecommerce sellers. https://merchcalc.com is our only official website. No other websites are authorized to represent MerchCalc. We are not affiliated with MerCalc app or any third‑party aggregator, scraped or auto‑listed content sites.
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