How to Calculate Reorder Points: The Complete Distributor Guide

The Basic Formula

The reorder point formula is straightforward:

When your on-hand quantity drops to this number, it's time to order. The first part — demand times lead time — covers what you'll sell while waiting for the new order to arrive. Safety stock is your buffer against variability in both demand and lead time.

Breaking Down Each Variable

Average Daily Demand

Take total units sold over a period and divide by the number of days. The key question is which period to use. A 30-day average reacts quickly to changes but is noisy. A 90-day average is smoother but slower to adapt. A 12-month average captures seasonality but may miss recent trend shifts. Most distributors benefit from a weighted approach where recent months count more than older months.

Lead Time

This is how many days between placing an order and receiving it. The critical mistake most distributors make is using a single lead time number when their actual lead times vary significantly. A vendor that averages 10 days but ranges from 5-20 days needs a very different safety stock than one that consistently delivers in 9-11 days. Always use actual lead times from your PO history, not the number your vendor quotes.

Safety Stock

Safety stock protects you against two types of variability: demand that's higher than expected, and lead times that are longer than expected. The standard formula is:

The Z-score represents your target service level. A 95% service level (Z = 1.65) means you want to have stock available 95% of the time. A 85% level (Z = 1.04) accepts more stockout risk in exchange for less capital tied up. Fast-moving products typically warrant 95% while slow movers can use 85%.

A Worked Example

Suppose you have a product with:

When on-hand inventory drops to 50 units, place a new order. The 42 units cover expected sales during the 14-day lead time, and the 8-unit buffer protects against demand spikes or delivery delays.

Why Static Reorder Points Fail

The formula above works for a snapshot in time. The problem is that demand and lead times change constantly. A product selling 3/day in January might sell 6/day in June. A vendor delivering in 14 days might start taking 21 days after a supply chain disruption. Static reorder points — the ones you calculated six months ago and never updated — become dangerously inaccurate over time.

This is where most distributors get burned. The reorder point that was right in Q1 causes stockouts in Q3 because demand increased, or causes overstock in Q4 because demand dropped.

The AI Approach: Dynamic Reorder Points

AI-powered systems recalculate reorder points automatically as new data comes in, using demand forecasting to stay ahead of changes. Every sale, every received PO, every day that passes updates the demand velocity, lead time estimates, and safety stock calculations for every SKU. If demand doubles, the reorder point adjusts upward within days — not months. If a vendor improves their delivery time, safety stock tightens and you free up capital.

For a distributor managing 1,000+ SKUs across multiple vendors with different lead times, manually recalculating reorder points is impractical. AI makes it automatic. You can try our free reorder point calculator to see how the math works for your products.

Common Mistakes to Avoid

Using vendor-quoted lead times instead of actual measured lead times is the most common error — vendors are optimistic, and your reorder points should be based on reality. Ignoring demand variability and using zero safety stock guarantees stockouts on any product with inconsistent sales. Setting the same service level for all products wastes capital — your top 100 SKUs deserve 95% service while the bottom 500 can tolerate 85%. And never set reorder points once and forget them — demand changes and your reorder points must change with it.