Inventory Forecasting 101: A Beginner's Guide
Key takeaway: Forecasting answers one question: how much will you sell before your next order arrives? The core formula is (Average Daily Sales × Lead Time) + Safety Stock. Good forecasts prevent both stockouts and the cash drain of overstock.
The One Question Forecasting Answers
Every inventory decision you make comes down to a single question: how much will I sell before my next order arrives?
Get it right and you have product on shelves when customers want it, with cash left over for growth. Get it wrong and you are either staring at a stockout notification or a warehouse full of product that is not moving. We think the stockout side is worse. You lose the sale, the ranking, and sometimes the customer permanently.
Whether you are managing Shopify inventory or running multi-channel operations, forecasting is the skill that separates sellers who grow from sellers who thrash. If you sell across more than one channel, the best multi-channel inventory software does this forecasting math across all of them at once.
The Formula
At its simplest:
Reorder Quantity = (Daily Sales x Lead Time) + Safety Stock - Current Inventory
Three inputs. That is it. The hard part is getting each one right.
Sales Velocity
How fast are you selling? Pull your last 30 days:
Last 30 days sold: 300 units
Daily average: 10 units/day
Simple division. But do not stop there. Is demand trending up or down? Say a product averages 10 units per day, then a competitor runs out of stock and you are suddenly selling 18. An order sized on the trailing average of 10 runs out long before the next one arrives.
Look at the trend, not just the average. And if a big promotion is coming, whether that is Prime Day or a site-wide sale, adjust upward manually. No algorithm will predict your own marketing calendar.
Lead Times
Lead time is not just shipping. It is every day between placing the order and having inventory ready to sell:
Supplier processing: 3 days
Shipping from China: 30 days
Customs + receiving: 7 days
Total lead time: 40 days
During those 40 days, you will sell approximately:
10 units/day x 40 days = 400 units
That is 400 units you need on hand just to survive the wait. Track actual delivery dates against quoted ones for every PO. A supplier that runs slower than it quotes is common enough to check for, and knowing by how much changes your math.
Safety Stock
Safety stock protects you when things go sideways. Demand spikes. Shipping delays. Supplier issues. A simple starting rule: keep two weeks of extra stock.
10 units/day x 14 days = 140 units
For a deeper look at the statistics behind this, see our safety stock calculation guide. But two weeks is a solid starting point for products with stable demand and reliable suppliers.
The Reorder Point
Put the pieces together and you get your trigger: the inventory level that means "order now".
Lead Time Demand + Safety Stock = Reorder Point
400 units + 140 units = 540 units
When you hit 540 units, place your next order. Not 500. Not "when it feels low." 540. Having a specific number removes the guesswork and the procrastination.
How Much to Order
The reorder point tells you when. Order quantity tells you how much. This depends on:
- Reorder frequency. Ordering monthly means larger orders than weekly.
- Minimum order quantities. Your supplier may require 500 or more units per order.
- Storage capacity. FBA limits and warehouse space put a ceiling on how much you can take in at once.
- Cash flow. A $20,000 PO is great math but bad if you cannot make payroll.
I default to ordering enough to cover one full reorder cycle plus safety stock. For most products, that means 30-60 days of supply per order. If you want the textbook answer that balances ordering costs against holding costs, run the numbers through our EOQ calculator and treat the result as a starting benchmark, not gospel.
Seasonality Changes Everything
E-commerce demand is not flat. If you sell the same amount in November as you do in February, you are either very lucky or selling something boring.
The patterns that matter most:
- Q4. Holiday shopping can push volume to several times a normal month.
- Summer. Some categories dip well below their yearly average.
- Prime Day and Black Friday. Concentrated spikes need pre-positioned inventory, not a same-week reorder.
- Category timing. Swimwear peaks in April, when people buy, not when they actually swim.
Pull last year's monthly sales. Divide each month by the average month. That ratio is your seasonal index, and it should adjust your forecasts forward.
When Spreadsheets Stop Working
Spreadsheets are fine for 20 SKUs. As the catalog grows, they start to drift: formulas reference the wrong rows, seasonal adjustments get skipped, and a velocity calculation keeps pulling data from a discontinued product.
A rough breaking point is around 50 SKUs. Below that, a well-maintained spreadsheet is adequate. Above it, the maintenance time exceeds the value.
We built ReplenishRadar to handle this transition. Connect your Shopify or Amazon store and forecasts generate automatically from your sales history. The system picks up seasonality, flags when velocity changes, and tells you when to reorder, all without a spreadsheet you have to babysit. If you sell on Shopify and want to compare your options first, we ranked the best Shopify inventory management software by forecasting depth, channel support, and pricing.
You can also connect an AI agent to your inventory and query your forecasts conversationally. Ask "what is the 30-day forecast for SKU-X?" and get an answer in seconds instead of digging through dashboards.
Try ReplenishRadar free for 14 days ->
Getting Started Today
- Export sales data: pull the last 12 months by SKU from your platform
- Calculate velocity: average daily sales per product for the most recent 30 days
- Document lead times: call your suppliers, get their quoted lead times, then add a few days
- Set reorder points: use the formula above
- Review monthly: forecasting improves with iteration, not perfection
The sellers who forecast well are not smarter. They just have better data and they actually look at it.
Related Reading:
- Best Inventory Forecasting Software: 7 Tools Compared
- Safety Stock Calculation Guide
- Demand Forecasting Whitepaper
- Reorder Point Calculator
- Stockout Cost Calculator
- Why One Formula Can't Forecast Every SKU
- Your Forecast Accuracy Score, Explained
- SKU-Level Demand Forecasting - per-product forecasting instead of category averages
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