Own-data Safety Stock Simulator

Illustrative estimate

These numbers are an illustrative estimate from a teaching simulation of the scenario you ran. They are not a forecast, a recommendation, or advice for your business.

The estimate uses three assumptions:

A single average weekly forecast stands in for the whole year.
Demand, lead time, and delivery quantity move inside fixed variability bands.
Each week's forecast is split evenly across seven days. Once a week the plan is rebuilt from the forecast and from orders already released. A new receipt cannot arrive until the full lead time has passed, and the plan keeps the normal lot quantity until the goods actually arrive.

The calculation runs in this browser. SKU names, forecasts, stock, and prices are not stored and are not sent to a server. After you edit, the page address holds those numbers after the #, in a #d= fragment. That part of the address stays on your device. Anyone with the link can read the numbers.

Practical Stock Planner is a separate Windows app that plans stock by weeks of cover. It does not calculate safety stock. This page is not a cut-down version of that app.

The teaching example, with its fixed prices, is still at the safety stock simulator.

Show explanations

Mode

SKUs

Up to 10 SKUs. Delivery quantity and the demand shock are shared. Everything else belongs to the SKU selected above. Switching SKU does not start a run.

Paste from a spreadsheet

Paste tab- or comma-separated rows, including a header. Preview the rows before they replace the SKUs on this page. Download an example.

Spreadsheet rows

Preview. Nothing is applied until you use these SKUs.
Row	SKU	Weekly forecast	Lead time	Current stock	Unit cost	Selling price	Check

This SKU

SKU name Weekly forecast Current stock Unit cost Selling price

The weekly forecast is one average for all 52 weeks. It replaces the Level, Seasonal, and Rising patterns on the teaching page. Current stock is the opening balance. It is not calculated from the forecast.

Replenishment

Lot size Fixed lot quantity Safety stock Fixed safety stock Lead time (weeks)

Each week's forecast is split evenly across seven days, and those days add up to the week. A receipt due on a day is in stock before that day's demand. A day with no receipt takes demand off the opening stock. At the start of each week the plan is rebuilt from the forecast and from orders already released, counted on the date they are still expected. A new receipt cannot arrive until the full lead time has passed. An order released in week 1 with a 5-week lead time is due on the first day of week 6, before that day's demand. Until the goods arrive the plan keeps the normal lot quantity. A miss is recognised when that day passes with no receipt. The plan then expects that same quantity one week later, and moves the expectation forward again if the goods still have not arrived. It does not learn the actual arrival date in advance. A short shipment is recognised only when the goods arrive. The release is as many normal lots as the forecast needs to finish the week at or above safety stock. An order already on the way is counted on its expected date, and another lot can be due that same week if the forecast still finishes below safety stock. Lead time is any whole number of weeks from 1 to 26. Lead-time variability adds whole extra weeks after that lead time and cannot bring the receipt forward.

Formula safety stock is round(Z × σD × √L). Z comes from the service level you pick: a higher service level uses a bigger Z, so the buffer is larger. σD is how far weekly demand usually sits from the forecast. L is the lead time in weeks. Multiplying by the square root of L spreads that weekly variation over the weeks you wait for an order. The result is extra stock on top of the forecast. It assumes a fixed lead time and random demand only. With demand variability off and a flat forecast, σ is 0, so the formula result is 0.

Variability

Demand for this SKU Lead time for this SKU Delivery quantity for every SKU

Demand variability defaults to Medium. Off uses the weekly forecast as the demand. Small and Medium stay nearer the forecast. Large is the extreme band, from none of the weekly forecast up to double it. The forecast is the expected mean. Across many runs those weeks average to the forecast. One year's realized mean can sit above or below it. These are the same bands as the teaching simulator. The draw is for the whole week, then that week is split evenly across its seven days. Delivery quantity is shared by every SKU.

Demand shock

Double three demand weeks

The shock is shared by every SKU. When it is on, three weeks from week 21 to week 52 are doubled. Those weeks are chosen from the sample number, at least four weeks apart.

One simulated year

No year yet. Press Run year at the end of the settings.

Illustrative estimate

Annual gross profit counts only units actually sold. Demand is filled from the stock available that morning, after a receipt due that day. Stock that is already below zero before the receipt is still a stockout. Inventory turns and working capital use the daily points on the chart. A plotted point below zero counts as zero, so working capital is not negative. The table stays one row per week. A week is out of stock if stock is below zero before a receipt due that morning, or after demand when nothing arrives.

Stock on hand
Safety stock
Zero

One year, one point per day: the stock after that day's demand. A receipt day also shows the stock just before the receipt. Monte Carlo does not draw this chart.

SOH means stock on hand, the units in stock. The table is one row per week. Beginning SOH is the stock at the start of the week. Pre-receipt SOH is the lowest stock before a receipt is added: the morning stock on a receipt day, and the stock after demand on every other day. It is not the lowest close after a receipt. Ending SOH is the stock at the end of the week. A week is out of stock if stock is below zero before a receipt due that morning, or after demand. Touching zero is not a stockout.

Weekly table

Weekly results for this SKU. Illustrative estimate.
Week	Base demand	Simulated demand	Beginning SOH	Pre-receipt SOH	Receipts	Ending SOH	Safety stock

These numbers are an illustrative estimate from a teaching simulation of the scenario you ran. They are not a forecast, a recommendation, or advice for your business.

Monte Carlo

No Monte Carlo batch yet. Press Run Monte Carlo at the end of the settings.

Illustrative estimate

Out-of-stock weeks, customer service, inventory turns, and average working capital show the median and the P10–P90 band. Annual gross profit (units sold) shows the median only. It counts only units actually sold. Demand that the morning stock, after a receipt due that day, could not fill is left out.

Per-run results

One row per retained year. There is no Monte Carlo chart.
Run	OOS weeks	Customer service	Inventory turns	Average working capital	Annual gross profit (units sold)	Safety stock

These numbers are an illustrative estimate from a teaching simulation of the scenario you ran. They are not a forecast, a recommendation, or advice for your business.

P10–P90 is the likely range. In 8 out of 10 simulated years, the result falls inside it.

All SKUs

Illustrative estimate

No results yet. Press Run year or Run Monte Carlo.

One row per SKU. The total adds working capital and gross profit only.
SKU	Out-of-stock weeks	Customer service level	Inventory turns	Average working capital	Annual gross profit (units sold)

In Monte Carlo, the table shows medians. P10–P90 is the likely range: in 8 out of 10 simulated years, the result falls inside it. The total adds those medians. It is not the median of one combined year. Out-of-stock weeks, customer service, and turns are not added across SKUs. Annual gross profit counts only units actually sold.

Illustrative estimate

Mode

SKUs

Paste from a spreadsheet

This SKU

Replenishment

Variability

Demand shock

One simulated year

Monte Carlo

All SKUs

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