Inventory Planning: The Complete Process for DTC and Ecommerce Brands

Inventory planning is the process of deciding how much stock to hold and when to reorder each SKU, so you avoid both stockouts and overstock. It takes a demand forecast and turns it into concrete numbers you can act on: a reorder point, a safety stock buffer, and an order quantity, all constrained by your real lead times and the service level you're targeting.

Inventory planning vs demand planning#

These two get used interchangeably, and that's where a lot of confusion starts.

Demand planning is about predicting what you'll sell and getting sales, ops, and finance aligned on that number. Inventory planning is what happens after that: given the forecast, how much do you actually buy, hold, and when do you reorder.

You can't plan inventory well on a bad forecast, and a great forecast does nothing if it never turns into a reorder point. The two processes are sequential, not interchangeable. If you haven't nailed down your forecasting method yet, start with demand forecasting before you build the inventory plan on top of it.

The four inputs you need#

Before you can calculate anything, you need four pieces of data per SKU:

1. A demand forecast. Expected units sold per period, ideally per SKU and per channel. See inventory forecasting for how to build this.
2. Lead time. How long from placing a reorder to having sellable stock in hand, including supplier production time, shipping, and any receiving delay.
3. Current stock position. On-hand inventory plus anything already on order (open purchase orders), minus anything already committed to unfulfilled orders.
4. Target service level. How much stockout risk you're willing to accept. A 95% service level means you're comfortable stocking out roughly 1 cycle in 20; 99% costs more in safety stock but stocks out far less often.

Get any one of these wrong and the plan is wrong. Lead time is the input operators most often get wrong, because they plan around the average instead of the worst realistic case.

The core levers#

Four numbers do almost all the work in inventory planning.

Reorder point is the stock level that triggers a new order. When on-hand inventory hits this number, you reorder. It's built from demand during lead time plus your safety stock buffer. Full walkthrough in how to calculate reorder point.

Safety stock is the buffer that absorbs demand spikes and lead time delays. It's what keeps a slightly-late shipment or a slightly-busy week from turning into a stockout. See safety stock formula for the full calculation, which uses your demand variability and lead time variability together.

Order quantity is how much you order each time you reorder. Order too little and you're placing orders constantly, eating shipping and handling costs. Order too much and cash sits in a warehouse. The economic order quantity formula balances ordering costs against holding costs to find the sweet spot.

Par level is a simpler target used alongside or instead of a strict reorder-point model: the stock level you always want on hand for a given SKU or location. It's common for brands managing inventory across multiple warehouses or channels where a full EOQ calculation per location is overkill. See par level inventory for when to use it instead of a calculated reorder point.

The step-by-step process#

1. Pull the demand forecast per SKU and per channel, not just a company-wide total.
2. Confirm lead times per supplier, and per channel if fulfillment differs (e.g., FBA replenishment vs. your own 3PL).
3. Check current position: on-hand stock plus open purchase orders, minus committed orders.
4. Set target service level per SKU tier. Your top sellers usually justify a higher service level than your long tail.
5. Calculate reorder point and safety stock using the formulas above.
6. Size the order quantity, balancing ordering cost against holding cost.
7. Place the order and track it against the lead time you planned around.
8. Review on a cycle. Weekly for fast movers, monthly for the rest. Demand shifts, so a plan that isn't revisited goes stale within a quarter.

Planning across Shopify, Amazon, and TikTok Shop#

Multi-channel brands can't run one inventory plan for the whole business, because each channel has its own dynamics:

• Shopify stock is usually yours to manage directly, with lead times set by your own suppliers and 3PL.
• Amazon, especially FBA, adds its own replenishment lead time on top of your supplier lead time, and stockouts there also hurt your listing's search ranking, not just that week's sales.
• TikTok Shop demand can spike hard and fast around a single viral video, which breaks a reorder point sized off calm historical averages.

The trap is treating "SKU 123" as one inventory position when it's really three separate stock pools with three separate lead times and three separate demand patterns. Never assume a SKU means the same thing across channels since the same product often carries different SKUs per channel; map it through a real cross-channel table rather than assuming equality. Plan reorder points and safety stock per channel, then roll them up to see total exposure per product.

Common mistakes#

• Planning off a single blended forecast instead of per-SKU, per-channel numbers. This hides the SKUs that are actually at risk.
• Using average lead time instead of accounting for lead time variability, covered above.
• Ignoring inventory turnover when setting order quantities. A high EOQ number that also means eight months of stock sitting on a shelf isn't actually efficient.
• Letting slow movers pile up. Stock that stops selling isn't just tying up cash, it's dead stock, and it needs a different plan than reorder-point math: markdown, bundle, or write off.
• Setting one service level for every SKU. A slow-moving, high-margin item and your best-seller don't deserve the same stockout risk tolerance.
• Skipping the review cycle. A reorder point calculated in January on Q4 demand data is wrong by March.

The bottom line#

Inventory planning is the decision layer that sits on top of your demand forecast: given what you expect to sell, how much do you hold, and when do you reorder. Get the four inputs right (forecast, lead time, current position, service level), calculate reorder point and safety stock per SKU and per channel, and revisit the plan on a real cycle instead of setting it once and forgetting it.

The formulas are simple. Running them accurately across every SKU, every channel, every week is the part that breaks down in a spreadsheet. See Enough Stock's features to see how the whole process runs automatically across Shopify, Amazon, and TikTok Shop.