The spreadsheet maths that decides stockouts and storage bills
Every inventory mistake is one of two numbers being wrong: reorder too late and you stock out — losing the listing's momentum, its reviews' recency and, on TikTok, the affiliate videos pointing at it. Reorder too much and the surplus ages into Amazon's storage and day-181 surcharge territory. Suites sell this as "inventory intelligence"; underneath, it is four formulas with your name on them. This calculator runs them openly: days of supply, reorder point from your full lead time (supplier + transit + the Amazon check-in days sellers always forget), safety stock, the order quantity that restores your target cover, and turnover — how hard the cash sitting in your stock is working.
Total lead time is the trap: a 20-day supplier and 10-day freight are obvious, but FBA check-in adds roughly 3–7 days before units can sell, and every one of those days is demand you must already own. Target cover of 45–90 days balances stockout risk against storage and ageing costs for most products.
Worked example
At the defaults — 300 units selling 10 a day, a 35-day all-in lead time — the reorder point is 420 units: higher than the stock on hand. That is not a bug; it is the calculator announcing that at this sales rate the next order is already late, and that a 720-unit order is what restores 60 days of cover plus the safety buffer.
How to use this calculator
- Use a sales rate you believe — 30-day average for stable products, 7-day for trending ones — and say which you picked.
- Build lead time from evidence: your last order's actual supplier, freight and check-in days, not the quote.
- Set safety days by consequence: a stockout on your hero product costs more than seven days of buffer stock.
- Cross-check the order quantity against the Storage Fee Calculator — a "safe" order that lands 200 units past day 181 is not safe.