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How to Calculate Inventory Runway (and Avoid Stockouts)
Running out of a bestseller is one of the most expensive mistakes in e-commerce: you lose the sales and, on Amazon, the hard-won organic rank that's slow to rebuild. Inventory runway is the early-warning number that stops it happening.
Here's how to calculate it, factor in growth and supplier lead time, and know exactly when to place your next order.
Inventory Runway Calculator
Your numbers
Stock on hand, sales pace, and your supplier lead time.
Units / day
%
Demand uplift
days
days
Buffer cover
Days of stock remaining
Reorder in 142d173days
Suggested reorder qty
682
Adjusted daily sales
22.0
with growthThe inventory runway formula
Days of stock = units in stock ÷ your daily sales rate. For a truer figure, adjust the daily rate for expected growth: if you sell 20/day and expect 10% more demand, use 22/day. So 3,800 units ÷ 22/day ≈ 173 days of runway.
The point isn't the exact number — it's watching it fall so you act before zero, not after.
When to reorder: lead time + safety stock
You must reorder before runway runs out — early enough that new stock arrives in time. Reorder when days of stock drop to your lead time plus a safety buffer. With a 21-day lead time and 10 days of safety cover, reorder when ~31 days of stock remain.
A suggested reorder quantity is roughly your adjusted daily sales × (lead time + safety days) — enough to cover the next cycle plus a buffer.
Growth and seasonality change everything
Runway based on flat average sales is dangerous around peaks. If demand is climbing or a promotion is coming, raise the daily rate; if lead times stretch over holidays, extend the reorder trigger. A little conservatism here is far cheaper than a stockout.
Frequently asked questions
How do I calculate inventory runway?
Divide units in stock by your daily sales rate (adjusted for expected growth). That's how many days of stock you have; reorder before it falls below your lead time plus safety buffer.
How is runway different from a reorder point?
Runway is the time (in days) your stock lasts; the reorder point is the stock level that should trigger a new order. You need both.
How much safety stock should I hold?
Enough to cover demand spikes and supplier delays for your product's volatility — often a set number of days of cover. Too little risks stockouts; too much ties up cash.
Track this on a live dashboard
Connect your store and watch these numbers update themselves — no spreadsheets.
Keep reading
How to Calculate Your Reorder Point (Never Run Out of Stock)
Your reorder point is the stock level at which you place a new order so you don't sell out. Learn the formula, how to set safety stock, and calculate yours in seconds.
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