What a reorder point is, and what it is not
A reorder point is a stock level, not a date. It answers one question: how low can inventory fall before a replenishment order must go out to arrive before the shelf empties? Cross that level and you order; stay above it and you do not. That is the whole of the continuous-review discipline, and it is why the reorder point is the single most useful number in a small product business — it converts an anxious judgement call into an arithmetic check anyone on the team can run.
It is not an order quantity. The reorder point tells you when; how much to buy is a separate calculation that trades ordering cost against carrying cost, and that is the job of the economic order quantity calculator. The two work as a pair: the reorder point fires the order, the order quantity sizes it. Together they are what operations texts call an (s, Q) policy — reorder at level s, order quantity Q.
It is also not a target stock level. A reorder point of 1,600 units does not mean you should hold 1,600 units; it means that when you are down to 1,600 you have exactly enough to sell through the lead time plus your buffer. Healthy inventory oscillates between the reorder point and the reorder point plus one order quantity.
The most consequential detail is what you compare it against. The trigger must be tested against inventory position — units on hand plus units already on open purchase orders — not against on-hand alone. Compare against on-hand and you will re-order every day the stock sits below the trigger while a container is already on the water, and you will end up with three times the inventory you planned.
Why the formula has the shape it has
The reorder point is built from two ideas that answer two different risks.
Lead-time demand, d × L, is the expected quantity you will sell between placing the order and receiving it. If you sell 40 units a day and the supplier takes 30 days, then 1,200 units will leave the building before the replenishment lands. Hold fewer than 1,200 at the moment you order and you will run out, no matter how efficient your supplier is. This term handles average demand, and it is pure bookkeeping: consumption during a known delay.
Safety stock, SS, handles everything the average does not. Demand is not the same every day, and lead times are not the same every shipment. A buffer of zero puts you at roughly a coin flip on every cycle: half the time actual lead-time demand comes in under the average and you are fine, half the time it comes in over and you stock out. Raising the buffer buys down that probability, at the cost of carrying inventory you may not need. Sizing it properly means taking your demand and lead-time variability and a target service level, which the safety stock calculator does explicitly.
The review period is a third term most sources omit, and it matters if you cannot order any day you like. Suppose you place orders every Monday. If your position dips below the trigger on Tuesday, the order does not go out for six more days, so the trigger has to cover the lead time plus that waiting interval: d × (L + R) + SS. Ignoring the review period is the most common reason a mathematically correct reorder point still produces stockouts.
One assumption is worth stating plainly: everything must be in the same time unit. Lead time in calendar days pairs with demand per calendar day. If you compute demand from selling days only — say 250 a year — but your supplier quotes 45 calendar days, your lead-time demand is understated by about 30%. This calculator uses calendar days throughout, and offers weekly and monthly input units that convert into them.
Worked example: 40 units a day, a 30-day supplier, 400 units of buffer
You sell a private-label product on your own store and on a marketplace. Over the last 60 days you have shipped 2,400 units, your supplier quotes 30 calendar days from purchase order to sellable stock, you hold 400 units of buffer, and this morning's count shows 1,800 units on hand with nothing on order.
- Average daily demand. 2,400 units ÷ 60 days = 40 units/day.
- Lead-time demand. 40 × 30 = 1,200 units. That is what will sell while you wait.
- Add the buffer. 1,200 + 400 = 1,600 units. This is your reorder point.
- Inventory position. 1,800 on hand + 0 on order = 1,800 units.
- Gap to the trigger. 1,800 − 1,600 = 200 units.
- Days until you must order. 200 ÷ 40 = 5 days. Order on or before day 5.
- Days of cover. 1,800 ÷ 40 = 45 days of stock at the current rate.
- Buffer in days. 400 ÷ 40 = 10 days. You can absorb a 10-day supplier delay, or ten days' worth of demand overshoot, but not both.
Now change one thing. Suppose you can only cut purchase orders at the start of each month, so the review period is 30 days. The trigger becomes 40 × (30 + 30) + 400 = 2,800 units — and your position of 1,800 is already 1,000 units below it. Same demand, same supplier, same buffer: the ordering calendar alone moved you from five days of slack to 1,000 ÷ 40 = 25 days late.
How to read the numbers
Start with days until you must order. Positive means you have slack, and the size of it tells you how urgent the purchase-order conversation is. Zero or negative means the trigger has already been passed and the only remaining questions are how much to buy and whether to expedite. A negative number of, say, −20 days means you crossed the trigger twenty days ago and a normal-lead-time order will now land twenty days' worth of demand late.
Then read days of cover against the lead time. This is the sanity check that catches genuine emergencies. Cover is reported on inventory position, so when you have nothing on order it is simply on-hand ÷ daily demand; when you do have stock on order, judge the stockout risk on the on-hand figure the step list gives you, because a container three weeks out does not fill today's orders. If on-hand cover is 20 days and lead time is 30, no order placed today can arrive in time; you are choosing between an air-freight premium, a partial stockout, or borrowing stock from another channel. If cover comfortably exceeds lead time plus buffer days, you have room to negotiate on price or consolidate the order with another SKU.
Safety stock in days is the most honest single measure of how much resilience you have bought. A buffer expressed in units is hard to judge; the same 400 units is 10 days of cover on a 40-a-day product and 2 days on a 200-a-day product. As a rule of thumb, practitioners aim for a buffer covering a meaningful fraction of the lead time — enough to survive the delay your worst supplier has actually inflicted on you in the past year. Look up that number rather than guessing: your own late-delivery history is better evidence than any benchmark.
Finally, remember that a reorder point built on a 60-day average is a statement about a stable product. It will mislead you badly on anything seasonal, newly launched, or currently being promoted. Recalculate the daily demand from the period you are about to live through, not the period you have just left.
Lead-time demand by daily sales rate and lead time
| Units/day | 7 days | 14 days | 30 days | 45 days | 60 days | 90 days |
|---|---|---|---|---|---|---|
| 5 | 35 | 70 | 150 | 225 | 300 | 450 |
| 10 | 70 | 140 | 300 | 450 | 600 | 900 |
| 25 | 175 | 350 | 750 | 1,125 | 1,500 | 2,250 |
| 40 | 280 | 560 | 1,200 | 1,800 | 2,400 | 3,600 |
| 75 | 525 | 1,050 | 2,250 | 3,375 | 4,500 | 6,750 |
| 150 | 1,050 | 2,100 | 4,500 | 6,750 | 9,000 | 13,500 |
| 300 | 2,100 | 4,200 | 9,000 | 13,500 | 18,000 | 27,000 |
Lead times of 45 to 90 days are normal for ocean-freighted imports once production time is included; domestic distributors are often inside 14 days. Use your own measured lead time, not the supplier's quote.
Measure lead time from the calendar, not the contract
Suppliers quote production time. Your lead time is longer, and it is the one that governs. It runs from the moment you approve the purchase order to the moment the stock is sellable, and it includes payment clearing, production, booking a slot, port dwell, sailing, customs clearance, drayage, receiving and putaway. For an imported product each of those adds days, and the clearance step can add unpredictable ones.
Pull the last five or ten purchase orders for the SKU, record the actual elapsed calendar days for each, and use the longest recent one rather than the mean if a stockout would be costly. Two of those steps have their own calculators: entry cost and clearance exposure in the import duty and customs fee calculator, and freight billing in the dimensional weight calculator.
Mistakes that make a reorder point fail
- Comparing the trigger to on-hand instead of inventory position. Open purchase orders count. Ignore them and you will re-order the same shortfall repeatedly.
- Mixing selling days with calendar days. Demand per selling day multiplied by a calendar lead time understates lead-time demand substantially.
- Using the supplier's quoted production time as the lead time. Transit, customs and putaway are yours, and they are often the larger half.
- Forgetting the review period. If you can only order weekly or monthly, the trigger must cover the lead time plus that interval.
- Averaging demand across a promotion or a seasonal peak. The average that includes Black Friday will not describe January, and vice versa.
- Leaving safety stock at zero. The plain formula covers average demand only, which by construction leaves about half of cycles short.
- Never revisiting the number. Demand drifts, suppliers change, and a reorder point set at launch is stale within a quarter. Recalculate monthly.
- Applying one reorder point to a whole product family. Triggers are per SKU per location, because demand and lead time are per SKU per location.
Where the reorder point sits among the alternatives
Continuous review with a reorder point is the right default when you can see stock levels in real time and order any day, which describes almost every business running a modern inventory system. Its two neighbours are worth knowing.
Periodic review (an R, S policy) checks stock on a fixed cycle and tops up to a target level rather than watching for a trigger. It suits businesses that consolidate purchase orders for freight reasons or that order from a supplier with a fixed shipping calendar. The review-period field in this calculator is what converts the continuous formula into something safe to use under that regime.
Min/max is the same idea wearing different clothes: the minimum is the reorder point, and the maximum is the level you top up to. If your ERP asks for min and max, compute the min here and set the max to min plus your economic order quantity.
Two financial checks belong alongside all of this. Inventory turnover tells you whether the policy as a whole is holding too much stock, and days inventory outstanding expresses the same thing in days so it can be compared against your payment terms. A reorder point that never stocks out but pushes turnover from eight to four has not solved a problem; it has moved it onto the balance sheet. And because ordering more often raises per-entry costs on imports, the true cost of a small, frequent order pattern shows up in the landed cost calculator, not in the reorder point itself.
Key terms
- Inventory position
- On-hand sellable units plus units on open purchase orders, minus any committed backorders. The quantity the reorder point must be tested against.
- Lead-time demand
- Expected sales between placing an order and receiving usable stock: average daily demand times lead time in calendar days.
- Safety stock
- Buffer inventory held to absorb demand variability and lead-time variability, sized from a target service level rather than from instinct.
- Days of cover
- Inventory position divided by average daily demand. How long current stock lasts at the current rate of sale.
- Review period
- The interval between opportunities to place an order. Zero for continuous review; seven days if you cut purchase orders once a week.
- (s, Q) policy
- Order a fixed quantity Q whenever inventory position falls to the reorder level s. The standard continuous-review policy this calculator supports.