1. Gross margin is not profit
Sales minus the cost of what you sold gives gross margin, and that is the calculation most shops actually run in their heads. Net profit is what remains after rent, salaries, electricity, freight, interest, breakage and everything else. The difference between the two is often the difference between a business that is working and one that is not, and it is invisible until the second set of costs is recorded.
2. Landed cost, not invoice value
What the stock cost you is the supplier invoice plus freight, loading, unloading and any shortage on delivery. In bulk material those are a meaningful share. Pricing off the invoice value alone is the commonest reason a shop finds its highest-turnover line is also its thinnest. Record freight against the purchase and your margin figures start describing reality.
3. The three costs shops never count
Interest on borrowed working capital, which in a trade with a sixty-day credit cycle is a monthly number rather than an annual one. The real cost of a vehicle — not just diesel but the driver, insurance, servicing and the fact that it is wearing out. And breakage, spillage and short delivery, which in cement, tiles and glass is a predictable percentage. Add those three and a lot of apparently profitable months look different.
4. Margin per item is where the answer is
A shop-wide margin figure tells you very little, because it averages a strong line against a weak one. Purchase rate against sale rate per item is the report that actually changes decisions, and it usually finds two or three fast-moving lines where a supplier rate rise has closed the gap without anyone noticing. Running it monthly takes fifteen minutes.
5. Three figures worth checking every month
Turnover on its own describes nothing, because a shop can grow turnover while losing money. The three that describe a month are gross margin on your main lines, the change in total outstanding compared with last month, and closing stock value against the month before. Rising turnover with rising outstanding and rising stock is not growth — it is working capital leaving the building.
6. What to do with the answer
Once you know margin by item, three actions follow. Re-price the lines where the gap has closed, or stop pushing them. Renegotiate with suppliers on the lines where you have volume. And look hard at the slow stock the same report exposes, because capital sitting in it is capital not earning anywhere. None of that requires new information — only the discipline of recording purchases as carefully as sales.