Inventory how-to

How to forecast demand for accessories that sell with a main product

Treat the accessory as its own SKU with its own sales history, but check its sales ratio against the main product regularly.

Short answer: Treat the accessory as its own SKU with its own sales history, but check its sales ratio against the main product regularly. Most of the time the accessory's own recent sales window is the best forecast input, adjusted when you know the main product's demand is about to change.

Why you cannot just copy the main product's forecast

Accessories rarely attach to every sale of the main product. A phone case might sell with 1 in 3 phones, and that ratio can shift with price changes, bundling promotions, or customers already owning one. If you forecast the accessory purely as "a fraction of the main product," you bake in whatever the historical attach rate was, even if it is drifting.

The safer default is to forecast the accessory from its own recent sales, the same way you would any other SKU. This captures the actual attach rate as it exists now, not a guess.

When to adjust manually

If you know something is about to change demand for the main product, like a launch, a discontinued model, or a seasonal push, the accessory's own sales history will not reflect that yet. In these cases, estimate the main product's expected unit sales for the period, apply your best estimate of the attach rate, and compare that to what a pure sales based forecast would suggest. Use the higher of the two if you want to avoid stockouts, or the average if you want to avoid overstock.

Also recheck the attach rate every few months. Pull total accessory units sold divided by total main product units sold over the last 60 to 90 days. If it has moved meaningfully from what you assumed, adjust your safety days or reorder quantities for the accessory rather than leaving it on autopilot.

Handle slow movers differently

Some accessories sell in low volumes even when the main product sells well. For these, a short sales window can be noisy, a single multi-unit order can make the trend look stronger than it is. Widen the window you use to judge demand, or lean more on safety stock than on a tight reorder point, since the cost of carrying a few extra cheap accessories is usually low.

Step by step

  1. List the accessory as its own SKU and track its sales independently from the main product.
  2. Calculate its attach rate: accessory units sold divided by main product units sold, over a recent period like 60 or 90 days.
  3. Use the accessory's own recent sales as the default forecast input, not a fixed percentage of the main product.
  4. Before a known demand shift in the main product, estimate the new main product volume, apply the attach rate, and compare to the sales based forecast.
  5. Recalculate the attach rate every few months and adjust safety days if it has moved.
  6. For low volume accessories, widen the sales window you judge demand from to avoid overreacting to one large order.
  7. Set slightly higher safety days on accessories if being out of stock on them causes lost sales of the main product, like a required cable or adapter.

Worked example

Say you sell a camera and a camera strap. Over the last 90 days you sold 180 cameras and 54 straps, an attach rate of 30 percent. Daily camera sales run about 2 units, so daily strap sales run about 0.6 units. The strap supplier has a 14 day lead time and you keep 5 days of safety stock, so the reorder point is 0.6 x (14 + 5) = 11.4, round up to 12 straps. If you know a new camera model is launching next month and expect camera sales to double to 4 a day, you would plan the strap reorder point at 1.2 x 19 = 22.8, round up to 23 straps, ahead of the sales data showing it.

How SteadyShelf helps

SteadyShelf forecasts each SKU, including accessories, from its own 28-day sales window and will build seasonality in once it has about 13 months of history, so it handles the steady-state attach rate automatically through normal sales data. It does not model bundles or link an accessory's forecast to a main product's expected future sales, so for known upcoming demand shifts like a launch, you need to manually raise the reorder quantity or safety days on the accessory SKU ahead of time.

$29/month flat, unlimited products and suppliers, 30-day free trial. Run the numbers now with the free reorder point calculator.

SteadyShelf is launching on the Shopify App Store. Join early access to get the install link first.

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FAQ

Should I bundle the accessory and main product as one SKU to simplify forecasting?

No, keep them separate. Bundling hides the real attach rate and makes it harder to reorder each item at the right quantity, since they likely have different suppliers, lead times, and case pack sizes.

What if the accessory also sells on its own, not just with the main product?

Then its own sales history is even more reliable as a forecast input, since it is not purely dependent on the main product. Keep tracking the attach rate as a sanity check, but trust the SKU's direct sales data as the primary signal.

Let SteadyShelf do this math for every SKU

Order-by dates per supplier, seasonal forecasts and draft POs for your Shopify store. $29/month after a 30-day free trial.

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