Lead times — why the MRP tells you to order today
Where lead time is stored, how the order-by date is worked out, and the two settings that quietly wreck your dates.
If the MRP keeps telling you to order things today, or hands you dates that are already past, it is almost always one missing number. This explains exactly how the order-by date is worked out, so you can see which one.
Part 01
A worked example
2 minutes
Acme orders 40 mounting brackets for 17 March. Each bracket needs one steel blank. You have 12 blanks free and none on order. Your supplier link for that blank says 14 days lead time and a minimum order of 50.
| The run works out | Result | How |
|---|---|---|
| What is needed | 40 | One blank per bracket, from the bill of materials. |
| What is short | 28 | 40 needed, 12 free, nothing on order. |
| What to order | 50 | The shortfall is 28, but the supplier will not sell fewer than 50, so it is rounded up. |
| When to order by | 3 March | 17 March minus 14 days lead time. |
So the run recommends buying 50 blanks, ordered by 3 March. Nothing is ordered until someone converts that recommendation into a purchase order — the date is telling you when the decision has to be made, not making it for you.
Part 02
Where lead time actually lives
1 minute
Lead time is not a property of the part. It sits on the link between a product and a supplier, alongside their minimum order quantity and price — because the same blank from a different supplier is a different wait.
A product with no supplier linked has no lead time, no minimum order and no price for the run to use. Linking the supplier is what turns a shortage list into a purchasing plan with dates on it.
Part 03
Three things that quietly wreck your dates
2 minutes
| What you see | What is actually wrong |
|---|---|
| It says order by the date I need it | No lead time on the supplier link, so it is treated as zero days. The maths still runs — it just assumes delivery is instant, so you will always be late. |
| It wants more than I need | The supplier's minimum order quantity is higher than the shortfall, so the quantity is rounded up to it. Correct, but worth knowing before you question the number. |
| It says order within a week, with no reason | Nothing gave that shortfall a deadline — no sales order date driving it — so a default of a week from today is used. That is a placeholder, not a calculation. |
Zero is a perfectly valid number to subtract, so the run completes and looks normal. The only symptom is order-by dates that match the needed date exactly. If you see that across several parts, the supplier links are the place to look.
Part 04
Setting numbers that hold up
2 minutes
- Use the wait you actually get, not the one you were quoted. If a supplier promises 10 days and delivers in 15, the honest number is 15. The MRP is not negotiating with them.
- Include your own handling. If goods sit two days before anyone books them in, that is part of the wait as far as the shop floor is concerned.
- Do not pad every part "to be safe". Padding buys stock you did not need and hides the suppliers that are genuinely slow. Fix the number on the part that was late instead.
- Revisit after anything unusual. Lead times move. A number set once at rollout and never touched is the most common reason a run stops being trusted.
You do not need every supplier link perfect. Set real lead times on the ten parts that stop a job when they are missing, and the run becomes useful immediately. The long tail can wait.