Inventory Forecasting for Small eBay and Shopify Sellers

What is inventory forecasting?

Inventory forecasting means predicting how much stock you will need over a future period — typically the next 30, 60, or 90 days — so you can order just enough to cover demand without tying up excessive capital in unsold goods.

For single-channel sellers this is already worth doing. For multi-channel sellers running eBay and Shopify simultaneously — or adding Amazon, Etsy, or TikTok Shop — it becomes essential. All of your sales channels are drawing from the same physical stock. A run of strong eBay sales can drain inventory that your Shopify store is also advertising. Without a forecast, you are reacting rather than planning.

The good news is that you do not need sophisticated software to get started. A few straightforward calculations cover most situations for sellers with up to a few hundred SKUs.

The simplest forecasting method: sales velocity

Sales velocity is just your average daily sales rate. Calculate it like this:

avg_daily_sales = units sold in last 30 days ÷ 30

Once you have that number, you can calculate how many days of stock you currently hold:

days_of_stock = current stock ÷ avg_daily_sales

A worked example: if you sold 90 units of a product over the last 30 days, your average daily sales rate is 3 units per day. If you currently have 45 units in stock, you have 15 days of stock remaining before you hit zero.

That 15-day number is what matters. Compare it to your supplier lead time. If your supplier takes 10 days to deliver, you have a 5-day window to place your order before you risk going out of stock. In practice you would want to reorder with a few extra days of buffer built in — more on that below.

Seasonal adjustment

Sales velocity calculated from the last 30 days is accurate for stable demand but misleading in the run-up to seasonal peaks. Q4 — October through December — typically sees 2 to 3 times normal sales volumes for eBay and Shopify sellers in the UK. If you calculate your daily rate in October and use it to plan November stock, you will run out before Black Friday.

The most reliable way to adjust for seasonality is to compare the same month last year. If you sold 200 units in November last year and 70 in October last year, your November multiplier is roughly 2.9×. Apply that to your current October velocity and you have a seasonally adjusted forecast.

If you are in your first year of trading and do not have prior-year data, a practical rule of thumb is to apply a 1.5× safety buffer to your Q4 orders compared to your summer baseline. You will carry some extra stock, but avoiding a stockout during peak season is almost always worth it — the cost of a missed sale in December is far higher than the carrying cost of a few extra units in January.

Reorder point formula

Your reorder point is the stock level at which you should place your next order. The formula is:

Reorder point = (avg daily sales × lead time in days) + safety stock

Safety stock is a buffer to absorb demand spikes or supplier delays. A common approach is to set safety stock equal to your average daily sales multiplied by half your lead time — essentially assuming your supplier could take 50% longer than usual, or that demand runs 50% higher than average, without causing a stockout.

A worked example: if you sell 3 units per day on average, your supplier delivers in 10 days, and your safety stock is 15 units (3 × 5 days), your reorder point is (3 × 10) + 15 = 45 units. When your stock level hits 45 it is time to order.

This number needs revisiting whenever your sales velocity changes significantly — a new product going viral, a price change, or a competitor going out of stock can all shift your average daily sales dramatically.

How Salync helps with forecasting

Salync calculates sales velocity automatically for every product in your catalogue. The stock page shows a days remaining column alongside current stock level — so instead of running calculations manually, you can see at a glance which products are running low.

You can set a reorder threshold per SKU. When stock drops to that level, Salync sends you a low-stock alert — by email or in-app — giving you time to place an order before you hit zero. Because Salync tracks stock across all your connected channels in real time (eBay, Shopify, and others), the days-remaining figure reflects actual available stock rather than channel-by-channel figures that may not account for sales happening elsewhere.

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Frequently asked questions

What is inventory forecasting?

Inventory forecasting is the process of predicting how much stock you will need over a future period based on historical sales, seasonality, and lead times. It helps you avoid stockouts and reduce the cost of holding excess inventory.

How do I forecast inventory for a small online business?

Start with sales velocity: divide your last 30 days of sales by 30 to get your average daily sales rate. Divide your current stock level by that rate to find how many days of stock you have remaining. Compare that to your supplier lead time to know when to reorder.

Does Salync do inventory forecasting?

Yes. Salync shows days of stock remaining on every product based on your last 30 days of sales velocity, and sends reorder alerts when stock hits your defined threshold. Stock levels are updated in real time across all connected channels.