How Many Days of Stock Should You Keep? A Guide for Ecommerce Stores

If you run an ecommerce store, you've probably asked yourself: "How much inventory should I keep?" Too little and you run out of stock, disappointing customers and losing sales. Too much and your cash gets trapped in unsold products.

The answer lies in understanding days of stock — a metric that tells you exactly how long your current inventory will last based on your sales velocity.

What Is Days of Stock?

Days of stock (also called days of inventory or days sales of inventory) measures the number of days your current stock will last at the current sales rate. It's a simple but powerful metric that helps you make data-driven purchasing decisions.

For example, if you have 100 units in stock and sell 5 units per day, your days of stock is 20 days. This means at current sales velocity, you'll run out of stock in 20 days.

Why Days of Stock Matters for Your Business

Understanding your days of stock isn't just about avoiding empty shelves. It directly impacts three critical areas:

• Cash flow: Every dollar tied up in inventory is a dollar you can't use for marketing, product development, or operations. Optimal stock levels keep your cash working for you.
• Customer experience: Stockouts mean lost sales and disappointed customers. In ecommerce, a stockout can cost you not just that sale, but potentially future purchases when customers go elsewhere.
• SEO and rankings: Consistent stockouts hurt your search rankings. Search engines favor products that are consistently available.

How to Calculate Your Optimal Days of Stock

The ideal days of stock varies by product and business model, but here's a general framework:

• Fast-moving products (high velocity): 14-21 days. These products sell quickly and need frequent reordering.
• Standard products: 21-30 days. Most products fall into this range.
• Slow-moving products (low velocity): 30-45 days. These products sell less frequently, so carrying too much inventory ties up cash unnecessarily.

Your ideal range depends on factors like:

• Supplier lead times (how long it takes to restock)
• Sales velocity consistency (do sales fluctuate seasonally?)
• Storage costs and warehouse capacity
• Your cash position and financing costs

What Is Safety Stock and Why Do You Need It?

Safety stock is extra inventory you keep as a buffer against unexpected demand spikes or supply delays. It's your insurance policy against stockouts.

For example, if your highest daily sales is 20 units (during peak season), your longest supplier lead time is 21 days, your average daily sales is 10 units, and average lead time is 14 days:

• Safety stock = (20 × 21) - (10 × 14) = 420 - 140 = 280 units

This buffer ensures you stay stocked even when demand exceeds expectations or deliveries are delayed.

The Problem with Manual Inventory Tracking

Most ecommerce stores track inventory in spreadsheets, updating quantities manually. This approach has three major flaws:

• No real-time visibility: Spreadsheets are outdated the moment you save them. You don't know your true days of stock until you manually recalculate.
• No automatic alerts: With spreadsheets, there's no warning when stock runs low. You discover stockouts after they happen.
• No velocity analysis: Spreadsheets don't automatically calculate sales velocity or predict when you'll run out.

How DaysOfStock Calculates This Automatically

DaysOfStock connects to your store and automatically calculates days of stock for every product. Here's what you get:

• Real-time inventory countdown: See exactly how many days of stock each product has, updated automatically.
• Velocity tracking: Understand your actual sales rate, not just your current quantity.
• Low stock alerts: Get notified before products run out, not after.
• Trend analysis: See if your days of stock is increasing or decreasing over time.