Inventory how-to

How to plan inventory for complementary products

Plan complementary products by looking at how their sales move together, then set reorder points that keep them in balance.

Short answer: Plan complementary products by looking at how their sales move together, then set reorder points that keep them in balance. If you sell grills and grill covers, or phone cases and screen protectors, you want enough of each so a customer never finds one in stock and the other out.

Why complementary items need extra attention

A standalone SKU just needs its own reorder point based on its own sales. A complementary SKU is different because part of its demand depends on another product being in stock and selling. If the main product runs out, the companion item's sales will slow down too, and a simple forecast based only on past sales can lag behind that change.

Match ratios between paired items

Start by checking the natural attach rate between the two products. If you sell roughly one grill cover for every two grills, your cover stock and reorder quantities should track that ratio. When you reorder the grill, check the cover's days of stock at the same time, even if the cover's own forecast has not yet triggered a reorder.

Watch for lead time mismatches

Complementary products often come from different suppliers with different lead times. If the grill has a 10-day lead time and the cover has a 30-day lead time, you need to order the cover earlier to have it ready when the grill arrives. Build your safety days around the slower supplier so the fast-moving item does not sit waiting on the slow one.

Review the pairing periodically

Attach rates drift. A cover might get discontinued by its maker, or a promotion might sell grills without covers. Check the ratio every few months using actual sales data, not assumptions, and adjust reorder quantities if the pattern has shifted.

Step by step

  1. List your main complementary pairs or sets manually, since this grouping has to be done by you.
  2. Calculate the attach rate: companion units sold divided by main product units sold, over a recent period.
  3. When you place an order for the main product, check the companion's days of stock at the same time.
  4. Set the companion's safety days to cover the longer of the two suppliers' lead times if they ship separately.
  5. Order the companion early enough that it arrives at or before the main product, not after.
  6. Recheck the attach rate every few months and adjust order quantities if it has changed.
  7. Watch for stockouts on the main product, since they will quietly depress the companion's sales too.

Worked example

A store sells camping stoves and fuel canisters. Over the last 28 days it sold 40 stoves and 65 canisters, an attach rate of about 1.6 canisters per stove. The stove supplier has a 7-day lead time, the canister supplier has a 20-day lead time, so safety days are set around the canister's longer lead time. Daily stove sales are 40/28 = 1.43, and canister daily sales are 65/28 = 2.32. Reorder point for canisters = 2.32 x (20 + 5 safety days) = about 58 units, so the store reorders canisters well before the stoves run low.

How SteadyShelf helps

SteadyShelf forecasts and reorders each variant on its own, it does not link SKUs as complementary pairs or apply attach rates automatically. You will need to track the pairing and ratio yourself, then use SteadyShelf's per-SKU order-by dates and suggested quantities as the base numbers you adjust against each other.

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

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FAQ

Can SteadyShelf bundle a main product and its companion into one purchase order automatically?

No. SteadyShelf drafts purchase orders per supplier based on each SKU's own forecast. If the main product and its companion share a supplier, their suggested orders can land on the same PO, but the quantities are not linked or balanced against each other automatically.

What should I do if the companion product has much lower sales volume and its forecast looks unreliable?

Low-volume SKUs naturally have noisier daily sales averages. Use the main product's sales trend as a sanity check on the companion's forecast, and lean on a higher safety day buffer for the companion rather than trusting a thin sales history alone.

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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