What’s the formula for calculating supplier lead time?

What's the formula for calculating supplier lead time?

Supplier lead time is the number of days between placing a purchase order and the stock landing on your shelf, averaged across your last several orders per vendor and item. Multiply that lead time by your average daily usage, add safety stock, and you have a reorder point you can trust.

Here is the scene that makes this math matter: a planner opens the purchasing screen on a Tuesday and sees 40 units of a gasket on hand. The last time they ordered that part, the supplier delivered in 9 days. This time it takes 16. The press line runs dry on day 12, and the only thing standing between "we caught it" and "we're apologizing to a customer" is whether anyone built that slower lead time into the reorder point in the first place.

How do you calculate supplier lead time in days?

Lead time is a subtraction problem before it is anything else: the date you received the stock minus the date you placed the order. The trouble is doing that subtraction once and treating it as gospel for the next two years.

1. Pull your last 10 to 20 completed purchase orders for one vendor and one item, not the vendor's whole catalog.
2. For each order, subtract the order date from the receipt date to get lead time in days.
3. Average those numbers for your typical lead time, and flag the longest one as your worst-case lead time.
4. Repeat per vendor and per item. A vendor who ships fasteners in 5 days might ship a custom-coated part from the same plant in 30.
5. Re-run the calculation every quarter, or every month if the vendor is on an unstable freight lane or a known capacity crunch.

Skip the per-item step and you end up with one lead time number standing in for a vendor's entire catalog, which is how a planner gets blindsided by the one item that actually takes three times as long to arrive.

How do you turn lead time into a reorder point?

The reorder point formula is: Reorder Point = (Average Daily Usage x Lead Time in Days) + Safety Stock. Each piece has its own small calculation underneath it.

Average daily usage is total units sold or consumed over a period, divided by the number of days in that period. Pull at least 90 days so a slow week or a promotional spike does not skew the number.

Safety stock is the cushion for the gap between what usually happens and what sometimes happens: Safety Stock = (Maximum Daily Usage x Maximum Lead Time) – (Average Daily Usage x Average Lead Time). It is built from your worst realistic case, not your best one.

A worked example

Say a part sells at an average of 40 units a day, with a worst day of 55 units. The vendor's average lead time is 10 days, with a worst-case lead time of 16 days.

1. Safety stock: (55 x 16) – (40 x 10) = 880 – 400 = 480 units.
2. Reorder point: (40 x 10) + 480 = 400 + 480 = 880 units.
3. Compare that to a reorder point built on the quoted lead time of 10 days with no safety stock: just 400 units, a number that runs out the moment the vendor has a bad week.

That 480-unit gap is not padding. It is the difference between a stockout that shows up as a missed ship date and an operation that absorbs a slow vendor without anyone noticing.

Where the math breaks down on a spreadsheet

A spreadsheet can hold this formula fine. What it cannot do is keep the lead time column current without someone remembering to update it after every delivery, for every vendor, for every item. In many shops that falls on whoever is least busy that week, which means it falls on no one.

1. Order dates and receipt dates live in different systems, so someone re-keys them to calculate lead time by hand.
2. A vendor's lead time creeps from 9 days to 14 days over six months, and the reorder point never moves with it.
3. Safety stock gets set once at product launch and never revisited, even after a stockout proves it was too low.
4. Reorder points live on one person's laptop, so the formula dies the week that person takes vacation.

Fishbowl is the platform. Fishbowl Inventory, Fishbowl Advanced Manufacturing, and Fishbowl Advanced Warehouse are purpose-built parts of Fishbowl, matched to how the business runs, and all three keep order dates and receipt dates in the same system that tracks on-hand quantity. That means the lead time behind a reorder point comes from your own purchase history, not a guess typed in once and forgotten.

In Fishbowl Inventory, the AI Operations Specialist is currently in beta and uses live Fishbowl data to prepare purchase orders, transfers, receipts, and adjustments for your approval. Nothing is saved or changed without the operator's approval. It works from the same lead time and usage history described above, not a separate guess. Fishbowl runs the purchasing math; QuickBooks or Xero still keeps the books.

Brian Garrett at Telluric Companies saves at least 45 minutes per purchase order. See more examples at fishbowlinventory.com.

Picture two versions of the same Monday morning. In the first, a purchasing clerk at a mid-size parts distributor opens a spreadsheet with 600 rows, scrolls to the gasket line, and tries to remember whether the last late shipment was an outlier or the new normal. She does not have time to check 15 purchase orders for every SKU below reorder point, so she eyeballs it, orders the usual quantity, and moves to the next row. Six weeks later the same part runs out two days before a scheduled production run, and the postmortem meeting spends an hour asking why nobody saw it coming. Nobody saw it coming because nobody had the time to look.

In the second version, the lead time for that gasket is already sitting next to the reorder point because every receipt against every purchase order for that vendor has been logged in the same system since the part was first stocked. The clerk does not need to reconstruct history from memory or a filing cabinet of packing slips. The number on the screen already reflects the vendor's actual performance, worst case included, so the purchase order goes out before the shortage becomes a production problem instead of after.

The difference between those two Mondays is not effort. The clerk in the first scene is not lazy, and the one in the second is not smarter. The difference is whether the lead time math lives in a system that updates itself from real transactions or in a column someone has to remember to touch. That is the whole argument for getting lead time out of a spreadsheet and into the same place purchase orders and receipts already happen.

Four questions to check your reorder point against reality

1. Is the lead time in this formula from your last 10 to 20 orders, or from a number a salesperson quoted once?
2. Does safety stock account for your worst-case lead time, or only the average?
3. Is this reorder point set per item and per vendor, or is one number covering a whole catalog?
4. Has anyone recalculated this in the last 90 days, or is it older than the vendor's current shipping lane?

Run your top 20 items through those four questions before you trust the number on the screen. A reorder point that passes all four is a decision you can defend. One that fails even one of them is a guess wearing a formula's clothes.

See how this runs on your own purchase history: get a demo.

Fishbowl does the work. You make the call.

Frequently asked questions

What is a safe lead time to use if my supplier's delivery times vary?

Use the longest lead time you have actually seen in the last 10 to 20 purchase orders for that vendor and item, not the quoted lead time on the vendor's website. The gap between average lead time and worst-case lead time is exactly what safety stock is built to cover.

How often should I recalculate supplier lead time?

Recalculate every quarter for stable vendors and every month for anyone on a fuel surcharge, overseas freight, or a known capacity problem. A lead time that was accurate in January can be wrong by June if a supplier changed ports, carriers, or production schedules.

Does Fishbowl automate reorder point calculations?

Fishbowl Inventory tracks receiving dates against order dates by vendor and item, so the lead time feeding your reorder point is built from your own purchase history. The AI Operations Specialist is currently in beta and uses live Fishbowl data to prepare purchase orders, transfers, receipts, and adjustments for your approval.

What's the difference between lead time and reorder point?

Lead time is a measurement: the days between placing an order and receiving it. A reorder point is a decision: the stock level at which you place the next order, calculated from lead time, average daily usage, and safety stock. One is an input, the other is the trigger.