The plain-language guide
How long does your supplier really take to deliver?
Ask a supplier for their lead time and you get one number. Write down your last five deliveries and you get a different one, and that second number is the one your order deadline should come from.
The short answer
Measure lead time from the moment your order leaves you to the moment the stock is counted and ready to sell. Take your last five deliveries. Plan the order deadline on the typical delivery, and hold safety stock for the gap between the typical one and the slowest. In the worked example below, a delivery that wobbles by a day or two needs more buffer than the day-to-day swing in sales does.
Where the clock starts and stops
A meat supplier quotes 2 days. Their clock starts when the order is keyed into their system. It stops when the truck is loaded. Your clock starts earlier and stops later. It starts when you send the order. It stops when the patties are checked and in your chiller.
The parts nobody quotes sit between the two clocks. An order sent at 4:10 pm against a 4:00 pm cutoff waits a full day before anyone reads it. A delivery that lands at 1 pm, in the middle of the lunch rush, may not be put away until 3 pm. Neither shows up in the supplier's 2 days. Both show up on your shelf, as a gap of 1 day or a few hours between what you planned and what you have.
Write down the last five deliveries
Count in whole days, from the day the order went out to the first day the stock could be sold. Five rows is enough to see a pattern. It is also recent enough to describe the supplier you have now. A supplier who changed drivers in March is a different supplier by June. The table below is an example: a burger stall and its patty supplier, 5 orders over 3 weeks.
| Order | Sent | Ready to sell | Lead time | What happened |
|---|---|---|---|---|
| 1 | Mon 3:00 pm | Wed | 2 days | On time. |
| 2 | Thu 2:30 pm | Sat | 2 days | On time. |
| 3 | Mon 4:10 pm | Thu | 3 days | Sent after the 4:00 pm cutoff. |
| 4 | Thu 1:45 pm | Sat | 2 days | On time. |
| 5 | Mon 3:20 pm | Fri | 4 days | Truck broke down. Arrived Thu night, shelved Fri. |
The quote said 2 days. The typical delivery, the middle row once the five are sorted, is 2 days. The average is 2.6 days. The slowest is 4 days. Each of those 4 numbers answers a different question. Mixing them up is how an order deadline ends up a day early on most weeks and a day late on the week that matters.
The late-day rule
Two numbers do the work. Set the order deadline from the typical lead time. Size safety stock in days to cover the gap between the typical lead time and the slowest. In the table that gap is 4 minus 2, or 2 days. At 9 patties a day, 2 days of cover is 18 patties.
Planning on the slowest delivery instead looks safer and costs more. Every order goes in 2 days early, so every week the stall carries 18 more patties than it needs for 4 days out of 5. In this example, chilled patties keep 4 days. Two days of extra stock on a 4-day item is half its shelf life spent waiting. Planning on the average fails the other way. A 2.6-day deadline is one nobody can use, and rounding it to 3 days still leaves the 4-day week uncovered.
One correction comes before any of this. Row 3 was late because the order went out 10 minutes after the cutoff. That day belongs to the stall, and it comes out of the supplier's record. Fix the habit and the slowest row is the breakdown, which is the risk safety stock is for.
Why a late truck needs more buffer than a busy day
The textbook safety stock formula puts the two kinds of surprise side by side. Safety stock is a service factor times the square root of two terms added together. The first term is the average lead time times the variance of daily sales. The second is daily sales squared times the variance of lead time. Most order sheets only carry the first term. Here are the patties again, with sales averaging 9 a day and swinging by about 3 either way.
- Sales term: 2.6 days times 3 squared is 23.4.
- Delivery term: the five lead times have a variance of 0.8 days squared. 9 squared times 0.8 is 64.8.
- Together: the square root of 88.2 is 9.4. At a 95% service level the factor is 1.65, so safety stock is 15.5 patties.
- With a supplier who is never late: only the sales term is left. The square root of 23.4 is 4.8, times 1.65 is 8 patties.
The delivery wobble takes the buffer from 8 patties to 15.5. In this example 73% of the variance comes from the truck and 27% from the customers. The late-day rule gave 18 patties, within 3 of the formula. That is close enough to skip the formula on most items and keep it for the 1 or 2 that carry the menu.
Try it with your numbers
2 days
typical lead time, the one to plan the deadline on
Whose day is it?
Most extra days have a cause you can name. Some are the supplier's and some are yours, and only the second kind can be fixed from your side of the counter.
| Cause | Adds | Whose fix |
|---|---|---|
| Order sent after the cutoff | 1 day | Yours. Send it before the cutoff, or set a reminder 30 minutes ahead. |
| No delivery on Sunday | 1 day on orders sent Friday or Saturday | Nobody's. Give each delivery day its own notice. |
| Stock lands mid-rush, shelved the next morning | Up to 1 day | Yours. Agree a delivery window outside the rush. |
| Frozen stock that needs a night to thaw | 1 day | Nobody's. Count the thaw as part of the lead time. |
| Truck breakdown or short delivery | 1 to 2 days | The supplier's. This is what safety stock covers. |
Run the stall's table through it. Row 3 is the stall's own late order, so it comes out. If the next delivery takes 2 days, the five rows read 2, 2, 2, 4 and 2. The typical stays at 2 days and the gap stays at 2 days, or 18 patties. If the breakdown does not repeat over the next 5 orders, it drops out too. The gap falls to 0 and the buffer only has to cover sales, about 8 patties.
Where ForeFlux picks up
In ForeFlux each supplier has delivery days, and each delivery day has its own days of notice and a daily cutoff. Supply turns those into a deadline per delivery, such as "Order by Tue 5PM", and shows "Cutoff passed" once it has gone. Safety stock is set in days: a default on Profile, and a per-item figure in the Safety stock (days) field on each stock item's page. Enter the typical lead time as the days of notice and the late-day gap as the buffer. ForeFlux does not measure lead time from your deliveries, so the five-row table stays yours to keep. The rule holds: we suggest, you decide.
Sources
- Wikipedia, Safety stock. The formula with both demand and lead-time variability, and the service factor of 1.65 for a 95% service level.
Questions owners ask
Past the basics.
What is the difference between lead time and delivery days?
Delivery days are the days the truck comes. Lead time is how far ahead you have to order to be on that truck, counted to the moment the stock can be sold. A supplier who delivers on Wednesday and needs the order by Monday has a lead time of 2 days for Wednesday. The same supplier may deliver on Saturday with only 1 day of notice. So one supplier can have a different lead time for each delivery day, and each one deserves its own row in your notes.
My supplier is late a lot. Should I switch?
Measure first, then price it. Five deliveries tell you the gap between a typical delivery and a slow one. Multiply the gap in days by daily sales and by the unit cost, and you have the stock you hold only because of that supplier. A 2-day gap on 9 patties a day at 40 pesos each is 720 pesos of stock sitting in the chiller on every order. If another supplier delivers on time and their price difference costs you less than that, the switch pays. If not, the buffer is the cheaper fix.
How often should I measure lead time again?
Keep the table rolling. Each new delivery goes in at the bottom and the oldest comes out, so the five rows always describe the last few weeks. Review it whenever something changes on the supplier side, such as a new driver or a new cutoff time.
Should prep time count as part of lead time?
Yes, when the stock cannot be sold the day it arrives. Frozen patties that land on Wednesday and need a night in the chiller to thaw are ready on Thursday, so the lead time for that order runs to Thursday. The same goes for anything that has to be portioned or marinated before service. The test is simple. Lead time ends on the first day a customer could buy the item.
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