Inventory how-to

How to plan inventory when freight times are unpredictable

Build the uncertainty into your lead time and safety days instead of using one fixed number.

Short answer: Build the uncertainty into your lead time and safety days instead of using one fixed number. Use your worst realistic lead time, not the average, and add safety days to cover the gap between best and worst case. This protects you from stockouts without you having to guess at the moment of ordering.

Why average lead time fails you

If freight sometimes takes 20 days and sometimes 45, using a 30-day average means you will stock out roughly half the time. The long delays are the ones that actually hurt you, so your planning number should lean toward the slow end, not the middle.

Track actual lead times per supplier

Write down the real time from order placed to stock received for the last 5 to 10 orders per supplier. Look at the longest ones, not just the average. If a supplier has had delays of 40, 35, 22, and 50 days recently, your planning lead time should be closer to 40-45, not 36.

Use safety days to absorb the rest

Even with a conservative lead time, add safety days on top for the remaining risk, especially for your best sellers. Thin-margin or low-volume SKUs can carry fewer safety days since a stockout there is less costly. Review and adjust these numbers every few months as freight conditions change.

Reorder earlier and in smaller, more frequent batches if possible

If lead times are swinging a lot, it can help to order more often in smaller quantities rather than one big order on a long cycle. This gives you more chances to react if a shipment is delayed, instead of betting everything on one forecast.

Step by step

  1. List your suppliers and pull the last 5-10 lead times for each
  2. Use the longest realistic lead time per supplier as your planning number, not the average
  3. Add extra safety days on top, more for your best-selling SKUs
  4. Recalculate your reorder point: daily sales x (lead time + safety days)
  5. Set order-by dates using this new reorder point so you order before you are tight on stock
  6. Reassess lead times every 2-3 months as freight conditions change
  7. Consider splitting large orders into smaller, more frequent ones if delays are common

Worked example

A store sells a jacket at 4 units a day. Freight from the supplier has ranged from 25 to 45 days over the last few orders, so they set planning lead time at 42 days instead of the 33-day average. They add 7 safety days for extra buffer, giving a reorder point of 4 x (42 + 7) = 196 units. They currently hold 160 units and have none on order, so they are already past the reorder point and need to order now.

How SteadyShelf helps

SteadyShelf lets you set lead time and safety days per supplier and calculates the order-by date and reorder point from those numbers automatically, so you do not have to redo this math by hand each time. It does not track individual shipment delays or predict freight disruptions, so you still need to update the lead time and safety day settings yourself as you learn how a supplier's freight actually behaves.

$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 use the same safety days for every SKU?

No. Use more safety days for high-volume or high-margin SKUs where a stockout costs you the most, and fewer for slow movers or low-margin items where carrying extra stock is not worth the cash tied up.

What if a supplier's lead time is wildly inconsistent, not just slow?

Plan for the worst case you have actually seen, not a guess. If delays are frequent and large, it is often safer to order more often in smaller quantities than to rely on one big order with a long, unpredictable wait.

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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