Most reorder mistakes start with a lead time that was guessed, or copied from a supplier's quote and never checked. This guide shows how to measure your real lead time from your own orders, and how to use the result.
What lead time includes
Lead time is the number of days from the moment you decide to order until the stock is on your shelf and ready to sell. The supplier's quoted production time is one part of it. The parts that get left out are usually the ones on your side of the dock.
- Order processing: Days until the supplier confirms your order
- Production: Days to make or pick the goods
- Transit: Days in shipping, including customs if you import
- Receiving: Days to count, check and shelve the delivery
Record two dates per order
You need two dates for every purchase order: the day you sent it and the day the stock was available to sell. Keep them in whatever you already use, as long as every order gets both. In Shopify, a purchase order lists the products, quantities, costs, payment terms and supplier details, and you can save it as a draft or mark it as Ordered once the supplier confirms. Note your two dates alongside it.
- ✅ Date the order was sent, not the date you started drafting it
- ✅ Date the stock was counted and available, not the date the truck arrived
- ✅ One record per supplier, since each has its own pattern
- ❌ The supplier's quoted lead time used as if it were measured
Work out the average and the spread
For each supplier, subtract the order date from the available date to get the lead time in days. Then look at two numbers: the average, and the longest. The gap between them tells you how reliable the supplier is.
| Order | Lead time |
|---|---|
| Order 1 | 12 days |
| Order 2 | 14 days |
| Order 3 | 13 days |
| Order 4 | 21 days |
| Order 5 | 15 days |
The total is 75 days across five orders, so the average is 15 days. The longest was 21 days, six days over the average. A handful of orders is a small sample, so keep adding to the record and recalculate as new deliveries arrive.
Use both numbers
The average and the longest lead time do different jobs. Use the average to work out how much you sell while you wait, and use the gap to size your buffer.
- Average lead time: Goes into your reorder point
- Gap to the longest: Feeds your safety stock, since it shows how late a delivery can run
- A wide spread: A reason to order earlier or hold more buffer for that supplier
When to measure again
Lead times drift. Recheck them whenever something in the chain changes, and at least before your busiest season.
- New supplier or factory: Start a fresh record
- New shipping method: Sea and air freight give very different transit times
- Peak season: Suppliers and carriers are often slower when everyone orders at once
Doing this at scale
StockSense forecasts per-SKU demand from your order history using velocity, trend, seasonality and lead-time-aware safety stock. It recommends a reorder point, quantity and date for each SKU, and drafts purchase orders grouped by supplier. See StockSense.