Data sources & methodology
Last updated: July 2026
MarginBase runs in your browser. The results are grade-school arithmetic with shop labels — markup, margin, break-even, sales tax split, and fee stacks. There is no black-box API and no rate table we scrape for your ZIP. This page lists the formulas, the defaults we start from, and what we deliberately leave out.
Default assumptions
- One product (or one blended average) at a time unless the page says otherwise
- Sales tax rate is typed by you — we do not look up city, county, or product rules
- Marketplace fee defaults are estimates; your seller dashboard overrides them
- Results are planning numbers for a price tag or listing — not books, filings, or audits
- Inputs stay on your device; closing the tab clears them
Core formulas
| Topic | Formula |
|---|---|
| Markup % | (price − cost) ÷ cost × 100 |
| Margin % | (price − cost) ÷ price × 100 |
| Price from markup | cost × (1 + markup ÷ 100) |
| Price from margin | cost ÷ (1 − margin ÷ 100) |
| Break-even units | fixed costs ÷ (price − variable cost) |
| Break-even revenue | break-even units × price |
| Add sales tax | pre-tax × (1 + rate ÷ 100) |
| Reverse sales tax | gross ÷ (1 + rate ÷ 100) |
| Food cost % | plate cost ÷ menu price × 100 |
| Menu from target food % | plate cost ÷ (target % ÷ 100) |
| Contribution margin | price − variable cost |
| Contribution margin ratio | (price − variable) ÷ price × 100 |
| Marketing / ad ROI | (revenue − spend) ÷ spend × 100 |
| Net profit | revenue − costs you enter |
| Price from profit $ | cost + desired profit |
Markup and margin use the same dollars with different denominators. A 50% markup on cost is a 33.3% margin on price. Mixing the words is how shops underprice without noticing.
Fee defaults currently used as starting points
| Item | Default on site |
|---|---|
| Etsy listing | $0.20 per listing (default) |
| Etsy transaction | ~6.5% of item price (default) |
| Etsy processing | ~3% + $0.25 (default) |
| Shopify processing | ~2.9% + $0.30 per order (default) |
| Amazon FBA referral | 15% starting point (edit by category) |
| Amazon FBA fulfillment | Flat $5.00 estimate you replace with fee preview |
Platforms change fees by category, country, and plan. If your statement disagrees with a default, trust the statement. Last reviewed: July 2026.
Known simplifications by tool
One cost, one price. No inventory layers, returns, or multi-currency. Shipping and marketplace fees are optional — include them in cost only if you want them in the unit math.
Single product (or a blended contribution you invent). No seasonality, no ramp, no debt service unless you put it in fixed costs yourself.
One rate, one amount. No ZIP lookup, no mixed exempt lines, no nexus rules. State pages (TX/CA/FL/NY/WA/IL) use the same math — you type the combined rate from your own receipts or tax software.
Common defaults for a quick estimate. Your seller dashboard and fee schedules win when they differ. Ads, storage, subscriptions, and shipping are usually out of scope unless noted on the page.
Ingredients / plate cost only — not labor, rent, or comps. Real weekly food cost still needs inventory and waste tracking.
Contribution ignores fixed costs. ROI uses the spend and revenue you enter — no automatic COGS. Net profit is not a filed P&L.
What we do not claim
- We are not your bookkeeper, CPA, or marketplace account manager
- We do not file sales tax, reconcile payouts, or store inventory
- We do not auto-detect the “right” markup for your niche
- Sales tax you collect is not revenue — keep it out of “what I made”
How we update
- Formula bugs get fixed as soon as we can reproduce them from your report.
- Fee defaults get a periodic pass when major platforms publish clear changes — still treat them as starting points.
- Email corrections to contact@shopmarginbase.com with the page URL and inputs. See also About and Contact.
Formulas and fee defaults last reviewed: July 2026. Sources & methodology
Found a wrong number? Email contact@shopmarginbase.com — corrections ship fast. See also About and Terms.