Days of Inventory Calculator
How long your current stock will last.
Results update as you type.
How it works
Days of inventory, also called days of supply or days on hand, tells you how many days your current stock will last at your typical selling pace. You calculate it by dividing the units you have on hand by your average daily sales. This single number turns an abstract pile of stock into a concrete runway, letting you see at a glance whether you are about to run out or sitting on far more than you need. It is one of the most practical inventory metrics because it connects the quantity in your warehouse directly to real demand rather than to a static reorder table. Too few days of supply and you risk stockouts, lost sales and disappointed customers who may not return; too many and you tie up cash in slow-moving goods, pay for storage, and expose yourself to obsolescence and markdowns. The right level depends on how long your supplier takes to replenish, how variable your demand is, and how much safety buffer you want to hold. Store owners and operations teams use this calculator to time purchase orders, to spot overstock and understock across a catalogue, to plan for seasonal peaks, and to keep working capital under control. Compare days of supply across products, watch it fall as a sale accelerates, and use it together with your supplier lead time to decide exactly when to reorder so you neither run dry nor drown in excess stock.
days of supply = inventory units ÷ average daily sales
The formula divides the number of units currently on hand by the average number of units sold per day, giving the number of days the stock will cover at that rate. The daily sales figure should reflect genuine recent demand, ideally averaged over a representative period, because a rate skewed by a one-off spike or a quiet week will over- or understate your true runway. The result is a forward-looking estimate, not a guarantee, since demand can change.
Source:Standard inventory management metric (days of supply)
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Worked example
How to use this calculator
- 1
Enter your values
Enter inventory on hand and average daily sales.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Days of supply updates instantly as you type — no button to press.
Frequently asked questions
What is a good number of days of inventory?
There is no universal target, because the right level depends on your supplier lead time, demand variability and storage costs. A rough rule is to hold enough to cover your replenishment lead time plus a safety buffer, so if a supplier takes two weeks you would want comfortably more than fourteen days of supply. Fast-moving perishable goods favour lower days on hand, while items with long or unreliable lead times justify holding more to avoid stockouts.
How is days of inventory different from inventory turnover?
They are two views of the same thing. Days of inventory measures how many days your stock lasts, while inventory turnover measures how many times you sell through and replace your stock over a period such as a year. A low days-on-hand figure corresponds to high turnover, and vice versa. Days of supply is often more intuitive for day-to-day reordering decisions, whereas turnover is a common efficiency ratio used in financial and annual reporting.
What daily sales figure should I use?
Use an average over a recent, representative period rather than a single day, so a one-off spike or a quiet day does not distort the result. For products with steady demand, a trailing thirty-day average works well. For seasonal items, base the figure on the relevant season rather than a flat annual average, because using the wrong period can badly under- or overstate your runway right when accurate planning matters most.
Why does days of inventory matter for cash flow?
Every unit sitting in your warehouse represents cash you have already spent but not yet recovered. Holding too many days of supply locks up working capital, adds storage costs, and risks markdowns if goods become obsolete or go out of season. Holding too few risks stockouts and lost sales. Tracking days of inventory helps you strike the balance that keeps shelves stocked while freeing up cash to invest in growth, marketing or faster-moving products.
How do I use days of supply to time reorders?
Compare your current days of supply against your supplier's lead time plus the safety buffer you want to hold. When days of supply falls to roughly that combined figure, it is time to place a new order so fresh stock arrives before you run out. Pairing this metric with a reorder point calculation makes the timing explicit, turning a gut feeling about running low into a clear, repeatable trigger that protects you from both stockouts and overstock.