Reordering is a timing problem dressed up as a math problem. Get the timing wrong and you either stock out and lose sales or overbuy and trap cash. Here is a method that does not live in a brittle spreadsheet.
Every reorder decision is really two questions asked at once: when do I buy, and how much do I buy. Answer them with gut feel and you swing between stockouts and dead stock. Answer them with a spreadsheet and you get a fragile model that breaks the moment a SKU is added, a lead time changes, or a channel spikes. There is a better way, a repeatable forecasting method built on three inputs you already have, that runs off your live inventory data instead of a workbook you have to babysit.
Forecasting is not fortune-telling. It is arithmetic on three knowable quantities:
Get those three right per SKU and the reorder point and reorder quantity fall out of them. The detail of the buffer math is in safety stock and reorder point (/glossary/safety-stock-reorder-point); this is the workflow that uses it.
Velocity is units sold per day, but the window matters. Too short and a single big day distorts it; too long and you smooth over a real trend. A common approach is a trailing 30 to 60 day average, adjusted if you know a season is shifting demand. The number that matters is daily units, because lead time and safety stock are also measured in days.
The trap for multi-channel sellers: velocity has to count sales across every channel for the SKU, not just one. If you forecast off Amazon velocity alone but the same SKU also sells on Shopify and Walmart, you will reorder too little and stock out. Pull velocity from total sales of the SKU, all channels combined.
Lead time is the full clock from "I place the PO" to "the units are sellable," not just the supplier's production time. It includes manufacturing, freight, customs if you import, receiving, and, for FBA, the time to ship into Amazon and have units checked in and available. Sellers routinely underestimate lead time by counting only the factory's quote and forgetting the two or three weeks of logistics on the back end.
Use your real, observed lead time, not the optimistic one. If recent reorders took 45 days to become sellable, plan on 45, even if the supplier says 30.
Safety stock is the cushion against the two things that go wrong: demand runs hotter than your average, or the reorder arrives later than planned. A simple, defensible approach is to hold safety stock equal to a number of days of average demand, sized to how volatile the SKU is and how unreliable the lead time has been. A steady SKU with a dependable supplier needs a thin buffer; a spiky SKU with a flaky supplier needs a fat one.
The reorder point is the on-hand level that should trigger a new order. The logic:
Reorder point = (Daily sales velocity x Lead time in days) + Safety stock.
If you sell 20 units a day, lead time is 30 days, and you hold 100 units of safety stock, your reorder point is (20 x 30) + 100 = 700 units. When on-hand stock across all locations drops to 700, you place the order. That timing means the reorder arrives just as you would otherwise be eating into the buffer.
| INPUT | EXAMPLE VALUE |
| Daily sales velocity (all channels) | 20 units |
| Lead time | 30 days |
| Safety stock | 100 units |
| Reorder point | 700 units |
The reorder point tells you when. The quantity tells you how much, and it balances tying up cash against the cost of ordering too often. Many sellers reorder enough to cover a target number of days of demand, say 60 or 90, adjusted for supplier minimums and any volume pricing. The goal is a quantity that keeps sell-through (/glossary/sell-through-rate) healthy without parking months of cash on a shelf.
The arithmetic is easy. Keeping it accurate by hand is not. Velocity changes weekly, lead times drift, and on-hand counts scatter across FBA, a 3PL, your warehouse, and in-transit stock. A spreadsheet forces you to refresh all of that manually, and the reorder point is only correct on the day you last updated it. Add a dozen SKUs and the model becomes a part-time job that is wrong most of the time.
The fix is to run the forecast off live inventory data. ConnectBooks tracks your sales velocity from actual settlement data across channels, and ConnectStock, its multi-location inventory feature, keeps a current on-hand count across FBA, 3PL, home warehouse, and in transit. With a live, complete count and real velocity, the reorder point is always current rather than as-of-last-Tuesday. ConnectStock is bundled with Platinum and available as an add-on on Gold and Diamond.
A SKU sells 20 units a day across Amazon and Shopify combined. Observed lead time is 30 days. You hold 100 units of safety stock. Reorder point is 700 units. On-hand across all locations drops to 690, below the trigger, so you place a reorder. You order 1,800 units, enough for about 90 days at current velocity, within the supplier's pricing break. The order arrives in 30 days, just as on-hand approaches the 100-unit buffer, and you never stock out and never overbought. That is the whole loop, repeated per SKU, kept current automatically.
Multiply your daily sales velocity by your lead time in days, then add safety stock. If you sell 20 a day, lead time is 30 days, and you hold 100 units of buffer, the reorder point is (20 x 30) + 100 = 700 units. When on-hand stock hits that level, you order.
All channels. If a SKU sells on Amazon, Shopify, and Walmart, base velocity on total combined sales. Forecasting off one channel understates demand and causes stockouts.
The full clock from placing the PO to units being sellable: manufacturing, freight, customs, receiving, and for FBA the time to ship in and have units checked in. Use observed lead time, not the supplier's optimistic quote.
Enough to cover demand spikes and lead-time slippage, sized to the SKU's volatility and the supplier's reliability. A steady SKU with a dependable supplier needs a thin buffer; a spiky SKU with an unreliable supplier needs more. Many sellers express it as a number of days of average demand.
ConnectBooks tracks real sales velocity from settlement data, and ConnectStock keeps a live on-hand count across FBA, 3PL, warehouse, and in transit, so the inputs to your reorder math stay current automatically instead of in a spreadsheet you update by hand. ConnectStock is bundled with Platinum and available as an add-on on Gold and Diamond.
Run reorder forecasts off a live, all-channel inventory count. See ConnectStock at /pricing.
Running an e-commerce business comes with plenty of challenges, but ConnectBooks is here to make your life easier. With real-time insights, seamless integrations, and detailed tracking of your profitability and inventory, you can stay ahead of the game. Whether you’re selling on Amazon, Shopify, Walmart, TikTok or eBay, ConnectBooks helps you manage your finances with 100% accuracy and confidence, so you can focus on growing your business.
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