Safety stock calculator, with reorder point and EOQ
How much buffer to hold, the stock level that should trigger an order, and how many to order when it does. Both safety stock formulas, your numbers, nothing to sign up for. Written for sellers whose stock is shared between more than one channel, because that is where the textbook version quietly goes wrong.
Across all the channels that share this stock, not one shop. The busiest day and the longest lead time you have actually seen, not the ones you fear.
=STDEV of a column of daily units. Leave blank to use the busiest-day method only.
How often you accept running out while you wait for a delivery.
Shipping, handling, your time. Not the goods.
Storage, insurance, damage, tied-up cash. 20–30% is usual for small goods.
Safety stock and reorder point
How much to order
Turnover and cover
What the calculator is doing
Copy any of these into a spreadsheet. The inventory control and multichannel templates below already carry the reorder point and status columns.
| Figure | Formula | In practice |
|---|---|---|
| Safety stock (busiest-day method) | (busiest day × longest lead time) − (average day × usual lead time) | Needs only four numbers you already know. Tends to run high, because it assumes the worst day and the worst delivery coincide. |
| Safety stock (statistical method) | Z × standard deviation of daily sales × √lead time | Z is 1.65 for a 95% service level. Needs a column of daily sales to take the deviation from; the templates below give you one. |
| Reorder point | (average day × usual lead time) + safety stock | Order when on hand, across every channel, drops to this number. Not when one shop’s count does. |
| Economic order quantity | √(2 × annual demand × cost per order ÷ holding cost per unit per year) | Holding cost per unit is unit cost × the yearly percentage. The answer is a quantity to order each time, not a stock level. |
| Inventory turnover | annual cost of goods sold ÷ average inventory at cost | Times a year the stock sells through. Average inventory is (opening + closing) ÷ 2, or on hand × unit cost if you only have today. |
| Days of inventory | 365 ÷ turnover | How long today’s stock lasts at the current rate. |
Four mistakes the arithmetic can’t see
One shop’s count
The reorder point is compared with the stock one channel shows, while two other channels sell the same units. The trigger fires late by exactly the sales it couldn’t see.
Lead time from the invoice
Lead time is order-to-shelf, not order-to-dispatch. Add the days the box sits unopened; for most small sellers that is the difference between 10 days and 14.
Average day, peak month
A yearly average daily sales figure is wrong for October. Run the calculator per season with that season’s busiest day, or the Q4 stockout is already in the numbers.
Safety stock as a round number
“Keep ten spare” is a reorder point with the maths removed. For a SKU selling one a week, ten is two months of cash; for one selling ten a day it is a single busy morning.
The templates that carry these columns
The inventory control template has a reorder point and reorder quantity per SKU and a status that turns amber when on hand drops to the point. The multichannel sheet adds a column per channel so the point is compared with the total, not one shop.
What sellers ask about safety stock
How do you calculate safety stock?
Two common ways. The simple one: (busiest day’s sales × longest lead time) − (average daily sales × usual lead time). The statistical one: Z × the standard deviation of daily sales × the square root of lead time, where Z is 1.65 for a 95% service level. The calculator above runs both when you give it a standard deviation, and the simple one when you don’t.
What is the difference between safety stock and reorder point?
Safety stock is the buffer you keep for the days when sales or deliveries don’t behave. The reorder point is the stock level that triggers an order: average daily sales × lead time, plus that buffer. You hold safety stock permanently; you hit the reorder point once per cycle.
What service level should I use?
95% is the usual default: you accept running out in about one replenishment cycle in twenty. Go higher for the 20% of SKUs that make 80% of your sales, lower for slow lines where the cash tied up costs more than the occasional stockout. Each step up from 95% roughly doubles the Z-value’s marginal cost in stock.
Why does the calculator ask for sales across all channels?
Because the stock is shared. If the same 90 candles sell on Shopify, Etsy and a Saturday market, the reorder point has to be compared with the total on hand, not with one shop’s count. Most oversells happen when each channel runs its own reorder point against a count the other channels have already drawn down.
How is EOQ different from the reorder point?
The reorder point says when to order; EOQ says how much. EOQ balances the cost of placing orders against the cost of holding stock: order more often and you pay more shipping and handling, order in bulk and you pay more storage and tie up more cash. It’s a quantity per order, and it doesn’t depend on lead time at all.
Do I still need safety stock with inventory sync software?
Yes, for supplier and demand variability, which software doesn’t change. What sync software removes is the other reason sellers pad stock: not knowing the true count. QuickSync writes a new count to every connected shop in under a second on eight of its twelve channels and checks eBay, Etsy and Squarespace stock every two minutes, so the buffer only has to cover the supplier, not the spreadsheet.
The buffer only has to cover the supplier
QuickSync keeps one count behind up to eight shops and corrects the others the moment one sells, so your safety stock covers late deliveries and busy Saturdays, not a spreadsheet that was right yesterday.
No credit card. After the trial, syncing stops if you walk away. Nothing is removed from your shops.