Stop guessing when to reorder. This calculator uses real demand variability and lead time to tell you exactly when to place the next PO — and how much safety stock you need to hit your service level.
Units sold per day for this SKU.
Days from PO to receiving stock.
Standard deviation as a % of avg demand. 25% = typical for stable brands; 50%+ for seasonal SKUs.
% of demand you commit to fulfill without stockouts. 95% is the standard target.
Units in inventory right now.
Place a new PO when stock hits this number.
Buffer for demand spikes during lead time.
Units you expect to sell during lead time.
At current pace, days until you hit the reorder point.
You're not at the reorder point yet. Keep monitoring — at the current sell rate, you'll hit it on schedule.
Most operators set a flat reorder threshold ("reorder when we hit 100 units") that ignores both lead-time variability and demand variability. The result: stockouts on hot SKUs and overstock on slow ones, often at the same time. Reorder point is supposed to be a *moving* number per SKU.
Reorder Point = (Avg Daily Demand × Lead Time) + Safety Stock. Safety Stock = Z-score × σ × √Lead Time, where Z is the service-level multiplier (1.65 for 95%, 2.33 for 99%) and σ is the standard deviation of daily demand. The √Lead Time term is the part most spreadsheets get wrong — variability scales with the square root of time, not linearly.
Service level is the % of demand you commit to fulfill without stocking out. 95% is the default for most DTC SKUs. For star products with high margin and high stockout cost, 97-99%. For long-tail or low-margin SKUs, 90% is fine — protecting them with deep safety stock ties up cash that would do more elsewhere.
The formula assumes daily demand is roughly normally distributed and stable. It breaks for: launches (no history), pre-orders (deterministic demand), seasonal spikes (use a seasonal multiplier), and SKUs with promotional volatility. For these, model the spike separately and add it on top of safety stock — don't just trust the σ from a quiet quarter.
Most teams use the supplier's *quoted* lead time (the optimistic one). Real lead time is quoted lead time + customs delays + production slips + transit variability. Track actual lead time from the last 3-5 POs and use the 80th percentile, not the average. Underestimating lead time is the #1 cause of stockouts.
Inventory Manager by Ecombone calculates reorder points, safety stock, and demand forecasts per SKU — across all your sales channels — and pings you before you stock out.