Updated March 2026
Days of Supply: What It Means for Inventory Management
Days of supply (also called days of stock or days of inventory) is how many days your current inventory will last at the current sell rate. When this number hits zero, you're stocked out.
The Formula
Days of Supply = Current Inventory ÷ Average Daily Sales
Divide what you have by how fast it's selling to get how many days until you run out.
Example
You have 240 units of a moisturiser in stock. You're selling an average of 8 units per day.
Days of Supply = 240 ÷ 8 = 30 days
You have 30 days of stock remaining.
At the current sell rate, you'll run out in 30 days. If your lead time is 21 days, you need to reorder within the next 9 days to avoid a stockout.
Why It Matters for Shopify Merchants
Days of supply is the simplest and most actionable inventory metric. It answers the question every merchant cares about: how long until I run out? For DTC brands managing hundreds of SKUs, scanning a days-of-supply column is the fastest way to spot problems. A SKU at 5 days of supply with a 21-day lead time is already in trouble. Tracking this across your entire catalogue daily — which is impossible manually — is what separates reactive merchants from proactive ones.
How Alertr Helps
Alertr calculates days of supply for every SKU automatically using your actual Shopify sales data. You see the number on your dashboard and get alerts when any product drops below your configured threshold — before it becomes a stockout.
Related Terms
Frequently Asked Questions
What's a good days of supply target?
It depends on your lead time. You want at least enough days of supply to cover your lead time plus safety stock. If your lead time is 21 days, aim for 30-40 days of supply minimum.
Is days of supply the same as days of inventory?
Yes. Days of supply, days of stock, and days of inventory outstanding (DIO) all measure the same thing — how long your current inventory will last at the current sell rate.
How often should I check days of supply?
Daily for your top 20% of SKUs (by revenue). Weekly for the rest. This is where automated alerts save time — you don't need to check if the system tells you when something needs attention.
What if my sell rate changes?
Days of supply is only as accurate as your sell rate data. Use a 30-day rolling average for stable products. For seasonal items or products in promotions, use a shorter lookback (7-14 days) to capture the current trend.
Skip the Manual Calculations
Alertr tracks sell rates and forecasts stock levels automatically from your Shopify data. Inventory forecasting and reorder alerts. Free tier available, no credit card required.
Join Waitlist