--- title: "Reorder Point Formula: How to Calculate It, With Safety Stock" description: "Reorder point = daily demand x lead time + safety stock. Worked example with three safety stock methods, z values, Excel formulas and periodic review." canonical: https://luckpanda.app/blog/reorder-point-formula author: "Dhrumil Barot" author_bio: "Dhrumil is a small-business operations and compliance expert. Dhrumil built LuckPanda so small businesses can run invoicing, inventory, accounts and compliance in one place." published: 2026-10-28 updated: 2026-10-28 category: "Purchasing and inventory" keywords: "reorder point formula" language: en --- # Reorder Point Formula: How to Calculate It, With Safety Stock Reorder point = average daily demand x lead time + safety stock. One item worked three ways, the z-value table, a spreadsheet layout and why reorder points fail. By [Dhrumil Barot](https://luckpanda.app/about#founder), Founder. Published 28 Oct 2026. Updated 28 Oct 2026. 7 min read. **TL;DR:** The reorder point formula is ROP = (average daily demand × lead time in days) + safety stock. When stock on hand plus stock already on order falls to that number, you raise a purchase order. The hard part is safety stock: the method you pick for it can move the answer by 50% or more for the same item. The reorder point (ROP), also called the reorder level, is the stock quantity at which you place the next replenishment order so the new stock arrives before you run out. It has two parts: lead time demand, which is what you expect to sell while the order is in transit, and safety stock, which covers selling faster than expected or a supplier delivering late. An item that sells 40 units a day with a 6-day lead time needs 240 units of lead time demand; add safety stock of 110 and the reorder point is 350 units. ## What is the reorder point formula? The reorder point formula is lead time demand plus safety stock: **Reorder point = (average daily demand × average lead time in days) + safety stock** | Term | What it means | Where to get it | | --- | --- | --- | | Average daily demand | Units sold or consumed per day | Units sold in the last 90 days ÷ 90 (use selling days if you close on Sundays) | | Lead time | Days from raising the PO to stock being usable on the shelf | Dates on your last 10 purchase orders and goods receipts, including inspection time | | Safety stock | Buffer for demand and delivery variation | One of the three methods below | Without safety stock the formula reduces to the basic reorder level, average daily usage × lead time. Wikipedia's [reorder point article](https://en.wikipedia.org/wiki/Reorder_point) gives the textbook case: 50 units a day and a 7-day lead time give a reorder level of 350 units. That number only works if demand and lead time never vary, which is why almost every real reorder point adds a buffer. ### Why compare stock on hand plus on order? Use the inventory position, not the shelf count: inventory position = on hand + on order − committed to customers. If you compare only what is on the shelf, you will reorder again the next day while the first order is still in transit, and double your stock. Committed units (sales orders not yet shipped) are already spoken for, so subtract them. ## How do you calculate safety stock for the reorder point? Safety stock is the extra quantity held so that normal variation in demand and lead time does not cause a stockout. There are three common ways to set it, from simplest to most precise. | Method | Safety stock formula | Data you need | Best for | | --- | --- | --- | --- | | Days of cover | average daily demand × buffer days | Average demand | New items, fewer than 3 months of history | | Max-max | (max daily demand × max lead time) − (average daily demand × average lead time) | Peak day and slowest delivery | Small catalogues, quick estimates | | Statistical | z × √(L × σd² + d² × σL²) | Daily sales history, lead time history | Items with 6+ months of data and steady demand | In the statistical formula, z is the service level factor, L is average lead time in days, σd is the standard deviation of daily demand, d is average daily demand and σL is the standard deviation of lead time in days. This is the type I service formula in Wikipedia's [safety stock article](https://en.wikipedia.org/wiki/Safety_stock). If your supplier's lead time is reliable (σL = 0) it shortens to z × σd × √L. The max-max formula is the one [inFlow's reorder point guide](https://www.inflowinventory.com/blog/reorder-point-formula-safety-stock/) uses. It is easy, but it assumes your busiest day and your slowest delivery happen together, which rarely occurs, so it usually overstocks. ### Which z value matches which service level? The z value is the number of standard deviations of buffer needed to hit a chosen cycle service level, the probability of not running out during one replenishment cycle. Values come from the inverse of the standard normal distribution; in Excel or Google Sheets use `=NORM.S.INV(0.95)`. | Cycle service level | z value | Stockouts expected | | ---: | ---: | --- | | 90% | 1.28 | 1 cycle in 10 | | 95% | 1.65 | 1 cycle in 20 | | 97.5% | 1.96 | 1 cycle in 40 | | 99% | 2.33 | 1 cycle in 100 | | 99.9% | 3.09 | 1 cycle in 1,000 | Going from 95% to 99% raises z by 41% (1.65 to 2.33), and safety stock rises by the same 41%. Reserve 99% for items whose stockout stops a sale of something else, such as a spare part a customer needs to finish a job. ## Reorder point example: one item, three methods A Pune hardware distributor stocks a 20 mm PVC elbow costing ₹38 per piece. Over the last 90 selling days: - Average daily demand d = 40 pieces, standard deviation σd = 12 pieces - Highest day: 60 pieces - Average lead time L = 6 days, standard deviation σL = 1.5 days, slowest delivery 9 days - Lead time demand = 40 × 6 = **240 pieces** | Method | Safety stock calculation | Safety stock | Reorder point | | --- | --- | ---: | ---: | | No buffer | 0 | 0 | 240 | | Days of cover (3 days) | 40 × 3 | 120 | 360 | | Max-max | (60 × 9) − (40 × 6) = 540 − 240 | 300 | 540 | | Statistical, 95% | 1.65 × √(6 × 12² + 40² × 1.5²) = 1.65 × √4,464 = 1.65 × 66.8 | 110 | 350 | | Statistical, 99% | 2.33 × 66.8 | 156 | 396 | The max-max reorder point is 190 pieces above the 95% statistical one. At a 20% annual holding cost that is 190 × ₹38 × 0.20 = ₹1,444 a year for one SKU. Across a catalogue of 500 items with similar numbers, the gap is about ₹7.2 lakh a year of carrying cost for protection you rarely use. Notice where the variation comes from: 6 × 12² = 864 comes from demand, and 40² × 1.5² = 3,600 comes from lead time. Four fifths of this item's safety stock exists because the supplier is unreliable. Cutting σL from 1.5 to 0.5 days drops 95% safety stock from 110 to 59 pieces. Ask for a firm delivery day before you buy more buffer. ## How do you calculate the reorder point in Excel or Google Sheets? Put daily sales in column B (one row per day for 90 days) and lead times in days from past purchase orders in column D. Then: \`\`` F1 Average daily demand =AVERAGE(B2:B91) F2 Std dev of demand =STDEV.S(B2:B91) F3 Average lead time =AVERAGE(D2:D11) F4 Std dev of lead time =STDEV.S(D2:D11) F5 Service level 0.95 F6 z =NORM.S.INV(F5) F7 Safety stock =ROUNDUP(F6*SQRT(F3*F2^2 + F1^2*F4^2), 0) F8 Reorder point =ROUNDUP(F1*F3 + F7, 0) `\`\` Include zero-sales days in column B. Leaving them out inflates average demand and understates its variation. ## Reorder point vs reorder quantity: what is the difference? The reorder point answers when to order; the reorder quantity answers how much. They are set separately. A common way to set the quantity is the economic order quantity, which balances ordering cost against holding cost; see the [EOQ formula guide](https://luckpanda.app/blog/eoq-formula). The reorder quantity also has to respect the supplier's minimum order quantity and case pack: if the elbow comes in cartons of 50, round the EOQ up to the next 50. Two related levels come from the same inputs: - **Minimum level** = reorder point − (average daily demand × average lead time). This equals the safety stock; stock below it means you are eating into the buffer. - **Maximum level** = reorder point + reorder quantity − (minimum daily demand × minimum lead time). Stock above it means you over-ordered or sales dropped. ## What if you review stock weekly instead of continuously? A periodic review system checks stock on a fixed schedule, such as every Monday, rather than after every sale. The reorder point formula assumes continuous review; on a weekly schedule stock can fall below the reorder point on Tuesday and nobody notices until the next Monday. Wikipedia's safety stock article notes that under periodic review the buffer must cover the review period plus the lead time. For periodic review, order up to a target level instead: **Order-up-to level = d × (R + L) + z × σd × √(R + L)** Where R is the review period in days. For the elbow with weekly review (R = 7) and a reliable supplier: 40 × 13 + 1.65 × 12 × √13 = 520 + 71 = 591 pieces. Each Monday, order 591 minus the inventory position. ## Why do reorder points fail? Most stockouts with a reorder point in place come from inputs, not the formula: 1. **Mixed units.** Demand in pieces, lead time in weeks, safety stock in cartons. Convert everything to the stock-keeping unit and days before multiplying. 2. **Stale lead time.** A supplier that took 6 days last year may take 10 days today. Recalculate from the last 10 receipts every quarter. 3. **Lead time measured to dispatch, not to shelf.** Add transit, unloading, quality inspection and putaway. If goods sit two days in receiving, your lead time is two days longer. 4. **Seasonal items on an annual average.** Wikipedia's safety stock article notes the normal formula consistently produces stockouts in peak season and waste off-peak when demand is seasonal. Use the last 8 to 12 weeks of the same season instead. 5. **Stock that is not really there.** Damaged, reserved or miscounted units make the reorder point trigger late. A regular [cycle counting](https://luckpanda.app/blog/cycle-counting) programme keeps the on-hand figure honest. 6. **Lumpy demand.** An item that sells 0 units most days and 200 on one day breaks the normal assumption. Set its buffer by days of cover or by the largest order you want to fill from stock. When a stockout does happen, decide whether to take a [backorder](https://luckpanda.app/blog/backorder-meaning) or let the customer go, and record it: lost sales do not show up in sales history and will make the next demand estimate too low. ## Tracking reorder points across items A spreadsheet works for a few dozen SKUs. Beyond that the effort is keeping the inventory position current: every sale, receipt and transfer has to update on hand and on order the same day. In the [LuckPanda Inventory module](https://luckpanda.app/modules/inventory), stock movements post from the invoices, bills and goods receipts you already create, so the on-hand figure the reorder point compares against comes from the same records as your sales and purchases. ## Questions ### What is the formula for reorder point? Reorder point = (average daily demand × average lead time in days) + safety stock. For 40 units a day, a 6-day lead time and 110 units of safety stock, the reorder point is 40 × 6 + 110 = 350 units. ### What is the reorder point formula without safety stock? Without safety stock, reorder point = average daily demand × lead time in days. It only works when demand and lead time are constant. Any variation means about half your replenishment cycles end in a stockout. ### How do you calculate reorder point with weekly demand? Convert both terms to the same unit. Either divide weekly demand by 7 (or by selling days per week) and use lead time in days, or keep weekly demand and express lead time in weeks: 280 units a week × 0.86 weeks (6 days) = 240 units. ### Is reorder point the same as reorder level? Yes. Reorder point and reorder level both mean the stock quantity that triggers a new purchase order. Cost accounting textbooks in India and the UK tend to say reorder level; US operations texts tend to say reorder point. ### How often should I update the reorder point? Recalculate quarterly for steady items and before each season for seasonal items, or whenever a supplier changes its lead time, minimum order quantity or delivery schedule. ### What service level should a small business use? Many businesses use 95% (z = 1.65) as a default, 90% for slow, low-margin items and 98% to 99% for items whose stockout loses a whole order. Each step up costs more safety stock, so set it per item category, not one number for the whole catalogue. ## More guides - [Average Inventory: Formula, Examples and When Each Method Fits](https://luckpanda.app/blog/average-inventory) - [Inventory Turnover Ratio: Formula, Example and What Is Good](https://luckpanda.app/blog/inventory-turnover-ratio) - [Purchase Price Variance (PPV): Formula, Examples and Journal Entries](https://luckpanda.app/blog/purchase-price-variance) ## Related tools - [Profit margin calculator](https://luckpanda.app/tools/profit-margin-calculator): Margin and markup from cost and price, or the price for a target margin. ## About the author **Written by [Dhrumil Barot](https://luckpanda.app/about#founder)**, Founder. Dhrumil is a small-business operations and compliance expert. Dhrumil built LuckPanda so small businesses can run invoicing, inventory, accounts and compliance in one place. 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