Reorder Point Calculator: When to Reorder Inventory

Get a whole-unit reorder trigger, safety-stock buffer, and a clear “reorder now” decision from your demand, lead time, and optional inventory position. This free calculator needs no sign-up and keeps every input in your browser.

Use a preset for a quick example or enter your own SKU data below.

Inputs

Scenario presets

Presets load illustrative values and calculate immediately; replace them with your own representative data.

Average demand

Units sold or consumed ÷ relevant days. Use 30–90 representative days, or a comparable seasonal period.

Average lead time

Include supplier processing, transit, inspection, and put-away until stock is usable.

Planning-day basis Use the same basis for demand and lead time. Operating days exclude non-working days using 261 days/year.

Unit label Name the counted item, such as units, cases, cartons, or kilograms.

Safety stock Buffer units held for demand or supplier variability. Keep manual mode for a known policy value.

Advanced: calculate safety stock

Safety-stock method Choose a method that matches the variability data you can support.

Manual formula: use the safety-stock quantity entered above.

Current inventory (optional)

Complete any field to compare inventory position with ROP. Blank optional fields count as zero.

Current on-hand Usable physical stock currently available at this location.

Units on order Confirmed incoming units not yet received; exclude unapproved suggestions.

Backorders Committed demand or shortages that must be deducted from available supply.

Results

Operational reorder point (round up) — Exact calculated value: —

Enter complete values to calculate ROP.

Add current inventory for a reorder decision Inventory position = on-hand + on-order − backorders.

Lead-time demand:—

Safety-stock buffer:—

Safety stock in days:—

Total coverage at ROP:—

Inventory position:—

Safety-stock method:—

How to use the reorder point calculator

Enter average demand and choose whether it is per day, week, or month.
Enter total replenishment lead time in days or weeks, including processing through put-away.
Use a known safety-stock quantity, or open the advanced section to calculate a buffer.
Choose calendar or operating days. The calculator normalizes demand and lead time to that same basis.
Optionally add on-hand, on-order, and backorders to see whether to reorder now.

Calculate each SKU and stocking location separately. Replenishment should trigger from inventory position = on-hand + on-order − backorders, not shelf stock alone.

Reorder point formula and variables

ROP = average daily demand × lead time in days + safety stock

The exact value is preserved for analysis. Because a fraction of a stocked unit is normally not actionable, the operational trigger uses Math.ceil(exact ROP). This prevents rounding down and ordering one unit late.

Variable	Meaning	How this calculator handles it
`d`	Average demand per planning day	Daily input stays daily; weekly and monthly input is divided by 7/30.4375 calendar days or 5/21.75 operating days.
`L`	Average replenishment lead time in planning days	Weeks are multiplied by 7 calendar days or 5 operating days.
`SS`	Safety-stock buffer	Entered manually or calculated using the selected advanced method, then kept exact inside ROP.
`IP`	Inventory position	`on-hand + on-order − backorders`; reorder when IP is at or below the whole-unit ROP trigger.

Advanced safety-stock formulas

Demand variability: SS = z × σd × √L, assuming independent, normally distributed demand by planning day and fixed lead time. A weekly or monthly period-total deviation is converted to a daily deviation using square-root-of-time.

Demand and lead-time variability: SS = z × √(L × σd² + d² × σL²), assuming independent demand and lead-time variation.

Maximum usage: SS = max daily demand × max lead time − average daily demand × average lead time. A negative result is limited to zero.

Worked reorder point examples

1. Basic fixed-buffer example

Average demand is 120 units/day, lead time is 14 days, and manual safety stock is 300 units.

Lead-time demand: 120 units/day × 14 days = 1,680 units.
Exact ROP: 1,680 units + 300 units = 1,980 units.
Operational trigger: ceil(1,980) = 1,980 units.

Inventory position: 2,100 on-hand + 0 on-order − 200 backorders = 1,900 units. It is 80 units below the 1,980-unit trigger, so reorder now. The order quantity still comes from EOQ, lot sizing, or another policy.

2. Example with no safety stock

Average demand is 50 units/day, lead time is 5 days, and safety stock is 0 units.

Lead-time demand: 50 units/day × 5 days = 250 units.
Exact ROP: 250 units + 0 units = 250 units.
Operational trigger: ceil(250) = 250 units.

Inventory position: 400 on-hand + 0 on-order − 0 backorders = 400 units. It is 150 units above the trigger, so no order yet; trigger replenishment when inventory position reaches 250 units.

3. Variable demand at a 95% service level

Average demand is 100 units/day, demand standard deviation is 20 units/day, fixed lead time is 10 days, and the 95% service factor is z = 1.645.

Safety stock: 1.645 × 20 units/day × √10 days = 104.04 units.
Lead-time demand: 100 units/day × 10 days = 1,000 units.
Exact ROP: 1,000 units + 104.04 units = 1,104.04 units.
Operational trigger: ceil(1,104.04) = 1,105 units.

Inventory position: 850 on-hand + 200 on-order − 25 backorders = 1,025 units. It is 80 units below the operational trigger, so reorder now.

Seasonal or intermittent-demand warning: a simple average can be unreliable when many periods have zero demand, promotions cause spikes, or the next season differs from the history window. Use a forward forecast or an intermittent-demand inventory model and validate the result SKU by SKU.

How to interpret the result

The operational ROP is the inventory-position threshold. If inventory position is at or below it, release or review a replenishment order now; if it is above, the displayed gap shows how much inventory position can fall before the trigger.

ROP determines when to order, not how much to order. Use EOQ, a fixed reorder quantity, order-up-to level, supplier minimum, or a planning-system recommendation for quantity. “Total coverage at ROP” includes both lead-time demand and safety-stock days.

Assumptions and limitations

The basic ROP assumes the entered average is representative of demand expected during lead time.
Calendar and operating periods are normalized consistently, but holidays and partial shifts are not modeled individually.
Service-level methods assume independent, approximately normal variability; they can misstate protection for skewed, correlated, or intermittent demand.
The combined method treats demand and lead-time variability as independent. Correlation requires a more advanced model.
Current inventory fields do not forecast new demand between now and an order-release decision.
Units must be consistent throughout. Convert cases, eaches, weight, or volume before calculating.

Methodology, review, and test checks

Last reviewed: 17 July 2026. Published and reviewed by Starlight Robotics. No individual professional credential is claimed.

This tool normalizes periods first, calculates safety stock using the displayed method, adds exact lead-time demand and safety stock, then rounds the actionable ROP upward. The decision compares that trigger with inventory position rather than on-hand stock alone. Calculations run locally; inputs are not uploaded or stored by this calculator. A share link includes the values in its URL only when you choose Share.

Verified reference cases

Case	Expected check
120 units/day, 14 days, 300 safety units	Exact and operational ROP = 1,980 units
700 units/week, 2 weeks, no safety stock	Daily rate = 100; lead time = 14 days; ROP = 1,400 units
100 units/day, σd 20, 10 days, 95% service	Safety stock ≈ 104.04; exact ROP ≈ 1,104.04; trigger = 1,105 units

Authoritative references

Oracle Inventory: Reorder Point Planning — ROP as safety stock plus forecast demand during lead time, available quantity, and lead-time components.
Oracle NetSuite: Inventory Optimization Calculations — ROP formula, normal-distribution assumption, and upward rounding.
Microsoft Business Central: Handling Reordering Policies — projected inventory, reorder triggers, safety stock, and separation of trigger from order quantity.

Frequently asked questions

What is a reorder point?

A reorder point is the inventory position at which replenishment should be triggered. It normally equals expected demand during lead time plus safety stock.

What is the difference between reorder point and safety stock?

Reorder point is the trigger for placing an order; safety stock is the buffer inside that trigger for demand or lead-time uncertainty. Safety stock is one component of ROP, not a separate order signal.

What is the difference between reorder point and EOQ?

Reorder point determines when to order. Economic order quantity (EOQ) estimates how much to order by balancing ordering and holding costs. Many inventory policies use both.

What is the difference between reorder point and minimum stock?

A reorder point anticipates demand during replenishment lead time, while minimum stock is a lower floor or buffer a business aims not to cross. Some systems use the terms differently, so document your local policy.

How do I calculate reorder point without safety stock?

Set safety stock to zero, then use ROP = average daily demand × lead time in days. For 50 units per day and 5 days, the exact ROP is 250 units.

Should I use on-hand stock or inventory position?

Use inventory position: on-hand + units already on order − backorders. Shelf stock alone can trigger a duplicate order because it ignores incoming supply and committed shortages.

How should I choose the demand period?

Use 30–90 days of recent, representative history when demand is reasonably stable, and match calendar or operating days consistently. Use a comparable seasonal period instead when the recent average would misrepresent upcoming demand.

How often should I update reorder point?

Review ROP whenever demand, supplier lead time, service targets, or fulfillment processes change. Monthly review suits many active SKUs; volatile, seasonal, or critical items may need weekly review.

How do seasonality and supplier delays affect reorder point?

Rising seasonal demand increases lead-time demand, while variable supplier delays increase the buffer needed. Use a forward-looking seasonal rate and a variability-based safety-stock method; a simple historical average can understate both risks.

Does this tool store my data?

No. The calculation runs in your browser. Inputs are not uploaded or stored by this calculator; sharing only places the current values in a URL you choose to send.