Safety Stock, Reorder Point & Inventory Turnover: The Formulas Every Seller Needs

Running out of stock kills sales rank. Overstocking ties up cash and racks up storage fees. The formulas in this guide help you hold exactly the right amount of inventory — no more, no less.

The Three Core Inventory Metrics

• Safety Stock — buffer inventory to cover demand spikes and supplier delays
• Reorder Point (ROP) — the inventory level that triggers a new purchase order
• Inventory Turnover — how efficiently you're selling through your stock

Safety Stock Formula

Safety stock is the cushion that prevents stockouts when sales spike or shipments are late.

Basic formula:

Safety Stock = (Max Daily Sales − Avg Daily Sales) × Max Lead Time (days)

Example: you sell on average 10 units/day, but at peak you sell 18. Your supplier's max lead time is 45 days.

Safety Stock = (18 − 10) × 45 = 360 units

This means you should always keep at least 360 units in reserve beyond your regular sales coverage.

Statistical formula (more accurate for variable demand):

Safety Stock = Z × σ × √Lead Time

Where Z is the service level Z-score (1.65 for 95%, 2.05 for 98%) and σ is the standard deviation of daily demand. This requires more data but is more precise for high-volume SKUs.

Reorder Point Formula

The reorder point tells you when to place a new purchase order so stock arrives before you run out.

Reorder Point = (Avg Daily Sales × Lead Time in Days) + Safety Stock

Example: you sell 10 units/day, lead time is 30 days, safety stock is 360 units.

ROP = (10 × 30) + 360 = 660 units

When your inventory drops to 660 units, place your next order.

For Amazon FBA sellers, lead time includes:

• Production time (supplier manufactures your order)
• Shipping time (ocean = 25–35 days, air = 5–7 days)
• Amazon check-in time (3–14 days to become available in FBA)

Total FBA lead time is typically 40–60 days for ocean freight from Asia. Plan your reorder points accordingly.

Inventory Turnover Ratio

Inventory turnover measures how efficiently you convert stock into sales.

Inventory Turnover = Cost of Goods Sold (annual) ÷ Average Inventory Value

Example: COGS for the year is $120,000. Average inventory value on hand is $15,000.

Turnover = $120,000 ÷ $15,000 = 8×

An 8× turnover means you sell through your entire inventory 8 times per year, or every 45 days.

Days of Inventory Outstanding (DIO)

DIO = 365 ÷ Inventory Turnover

DIO = 365 ÷ 8 = 45.6 days

DIO is often easier to reason about than turnover. A 45-day DIO means you're holding 45 days of stock on average. For FBA sellers, 30–60 day DIO is common; over 90 days and you're accumulating storage fees that hurt margin.

Inventory Benchmarks by Channel

Amazon's IPI (Inventory Performance Index) score penalizes you for excess inventory relative to sales. Keeping DIO under 60 days protects your IPI score and storage limits.

Frequently Asked Questions

What is safety stock and how do you calculate it?

Safety stock = (Max Daily Sales − Avg Daily Sales) × Max Lead Time. It's the buffer inventory that prevents stockouts during demand spikes or supplier delays.

What is a reorder point?

ROP = (Avg Daily Sales × Lead Time) + Safety Stock. When inventory drops to this level, place your next purchase order to avoid running out.

What is inventory turnover ratio?

Turnover = Annual COGS ÷ Average Inventory Value. Higher is better — it means faster-moving stock and less capital tied up in inventory.

What is days of inventory outstanding?

DIO = 365 ÷ Turnover. How many days on average your stock sits before being sold. For FBA, target 30–60 days to keep storage fees low.