Inventory & Stock Calculator
Know how many days your stock will last, its value, when to reorder and how much to order — before you run out during a winning campaign.
How to calculate stock requirements
Days of stock = units in stock ÷ daily sales. Reorder point = daily sales × (supplier lead time + safety days).
How to use the inventory calculator (formula)
What each field means
- Units in stock — Units currently in stock.
- Cost per unit — Cost per unit.
- Average daily sales (units) — Average units sold per day.
- Supplier lead time (days) — Days for new stock to arrive.
- Safety stock (days) — Extra days of stock as a buffer.
Worked example
Using these numbers: units in stock 600, cost per unit $9.00, average daily sales (units) 18, supplier lead time (days) 14, safety stock (days) 7.
The days of stock left is 33 days.
- Inventory value: $5,400
- Reorder when stock reaches: 378
- Units to order now (30-day cover): 318
How much safety stock should I keep?
A week of sales is a common starting point; increase it if supplier lead times are unreliable or you are scaling ads.
Tips to improve your result
- Recalculate daily sales when you scale ads.
- Stock-outs reset ad learning — plan stock with budgets.
- Overstock ties up cash; order in smaller batches if lead time allows.
- Track slow movers separately.
Frequently asked questions
How do I calculate days of inventory?
Units in stock ÷ average daily sales.
What is a reorder point?
The stock level at which you must reorder to avoid running out.
How does overstocking hurt?
It ties up cash and risks unsold stock.
How does understocking hurt?
You lose sales and paid campaigns lose momentum.
You might also need
E-commerce Profit CalculatorNet profit and margin per order after product, shipping, fees and ads.
Want help improving these numbers?
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