My supplier quotes 5 working days but my shop sells every day: do I enter lead time in working days or calendar days?
Enter calendar days. The calculator multiplies your average daily sales by the number of lead-time days, so both have to be the same kind of day. Your online shop keeps selling over the weekend while the supplier is closed, and five working days usually stretch across seven calendar days. Those two extra days carry real sales that belong in the reorder point.
Where the mistake hides
When a supplier's quote says 'delivery within 5 working days', it counts the days it works. Your average daily sales, though, are usually worked out over every day of the week, because an online shop never closes, and the weekend may even be your busiest time. Enter 5 with daily sales based on 7-day weeks and the calculator assumes you will sell for only five days while you wait, when in fact you will sell for seven.
An example in numbers
You sell 40 units a day on average across the whole week, keep 80 units of safety stock, and the supplier delivers in 5 working days, which is 7 calendar days when a weekend falls in between.
Working days by mistake, 5 days: demand during lead time 40 × 5 = 200, reorder point 200 + 80 = 280 units.
Calendar days, 7 days: demand during lead time 40 × 7 = 280, reorder point 280 + 80 = 360 units.
The gap is 80 units, the entire safety stock in this example.
With the first figure you order at 280 units and sell 280 before the goods arrive. The safety stock is gone on an ordinary wait, and the order lands with you at zero, without the supplier being a single day late.
Converting working days to calendar days
- Ask the supplier: does the count start on the order day or the day after, and does it include the delivery day?
- Put it on a calendar: from the day you send the order, count working days to delivery, then see how many calendar days actually passed.
- Add what comes after delivery: receiving, checking and entering the stock until it is on sale.
- If the day you usually order varies, take the longest case, or take the usual one and use the maximums option for the longest.
- Enter the calendar days in the Inventory reorder-point calculator, with daily sales calculated over every day of the week.
The reverse holds too
If your own shop closes two days a week and sells nothing on them, and your daily sales figure is based on your trading days only, then enter lead time in your trading days, not the supplier's working days. The rule is the same either way: the kind of day in the daily sales field is the kind of day in the lead time field. The calculator cannot tell which you mean; it multiplies the two numbers as they are.
Around long holidays such as the Eid break, five working days can stretch over two weeks. Recalculate for that period with the actual calendar days, and send the order early with the Purchase-order generator before the supplier closes.
Common questions
The supplier does not say working or calendar days. How do I know?
Ask directly, or work it out from past shipments: the date you sent the order and the date the goods arrived. The gap in calendar days is what you need.
Does the calculator accept a lead time such as 6.5 days?
Yes, it accepts decimals. But with calendar days, rounding up to a whole day is the safer choice if you are unsure.