Managing online stockouts: preventing and reacting

Managing online stockouts comes down to two things: stopping a product from running out before the replenishment arrives, and knowing what to do when it runs out anyway. For the first, you need real, synchronised stock, a reorder point calculated per SKU and alerts before you hit zero. For the second, a protocol that saves the sale and the campaign.

A stockout doesn't just block one order. It can stop a campaign that was working, burn through advertising budget and send your best customer to a competitor. The solution isn't buying without limits, but deciding with reliable data. Below you'll find the formulas, a worked example with indicative figures, the warehouse mistakes that cause the most stockouts and a weekly checklist.

What is a stockout

A stockout is the moment a product has no units available to sell or fulfil, even though there's demand. In an online shop it can be visible (the product page shows sold out) or invisible (the website sells a unit that doesn't exist and the order can't be prepared). Both cost money; the second also costs you an angry customer.

It's worth distinguishing between a commercial stockout, when there's genuinely no stock left, and an operational stockout, when the stock is in the warehouse but can't be sold: not located, poorly received, reserved for another order or damaged. In the day-to-day running of a warehouse, the second is much more common than people think.

What a stockout really costs

The obvious loss is the order that doesn't go through. But if the customer arrived from an advert, you've already paid for their acquisition. Without an alternative or a reliable restock date, they're likely to buy elsewhere and never come back.

Stockouts also throw the team off balance. Marketing launches a promotion or a creator campaign, stock can't keep up, operations reacts too late and customer service gets flooded with queries. The result is compensation discounts, cancellations and, on marketplaces, penalties for unfulfilled orders.

And there's a less visible cost: fear of running out leads to over-ordering. If you don't distinguish between a high-turnover product and a one-off spike, you go from stockout to excess inventory. Capital gets tied up, you use more warehouse space and the risk of obsolescence rises in fashion, cosmetics, electronics or seasonal products.

Most common causes of stockouts in an online shop

• Forecasting based only on historical averages, without accounting for campaigns, seasonality or a new channel.
• Poorly estimated lead time: counting only how long the supplier takes to manufacture, but not transport, customs or warehouse receiving.
• Stock out of sync between the shop, marketplaces and warehouse, causing overselling.
• Slow or unvalidated goods-in: the stock has arrived but takes days to show as available.
• Inventory discrepancies from picking errors, damaged units not logged, or swapped references.
• A supplier that delays without warning, and no one tracking it until stock hits zero.
• Monitoring the product family instead of the SKU: there are 900 T-shirts, but the black size M only has 6.

Managing online stockouts starts with measuring properly

You can't protect what you can't see. You need a single source of truth for inventory, and the stock shown on Shopify, WooCommerce, PrestaShop or your marketplace has to come from there, not from a spreadsheet someone updates at the end of the day. If you work with several platforms, check how they connect to your warehouse on the online shop, marketplace and courier integrations page.

The figure that matters isn't physical stock, but sellable availability. Split out at least these quantities:

• Available: what can be sold right now.
• Reserved: units already committed to orders pending preparation.
• Damaged or under review: unreviewed returns, breakages, units in quarantine.
• In transit: ordered from the supplier, not yet received.

And always by SKU. Every variant (size, colour, format) has its own sales pace, cover and reorder point. In catalogues with many variants, looking at the family total hides the stockouts that hurt the most. If your inventory doesn't match reality, start with the guide to getting accurate stock in your ecommerce.

The four metrics that should trigger decisions

• Sales velocity: units sold per day, by SKU and by channel.
• Stock cover: days you can keep selling at the current pace (available stock divided by daily sales).
• Lead time: total days from placing the order with the supplier until the stock is located and ready to ship.
• Safety stock: the buffer that absorbs demand spikes, delays and errors.

Review these by SKU and by channel. A product can have healthy cover on organic sales and fall short as soon as it's featured in a marketplace promotion.

How to calculate the reorder point and safety stock

The reorder point is the stock level at which you need to trigger replenishment. It's calculated as: average daily sales multiplied by lead time, plus safety stock. If you order once you're already below that, a stockout is almost guaranteed.

For safety stock, a simple and conservative formula is: (maximum daily sales × maximum lead time) minus (average daily sales × average lead time). Use data from recent comparable months and strip out anomalous days, such as a pricing error that spiked sales.

Worked example with indicative figures

A cosmetics shop sells an average of 18 units a day of its flagship serum. On strong days it reaches 28. The supplier usually takes 20 days including transport and receiving, but has occasionally taken 26.

• Consumption during replenishment: 18 × 20 = 360 units.
• Safety stock: (28 × 26) − (18 × 20) = 728 − 360 = 368 units.
• Reorder point: 360 + 368 = 728 units.

When available stock drops below 728, it's time to order. This is a cautious calculation designed for a bestselling product with an unreliable supplier. If your supplier is domestic and reliable, the margin can be much smaller; if the product has a shelf life, stockpiling this much makes no sense, and it's better to shorten the lead time with smaller, more frequent orders.

Three alert levels before hitting zero

1. Planning alert: stock crosses the reorder point. Purchasing places the order and requests a delivery date.
2. Confirmation alert: half the safety stock remains. You need a confirmed arrival date and tracking of the supplier's shipment.
3. Commercial alert: only a few days of cover remain. Cut ad spend on that SKU, push alternatives, limit visible stock on some channels or open a pre-order with a realistic date.

Waiting until stock hits zero leaves you with none of these options.

Adjust forecasting for campaigns, seasonality and channels

Historical data is the starting point, not the answer. A supplements brand has recurring demand; a fashion brand depends on collections, sales and sizes; a gift shop concentrates sales into a few weeks. Forecasting needs to factor in the commercial calendar.

Before each campaign, marketing and operations should answer three questions together: which SKU is being promoted, how many units can it sell without leaving orders unfulfilled, and what happens if demand exceeds forecast. For dates like Black Friday it's worth planning weeks in advance, as explained in the guide to preparing Black Friday logistics.

If you sell across several channels, decide how you split stock. Showing all your inventory on every marketplace increases the risk of overselling; reserving allocations per channel protects your most profitable orders but reduces flexibility. It depends on margin, the cancellation penalty on each channel and how often you sync. This is covered in the guide to syncing stock across channels without overselling.

Stop warehouse errors turning into stockouts

Many stockouts don't originate in purchasing, but in the warehouse. Mistakes we often see:

• Goods received that sit on the dock for two or three days without being logged. Meanwhile, the website shows sold out with the product ten metres away.
• Receiving checked against the supplier's delivery note without counting: 480 units arrive, 500 are logged in, and the difference shows up as overselling weeks later.
• Picking the wrong variant: size L is shipped instead of M, and both end up out of balance.
• Returns piling up unreviewed, never going back into sellable stock.

The recipe is well known, but it has to be applied every single day. Every goods-in is checked against the purchase order (quantity, reference, condition and, where relevant, batch or expiry) and located the same day. Picking is done with locations and scanning. And instead of an annual stocktake, cyclical counts of the highest-turnover, highest-value or most problematic SKUs. The sooner you spot a discrepancy, the fewer orders it affects.

At Furgox we solve this with a dashboard connected to the warehouse: stock updates in real time with every order, goods-in and return, and reviewed returns go back into stock within 48 hours of arrival. You can see how it works on the real-time stock and order dashboard. Technology doesn't replace purchase planning, but it removes most of the manual error that feeds operational stockouts.

What to do when a product has already run out

1. Don't hide the product page. Keep it visible with an availability notice to capture emails from interested customers.
2. Communicate a sensible date. If you know the restock date, give an estimate with some margin; if you don't, don't promise a timeframe.
3. Offer a relevant alternative: another variant, another format or an equivalent bundle. A lower-value substitute that doesn't fit is worse than offering nothing.
4. Pause or redirect advertising. Keeping ads running towards a sold-out SKU wastes budget and muddies your metrics.
5. Only consider pre-orders if the arrival date is confirmed and you can meet the delivery timeframe you're advertising.
6. Check your marketplaces: make sure stock shows zero everywhere to avoid a pile-up of cancellations.

Afterwards, document the cause. Did forecasting fail, did the supplier delay, did an unexpected channel sell more, or was there a discrepancy? Don't look for someone to blame: look for the point in the process that needs to change so the next alert arrives sooner.

How to measure stockouts

If you don't measure it, you won't know if you're improving. Three simple indicators:

• Stockout rate: days each SKU has been out of stock divided by the days it should have been available for sale. Calculate it for your core SKUs.
• Affected orders: percentage of orders with at least one line out of stock when being prepared. Measures operational stockouts.
• Estimated lost sales: days out of stock × average daily sales × margin per unit. It's indicative, but it puts a figure on the problem.

Alongside these, track inventory accuracy and receiving time. You'll find more ideas in the article on key indicators for a logistics operation.

Weekly checklist against stockouts

• Days of cover reviewed for your top 20 best-selling SKUs.
• Supplier orders placed for everything below the reorder point.
• Arrival dates confirmed for orders in transit.
• This week's goods-in located and logged.
• Returns reviewed and put back into sellable stock.
• Cyclical count of the SKUs with the most issues.
• Stock synchronised with no discrepancies between shop, marketplaces and warehouse.
• Upcoming weeks' campaign calendar cross-checked against available stock.
• Active ads reviewed: none pointing to products with no cover.

Turn inventory into a commercial decision

The goal isn't 100% availability at any cost, but profitable availability. Months of dead stock can do as much damage as losing sales to stockouts. The balance depends on margin, shelf life, supplier reliability, storage cost and how quickly you can restock.

When purchasing, marketing and logistics work from the same data, a stockout stops being a last-minute surprise and becomes a signal: adjust the promotion, bring forward a restock or prioritise the right SKU.

Frequently asked questions

How can you prevent a stockout in an online shop

With three pieces: reliable, synchronised inventory across all channels, a reorder point calculated per SKU that includes safety stock, and alerts that trigger before you hit zero. Add fast receiving and cyclical counts so the system's stock matches what's actually on the shelf.

What causes a stockout

The most common causes are forecasting that ignores campaigns or seasonality, poorly estimated lead times, supplier delays, overselling from stock out of sync between channels, and warehouse errors such as slow receiving or incorrect picking. Often several combine at once.

What is safety stock and how is it calculated

It's the extra quantity you keep to cover demand spikes and replenishment delays. A simple way to calculate it is subtracting the product of average sales and average lead time from the product of maximum daily sales and maximum lead time. Review it every few months or when the supplier or sales pace changes.

What should I do if I sell a product I don't have in stock

Notify the customer as soon as possible, explain the real timeframe and offer them the choice to wait, switch to an alternative, or cancel with an immediate refund. Then review why the visible stock didn't match reality: it's usually a sync issue or a warehouse discrepancy.

Does a logistics operator help prevent stockouts

It can help a great deal with operational stockouts: goods-in validated and located the same day, scan-based picking, returns that go back into stock quickly, and inventory connected to your shop in real time. Forecasting and purchasing are still your responsibility, but you work with data you can trust.

If your stockouts come more from the warehouse than from purchasing, it may be worth handing order fulfilment to a team with stock connected to your shop. The cost depends on your order volume, product type and destinations; you can request a quote for your case on the pricing page or see the dashboard in action with a demo with our team.