Revenue is rising, supplier prices have not changed, and retail prices are stable. Yet gross trading margin is falling. One explanation may be the composition of sales: product groups generating less margin per euro sold now account for a larger share of revenue.
Product mix describes the distribution of sales across items or categories. For an esoteric shop combining ritual materials, books, and accessories, it explains why an average across the catalogue is insufficient to forecast the amount available to fund the business.
Overall margin depends on the products actually sold
Bpifrance Création's definition of gross trading margin compares merchandise sales with the purchase cost of goods sold. It is not the theoretical margin on every product displayed on the shelves.
A shop may stock many items with high gross margin rates on sales yet generate most of its revenue from a few products with lower rates. A simple average of the rates on all product pages would give a misleading picture of actual performance.
The weighting must follow each product's sales excluding VAT. An item sold for 5 € excluding VAT and one sold for 50 € do not contribute equally to total sales. Giving them the same weight does not produce the overall gross margin rate.
An example where growth reduces gross trading margin
The following table compares two hypothetical periods on a consistent basis. Group A keeps a gross margin rate of 70% on sales, and group B a rate of 40%. These figures are educational assumptions: they do not describe any Esotericæ product family or industry average.
Sales are net of discounts and refunds. Purchase costs are entered consistently. The model isolates gross trading margin and excludes other selling and operating expenses.
| Group | Gross margin rate on sales | Sales ex. VAT, period 1 | Margin, period 1 | Sales ex. VAT, period 2 | Margin, period 2 |
|---|---|---|---|---|---|
| A | 70 % | 6 000 € | 4 200 € | 3 300 € | 2 310 € |
| B | 40 % | 4 000 € | 1 600 € | 7 700 € | 3 080 € |
| Total | Depending on sales mix | 10 000 € | 5 800 € | 11 000 € | 5 390 € |
Revenue rises by 10%, while gross margin falls by 410 €, or about 7.1%. The overall rate declines from 58% to 49%. Neither group rate has changed; their relative share of revenue explains the difference.
In the first period, A accounts for 60% of sales; in the second, it accounts for only 30%. The simple average of 70% and 40%, or 55%, correctly describes neither period.
Calculate a weighted rate rather than averaging product pages
The most direct method is to add gross margin amounts in euros and divide by total revenue excluding VAT. For period 1, 5 800 ÷ 10 000 = 58%. For period 2, 5 390 ÷ 11 000 = 49%.
Alternatively, multiply each group's rate by its share of sales. In period 1: 70% × 60% + 40% × 40% = 58%. In period 2: 70% × 30% + 40% × 70% = 49%. The methods should agree when the data and rounding are consistent.
An average pricing multiplier does not replace this calculation. It combines purchase and sale amounts differently, and its meaning can also be affected by VAT rates. Return to actual revenue excluding VAT before aggregating the catalogue.
Check individual products before aggregating results
The Esotericæ margin calculator helps check a single item's figures using its ex-VAT purchase price, multiplier, and VAT. To reproduce an existing retail price, divide the price including VAT by the purchase price excluding VAT to obtain the multiplier.
This remains a unit-level check. Overall gross margin must be calculated separately from actual quantities and prices charged. The tool does not import sales history or automatically weight several categories.
Include discounts in net sales and costs corresponding to the items actually sold. Recalculating an entire past period using new supplier prices may confuse historical performance with future replenishment profitability.
Separate the effect of sales volume from changes in product mix
In our example, if period 2 had retained the original overall margin rate of 58%, sales of 11 000 € would have produced 6 380 € in gross margin. Revenue growth would therefore have added 580 € compared with the original 5 800 €.
The actual product mix produces only 5 390 €, or 990 € less than it would at the original rate. The 580 € gain from increased revenue and −990 € from the mix together explain the final −410 € difference.
This is a mathematical breakdown, not a complete commercial explanation. To understand why A lost share, examine availability, product visibility, customer requests, and any activities focused on B.
Investigate causes before changing the assortment
Stockouts affecting several A items can move purchases toward B. A promotion for B may change the mix temporarily. Changes in customers or sales channels can also produce different baskets. Test these explanations against the shop's data.
The Service Public Entreprendre guide to management dashboards connects sales tracking by category to analyzing differences and taking corrective action. For your business, record events that help explain the figures.
ABC analysis can identify the most influential items. The guide to best sellers and long-tail products helps assess their roles without reducing the catalogue to a ranking of percentages.
Do not automatically remove lower-margin categories
A lower rate does not make a product useless. An item can generate a substantial margin in euros, require little inventory, or meet demand that other categories cannot satisfy. Removing it does not automatically transfer its sales to products with higher contribution.
Also examine costs after purchasing. A category with a strong gross margin rate may require more advice, repacking, or shipping. Contribution after those expenses and gross margin relative to average inventory add context to the gross margin rate.
Products purchased together provide another avenue. If two categories appear in the same orders, that does not prove one caused sales of the other. The association can, however, guide a display test, evaluated on the whole basket rather than one item.
Build a margin forecast that reflects the assortment
To forecast the following month's margin, estimate sales mix as well as total revenue. A campaign focused on a different category can alter the overall margin rate even when unit purchase costs and prices remain unchanged.
Include a scenario where lower-contribution categories account for more sales. The forecast will then show how sensitive the result is to product mix. This is more accurate than keeping the same average rate throughout the year with no reference to the retail calendar.
Check unit margins, then their actual weight
Verify important products with the free margin calculator. Add their actual sales and margins to your management table to calculate a weighted rate. The first calculation checks pricing; the second explains overall catalogue performance.
Profitable growth depends as much on what sells as on total revenue. Monitoring product mix reveals this before it disappears behind an average. It helps retailers adjust purchasing, presentation, and margin objectives.
Sources and references
Bpifrance Création — Gross trading margin; Service Public Entreprendre — Setting up a management dashboard. Accessed September 17, 2026. Groups, rates, and periods are hypothetical. Weightings and breakdowns are educational calculations, not a profitability ranking of Esotericæ product families.