Reorder point: the formula, a calculator, and how much safety stock a supplement brand needs

The reorder point is the stock level at which you place your next order: average daily sales × lead time in days + safety stock. A brand selling 150 bottles a day, with a 45-day lead time and 15 days of safety stock, reorders at 150 × 45 + 2,250 = 9,000 bottles. Lead time means the whole wait, from placing the order to stock you can ship.

The method fits in three lines, and the rest of this tutorial is about getting each input right:

The reorder point in three lines

Reorder point=average daily sales × lead time in days + safety stock

Safety stock (days of cover)=average daily sales × days of buffer

Order quantity=average daily sales × days of sales to order

Illustrative: 150 × 45 + (150 × 15) = 6,750 + 2,250 = 9,000 bottles. Order 150 × 90 = 13,500 bottles.

The base of the formula is simple: the stock you'll sell while you wait for the next batch, plus a buffer. A Virginia Cooperative Extension publication writes it as RP = (D × S) + SS, where D is demand, S is the supplier lead time in days and SS is safety stock (Virginia Tech). Because S is in days, D is daily sales. Get any of the three inputs wrong and the reorder point is wrong with it: a lead time that's too short, a daily sales figure from the wrong month, a buffer chosen by habit.

Step 1: Measure your real lead time

Lead time is the gap between the day you place an order and the day the bottles are sellable in your warehouse. For a supplement brand that's several waits in a row, and a manufacturer's quote may cover only one of them. Here is how 45 days can add up:

Where 45 days of lead time go

Illustrative supplement production order, in days

• Waiting for a production slot14 days31%
• Production and packaging10 days22%
• Testing and release12 days27%
• Freight to your warehouse6 days13%
• Receiving and put-away3 days7%

Measure it from your own history rather than from quotes. Take your last two or three production orders and write down the date you sent each purchase order and the date its stock was available to ship. If they differ, plan on the longer one until you know why.

Two parts of the wait deserve attention. The production slot depends on your manufacturer's schedule, so ask how far ahead they are booked before you ask how long a batch takes; the manufacturer post has the questions to ask. Testing and release is the easiest wait to forget: the lab results and paperwork a lot needs before it can ship.

Packaging has its own clock. Bottles, caps, labels and boxes come with lead times and minimums of their own. If the manufacturer supplies them, ask whether the quoted lead time assumes they're in stock; if you supply them, they sit on your critical path. A label change adds a step: the new artwork has to be approved and printed before the batch can be packed.

Step 2: Work out daily sales from your ad plan

The textbook takes average daily sales from history. A brand that buys its traffic has a better source: the plan. Your orders follow your ad budget, so the sales you'll make during the next lead time are the ones your next budget buys.

The arithmetic: planned ad spend per day ÷ cost per acquisition = new orders per day; multiply by bottles per order; add repeat and subscription bottles.

Say the brand spends $4,500 a day at a $50 CPA. That's 90 new orders a day, and at 1.5 bottles per order, 135 bottles. Subscriptions and reorders add 15 more, so the plan sells 150 bottles a day. Take the CPA from your store's own new-customer orders rather than the ad platform's report; the target CPA post explains why the two differ.

Now the brand decides to scale to $7,500 a day. If CPA holds at $50, that's 150 orders, 225 bottles, plus the 15: 240 bottles a day. The reorder point at the new rate is 240 × 60 days of lead time and safety stock = 14,400 bottles. The brand holds 12,000 today, so the moment the budget goes up it's already below its reorder point: the order has to go in before the scale-up, not after the first stockout. Use the CPA you expect at the new budget: if it climbs to $60, the same $7,500 buys 125 orders and about 203 bottles a day.

Bottles per order is the input that moves without anyone noticing. When an offer sells one, three and six bottles, a change in the mix changes demand even when orders don't: if a new bundle layout lifts bottles per order from 1.5 to 2, the same 90 orders a day ship 180 bottles instead of 135. Recompute it from the last two weeks of orders whenever the offer changes.

Subscription shipments are the most predictable part of demand. You know next month's renewals today, minus the share that cancels, so count them separately from ad-driven orders. The supplement subscription post shows how to estimate that churn.

Step 3: Choose your safety stock

Safety stock is the buffer for what the plan gets wrong. NC State's Supply Chain Resource Cooperative defines it as the additional inventory beyond what you need to meet average demand during the lead time, held against uncertainty in demand or lead time (NC State). There are two common ways to size it:

Two ways to size safety stock

Illustrative numbers for the 150-bottle example

Days of cover

Safety stock = average daily sales × a number of days you choose.

• Example: 150 bottles × 15 days = 2,250
• Simple to explain and to check
• Grows automatically with sales
• Works with no sales history

The catch: it ignores how much your sales actually swing, so a steady product and a volatile one get the same buffer.

Statistical

Safety stock = Z × the standard deviation of demand during the lead time, where Z is the number of standard deviations you hold above average demand.

• Example: 1.65 × a standard deviation of 900 bottles = 1,485, about 10 days of sales
• NC State’s tutorial pairs Z = 1.65 with a 95% cycle service level
• Sized to the swings you have measured
• The tutorial also covers lead times that vary

The catch: it needs months of steady history, and a planned change in ad spend isn’t random noise.

The statistical method, in plain words: look at how many bottles you sold in each 45-day stretch over past months. If those totals barely move, a small buffer covers the swings. If they jump around, you need more. The standard deviation measures that spread, and Z decides how many spreads to hold above the average.

NC State's tutorial pairs Z = 1.29 with a 90% cycle service level, 1.65 with 95% and 2.33 with 99%: the more certain you want to be of not running out, the more stock you carry. In the illustrative example, a standard deviation of 900 bottles at Z = 1.65 gives 1,485 bottles, about 10 days of sales: the 15 days the brand picked holds about 50% more than its measured swings call for.

For an ad-driven brand, the statistical method has a blind spot. It assumes tomorrow swings the way yesterday did, but your biggest swings aren't random: they're budget decisions, and you know about them in advance. Put planned changes into daily sales (Step 2) and use safety stock for what you can't plan: a creative that takes off, a lot that fails testing, a carrier backlog.

Lead time eats safety stock too. A batch that arrives 10 days late at 150 bottles a day uses 1,500 of your 2,250 bottles of buffer before the new stock lands.

What running out costs when you buy your traffic

For most stores, a stockout means lost sales. For a brand that buys every visitor, it means a choice between two bad options: pause the ads and lose the customers they would have brought, or keep them running and take orders you can't ship on time.

Put rough numbers on the first option. The example brand gets 90 new orders a day from ads; say each one leaves $15 of profit after its $50 ad cost. Ten days out of stock is 900 new customers who never arrive, $13,500 of first-order profit, before counting the reorders they would have placed. Five more days of safety stock, by contrast, is 750 bottles: at an illustrative $4.20 a bottle, $3,150 of cash that comes back when the bottles sell. Extra buffer costs cash you get back. A stockout costs customers you don't.

The second option has rules. The FTC's Mail Order Rule requires a reasonable basis to ship within the time you state, or within 30 days if you state none, and a delay notice that offers the right to cancel when you can't (FTC). Keep selling stock you don't have under a ship time you can't meet, and that reasonable basis is gone; the Mail Order Rule post covers delays and notices. This is a summary, not legal advice.

Step 4: Calculate your reorder point and watch the cycle

Put the numbers together and the cycle looks like this. The brand has 12,000 bottles today, so at 150 a day it reaches the 9,000-bottle reorder point in 20 days. It orders 90 days of sales, and 45 days later the delivery lands:

Stock on hand for one bottle SKU

Illustrative: 150 bottles a day, 45-day lead time, 15 days of safety stock, orders of 13,500

• Sales as planned (150 a day)
• Sales 20% faster after the order (180 a day)
• Reorder point

04,0008,00012,00016,000020406080100120Order placedStock arrives2,250900

Days from today

Show the numbers

Days from today	Sales as planned (150 a day)	Sales 20% faster after the order (180 a day)	Reorder point
0	12,000	12,000	9,000
5	11,250	11,250	9,000
10	10,500	10,500	9,000
15	9,750	9,750	9,000
20	9,000	9,000	9,000
25	8,250	8,100	9,000
30	7,500	7,200	9,000
35	6,750	6,300	9,000
40	6,000	5,400	9,000
45	5,250	4,500	9,000
50	4,500	3,600	9,000
55	3,750	2,700	9,000
60	3,000	1,800	9,000
65	2,250	900	9,000
70	15,000	13,500	9,000
75	14,250	12,600	9,000
80	13,500	11,700	9,000
85	12,750	10,800	9,000
90	12,000	9,900	9,000
95	11,250	9,000	9,000
100	10,500	8,100	9,000
105	9,750	7,200	9,000
110	9,000	6,300	9,000
115	8,250	5,400	9,000
120	7,500	4,500	9,000

The second line shows what the buffer is for. When sales run faster than planned after you order, the safety stock absorbs it, and the delivery still lands before the shelf is empty. It also shows how fast a reorder point goes stale: at 180 bottles a day, it should rise to 180 × 60 = 10,800. A reorder point is a number for today's plan, not a setting you choose once.

Here is the calculator, set to the same example. Change daily sales, lead time or safety days to see the reorder point move:

Calculator

Reorder point calculator

When to place the next production order so the stock arrives before you run out.

Units sold per dayLead time (production, testing, freight, receiving)daysSafety stockdaysDays of sales to orderdaysUnits on hand now

Reorder point

9,000

units on hand when you place the order

Safety stock

2,250

Order quantity

13,500

90 days of sales

Days until the reorder point

20

Lead time is the whole wait: the manufacturer’s production slot, testing and release, freight and receiving. Raise the safety stock before you scale ad spend, because demand follows the budget faster than production can.

Reset to the example

The four results are a plan: when to order (20 days from now), how much (13,500 bottles) and what should be left when the delivery lands if sales go to plan (the 2,250-bottle buffer). Now try the scale-up from Step 2: set daily sales to 240 and the calculator says to order now, because the 12,000 bottles on hand are already below the new reorder point of 14,400. Put the order date in your calendar and move it whenever an input moves.

Step 5: Decide how much to order: MOQs, cash and dates

When to order and how much to order are separate decisions. The calculator sizes the order in days of sales: 90 days at 150 a day is 13,500 bottles. Three limits push that number around.

The manufacturer's minimum order quantity. If the minimum is 20,000 bottles, you buy 133 days of sales at 150 a day, whether you want to or not.

Cash. At an illustrative $4.20 per bottle, 13,500 bottles tie up $56,700 until they sell, and 20,000 bottles tie up $84,000. Use your landed cost per unit, which includes freight and duties, not the supplier's price; the landed cost post shows how to work it out.

Dates. FDA's CGMP rule for supplements doesn't require an expiration or "best if used by" date, but FDA says that if you put one on the label, you should have data to support it (FDA). Whatever date you print, the order has to sell through well before it. The MOQ is where that bites: a slow SKU selling 20 bottles a day needs 1,000 days, nearly three years, to sell a 20,000-bottle minimum, which may be longer than your date allows. The supplement expiration date post covers how a date is set and backed up.

For a new product, those limits argue for a small first batch: launch small, reorder on data, and let the reorder point take over once real orders replace the plan.

Step 6: Ship the oldest date first, and recheck every week

Every delivery puts a new lot next to what's left of the old one: in the example, 13,500 fresh bottles beside the 2,250 bottles of safety stock. Ship by FEFO, first expired, first out: the warehouse picks the lot with the earliest date before the newer one. Without it, the buffer you paid to hold is the stock that ages on the shelf.

Then make the reorder point a weekly number. Recalculate it whenever ad spend, CPA, lead time or bottles per order change, and before any planned scale-up. It takes five minutes, and the alternative is buying traffic to an offer you can't ship.

Before you place the next production order

Run through this list with your ad plan and your last delivery dates open:

Before the next production order

• Lead time measured from your last orders: purchase order sent to stock ready to ship
• Daily sales taken from next month’s ad plan, not last month’s sales
• CPA in the plan taken from your store’s new-customer orders
• Subscription and repeat shipments counted separately
• Safety stock set, and raised before any planned scale-up
• Reorder point recalculated with today’s numbers
• Order quantity checked against the MOQ, your cash and the product’s date
• Warehouse set to ship the earliest-dated lot first (FEFO)
• A date in the calendar to recheck the reorder point

0 of 9 done