The reorder point formula itself is one line. Getting a trustworthy number out of it takes a bit more care, because two of its three inputs have to be gathered — and one of them calculated — before you ever touch the final equation.
Step 1: Find Your Average Daily Demand
Pull actual sales history for the item — ideally 6 to 12 months, long enough to smooth out any single unusually busy or quiet stretch. Divide total units sold by the number of days in that period.
Average Daily Demand = Total Units Sold ÷ Number of DaysTwo things to watch for here. First, use a period that reflects normal operating conditions — if the last 3 months included a one-off bulk order from a single customer, that will skew the average upward for reasons that won’t repeat. Second, if the product has a genuine seasonal pattern, a single annual average blurs peak and off-peak together; in that case, calculate a separate average (and a separate reorder point) for each season rather than forcing one number to describe both.
Step 2: Find Your Lead Time
Lead time is the number of days between placing an order with your supplier and having usable stock in hand — not the day the supplier ships, the day it actually arrives and is ready to sell. Look back at your last several purchase orders for this item or supplier and calculate the average number of days from order date to receipt date.
Lead Time = Average(Order Date to Receipt Date, across recent POs)Don’t use the supplier’s quoted lead time if your own purchasing history tells a different story — quoted lead times are frequently optimistic. And don’t use a single fast or single slow order as your only data point; average across several recent ones.
Step 3: Calculate Safety Stock First
This is the step most people skip or shortcut, and it’s the one the reorder point formula genuinely cannot do without. Reorder point does not calculate safety stock internally — it expects a safety stock figure as an input you’ve already worked out. That means before you can finish a reorder point calculation, you need:
- The standard deviation of daily demand for this item (how much daily sales actually swing above and below the average you calculated in Step 1)
- Your lead time (already have this from Step 2)
- A target service level, expressed as a Z-score (1.65 for 95%, 2.33 for 99%, and so on)
Safety Stock = Z × Demand Std Dev × √(Lead Time)How to Calculate Safety Stock covers gathering and computing each of these three sub-inputs in full detail — worth reading in full if you haven’t calculated a safety stock figure for this item before.
Step 4: Combine Everything Into the Reorder Point
With all three inputs in hand — average daily demand, lead time, and safety stock — the final step is a single addition:
Reorder Point = (Average Daily Demand × Lead Time) + Safety StockA Full Walkthrough
Say you’re calculating a reorder point for an item with the following, gathered exactly as described above:
- Average daily demand (from 9 months of sales history): 45 units/day
- Lead time (averaged across the last 6 purchase orders): 9 days
- Daily demand standard deviation: 20 units
- Target service level: 95% (Z = 1.65)
Step 3 first — safety stock:
Safety Stock = 1.65 × 20 × √9 = 1.65 × 20 × 3 = 99 unitsStep 4 — reorder point:
Reorder Point = (45 × 9) + 99 = 405 + 99 = 504 unitsThe resulting policy: when on-hand inventory drops to 504 units, place the next order.
Common Mistakes in This Process
Skipping straight to reorder point without a real safety stock figure. Guessing a round-number buffer, or setting it to zero by default, quietly removes the entire protective purpose of the formula — you end up with a number that only works if every order arrives exactly on time and demand never exceeds average.
Using quoted lead time instead of measured lead time. A supplier’s stated lead time and your actual, historically observed lead time are often different numbers. Use the one your own purchase order history actually shows.
Averaging demand over too short a window. A single month, especially a recent one, can be skewed by one large order or one unusually slow stretch. Six months or more gives a steadier average.
Recalculating rarely, or never. Demand shifts, suppliers get faster or slower, and service level priorities change. A reorder point calculated once and left untouched for years drifts away from the conditions it was built for — see Reorder Point Decision-Making in Practice for how to recognize when a recalculation is overdue.
Try It With Your Own Numbers
Once you have all three inputs, the Reorder Point Calculator does the final combination instantly — useful for testing how sensitive your reorder point is to a slower supplier or a demand swing before committing to a purchasing policy built on it.