Safety Stock Calculator

Calculate the right safety stock level for each SKU — the buffer inventory you hold to absorb demand spikes and supply delays without going out of stock. Too little and you lose rank every time you stock out. Too much and you're paying storage fees on idle inventory. This prompt finds the right number.

Operations
Amazon < 30 mins Intermediate Error reductionBetter decisions
Last updated: April 20, 2026
The Prompt
Copy and paste into your AI tool 
You are a senior Amazon inventory management analyst. You know that
safety stock is one of the most important and least-calculated
numbers in an Amazon business. Most sellers set safety stock by
gut feel — "keep 30 days on hand" — without accounting for their
specific demand variability or lead time variability. The result
is either chronic stockouts (rank loss, lost revenue) or chronic
overstock (cash tied up, storage fees). Your job here is to
calculate the correct safety stock level for each SKU provided.

I'm going to provide inventory and demand data. Calculate safety
stock and reorder points.

SAFETY STOCK FORMULA:
Safety stock = Z × √(lead time) × demand standard deviation

Where:
Z = service level factor (how confident you want to be that you
  won't stock out):
  - 90% service level: Z = 1.28
  - 95% service level: Z = 1.65
  - 99% service level: Z = 2.33
  Default to 95% unless specified otherwise.

Lead time = average lead time in days (supplier production +
  ocean/air freight + customs + Amazon receiving)

Demand standard deviation = standard deviation of daily units
  sold over the measurement period

If daily demand standard deviation is not directly available,
calculate it from weekly or monthly data:
  Daily StdDev ≈ weekly StdDev ÷ √7
  Daily StdDev ≈ monthly StdDev ÷ √30

REORDER POINT FORMULA:
Reorder point = (average daily demand × average lead time)
  + safety stock

This is the inventory level at which you should place your
next order so that you receive new stock before hitting zero.

DAYS OF SUPPLY AT SAFETY STOCK:
Safety stock days = safety stock ÷ average daily demand
This tells you how many days of cover your safety stock provides.

LEAD TIME VARIABILITY ADJUSTMENT (advanced):
If lead time is itself variable (not consistent), use the
extended formula:
Safety stock = Z × √((average lead time × demand variance)
  + (average daily demand²× lead time variance))

Use this formula if lead time standard deviation is provided
and is > 20% of average lead time.

STEP 1: CALCULATE PER SKU
For each SKU, calculate:
- Average daily demand
- Demand standard deviation (daily)
- Safety stock units
- Reorder point
- Days of supply at safety stock
- Current safety stock vs. recommended (over/under)

STEP 2: CASH IMPACT
For each SKU:
- Current safety stock inventory value = current SS × COGS
- Recommended safety stock inventory value = recommended SS × COGS
- Cash impact of change (positive = cash released, negative =
  additional cash needed)

STEP 3: SERVICE LEVEL SENSITIVITY
Show how safety stock changes at 90%, 95%, and 99% service levels
for each SKU — so the seller can make an informed trade-off
between stockout risk and capital cost.

Output format:

SAFETY STOCK ANALYSIS

SKU RESULTS TABLE
| SKU | Avg Daily Demand | Demand StdDev | Lead Time (days) |
Safety Stock | Reorder Point | SS Days | Current SS | Status |

CASH IMPACT TABLE
| SKU | Current SS Value | Recommended SS Value | Cash Impact |

SERVICE LEVEL SENSITIVITY
| SKU | SS @ 90% | SS @ 95% | SS @ 99% |

BEFORE YOU EXECUTE:

1. If any required input is missing, unclear, or looks malformed,
   stop and ask me a specific clarifying question before proceeding.
   Do not guess or fill in plausible values.

2. If I haven't provided demand variability data (standard deviation
   or enough historical data to calculate it), ask. A safety stock
   calculation without demand variability is just a fixed buffer —
   flag this clearly if you have to estimate.

3. If lead time is highly variable (> 20% coefficient of variation),
   recommend using the extended formula and ask for lead time
   standard deviation.

4. If you are less than 95% confident you understand what I'm asking
   for, ask me to clarify before executing the task.

5. Verify every arithmetic calculation by working it twice. Round
   safety stock figures up to the nearest whole unit.

=====

PASTE YOUR INVENTORY DATA BELOW. For each SKU include: SKU name,
average daily units sold, daily or weekly demand standard deviation
(or enough historical sales data to calculate it), average lead
time in days (total from order placement to Amazon receiving),
lead time standard deviation if known, COGS per unit, and current
safety stock level. Also specify your desired service level (90%,
95%, or 99%) or leave blank to use the 95% default.

[YOUR DATA HERE]
Example Input
What you'd paste after the divider 
Desired service level: 95%

SKU: SPAT-3PK
Average daily units: 14.2
Weekly sales (last 12 weeks): 98, 105, 87, 112, 94, 118, 101, 96,
  122, 88, 109, 103
Average lead time: 52 days (14 days production + 28 days ocean
  freight + 5 days customs + 5 days Amazon receiving)
Lead time variability: sometimes 45 days, sometimes 62 days
COGS per unit: $7.35
Current safety stock: 120 units

SKU: BOWL-SET
Average daily units: 6.1
Weekly sales (last 12 weeks): 44, 39, 48, 41, 52, 38, 47, 43,
  55, 36, 49, 42
Average lead time: 58 days
Lead time variability: fairly consistent, ±5 days
COGS per unit: $11.20
Current safety stock: 80 units
Tips
01

Safety stock is not the same as your reorder point. Safety stock is the buffer you hold against uncertainty — it's the inventory you hope never to touch. The reorder point is the trigger for your next order. Confusing the two leads to either ordering too late or holding too much idle stock.

02

Demand variability matters more than average demand for safety stock calculation. A SKU selling 10 units/day with a standard deviation of 1 needs far less safety stock than one selling 10 units/day with a standard deviation of 5. If you set safety stock based on average demand alone, you're systematically under-protected during volatile weeks.

03

Lead time is almost always longer than sellers think when you include Amazon's receiving time. A 28-day ocean shipment that spends 8 days at an Amazon FC before being received into available inventory is actually a 36-day lead time for planning purposes. Use the total time from order placement to "available to sell" — not just transit time.

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