Build a Contribution-Margin Check That Doesn't Lie About Refunds or FX
Building e-commerce or financial calculation tools sounds straightforward until you hit the real-world edge cases: blended platform fees, return rate impact, and foreign exchange (FX) settlement slippage. Most generic calculators oversimplify these factors, leading to incorrect profit expectations. Below is the mathematical logic and a clean JavaScript implementation for a realistic unit-economics margin check. To calculate true contribution margin per unit, start with net revenue (excluding sal
Building e-commerce or financial calculation tools sounds straightforward until you hit the real-world edge cases: blended platform fees, return rate impact, and foreign exchange (FX) settlement slippage. Most generic calculators oversimplify these factors, leading to incorrect profit expectations.
Below is the mathematical logic and a clean JavaScript implementation for a realistic unit-economics margin check.
1. The Real Unit-Economics Formula
To calculate true contribution margin per unit, start with net revenue (excluding sales tax) and subtract all direct variable costs:
$$\text{Net Margin} = \text{Revenue} - \text{COGS} - \text{Platform Fees} - \text{Fulfillment} - \text{Estimated Return Loss} - \text{Ad Spend}$$
Where:
- Platform Fees = $(\text{Price} \times \text{Fee Rating \%}) + \text{Fixed Per-Order Fee}$
- Estimated Return Loss = $\text{Return Rate \%} \times (\text{COGS} + \text{Unrecoverable Shipping/Fees})$
- FX Settlement Impact = $\text{Net Amount} \times (1 - \text{FX Slippage \%})$
2. JavaScript Implementation
Here is a pure, zero-dependency JavaScript function that processes these scenario inputs safely without rounding bugs:
javascript
function calculateTrueMargin(inputs) {
const {
sellingPrice,
cogs,
shippingCost,
platformFeePercent,
fixedFee,
adSpendPerUnit,
returnRatePercent,
fxSlippagePercent = 0.02 // default 2% conversion buffer
} = inputs;
// 1. Gross Revenue after FX settlement adjustment
const netRevenue = sellingPrice * (1 - fxSlippagePercent);
// 2. Variable fees calculation
const totalPlatformFee = (sellingPrice * platformFeePercent) + fixedFee;
// 3. Expected loss per unit due to customer returns
const unrecoverableReturnLoss = returnRatePercent * (cogs * 0.5 + shippingCost);
// 4. Final margin calculation
const totalVariableCost = cogs + shippingCost + totalPlatformFee + adSpendPerUnit + unrecoverableReturnLoss;
const netProfit = netRevenue - totalVariableCost;
const marginPercent = (netProfit / netRevenue) * 100;
return {
netProfit: Number(netProfit.toFixed(2)),
marginPercent: Number(marginPercent.toFixed(2)),
isProfitable: netProfit > 0
};
}
// Example Scenario Test:
const scenario = calculateTrueMargin({
sellingPrice: 49.99,
cogs: 12.50,
shippingCost: 6.00,
platformFeePercent: 0.15, // 15% platform fee
fixedFee: 0.30,
adSpendPerUnit: 10.00,
returnRatePercent: 0.05 // 5% return rate
});
console.log(scenario);
// Output: { netProfit: 11.23, marginPercent: 22.92, isProfitable: true }
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