Calculate Inventory Level

Purpose

This calculator shows how a stock level would move over time under a simple reorder-point ordering policy: whenever available stock (on hand plus already on order) drops to or below a reorder point, a fixed-size order is placed, and it arrives after a lead time. It helps you see how current stock, pending orders, and demand interact over a planning period, and whether a chosen reorder point and order quantity keep stock from running out.

Background

Reorder point and order quantity

Two decisions drive the simulation: the reorder point — the stock level that triggers a new order — and the reorder quantity — the fixed amount ordered each time. Demand steadily reduces stock on hand every day. Once the projected stock position (stock on hand plus quantity already on order but not yet received) falls to or below the reorder point, a new order for the reorder quantity is placed. That order arrives after the stated lead time, adding to stock on the day it is received.

Inputs

Demand

The rate at which the item is consumed, expressed per year by default and applied evenly across every day of the simulation.

Current Stock

The quantity on hand at the start of the simulation (Day 0).

Reorder Point

The stock position at or below which a new order is triggered. The calculator compares this against Current Stock plus any quantity already on order but not yet received, not against on-hand stock alone.

Reorder Quantity

The fixed quantity ordered every time the reorder point is reached.

Lead Time

The number of days between placing an order and receiving it.

Time Horizon

The total length of the simulation.

Calculate Every

How often, at minimum, a row of results is produced (for example, weekly). The calculator also adds an extra row on any day an order is placed or received, even if that day falls between the regular intervals, so events are never missed from the results.

Show

Choose whether to display a Table, a Chart, or Both of the simulated results.

Results

Current Stock

The simulated quantity on hand for each reported day. It decreases daily with demand and increases on days an order is received.

Reorder Point

The reorder point value repeated on every row, shown alongside Current Stock so you can see how close stock is to triggering a new order.

Stock Promised

The quantity currently on order but not yet received (0 once no orders are outstanding). This rises by the Reorder Quantity each time an order is placed and falls by the same amount when that order is received.

Chart / Table of results over time

The table and chart plot Current Stock, Reorder Point, and Stock Promised against Day. A sudden rise in Current Stock marks an order being received; a rise in Stock Promised marks an order being placed. The results do not include a separate marker column for these events — they are visible only as these changes in the plotted values.

Understanding the Calculation

The calculator steps through each day of the Time Horizon:

1. Subtract one day's Demand from Current Stock.
2. If an order placed earlier is due to arrive (its lead time has elapsed), add the Reorder Quantity to Current Stock and remove that amount from Stock Promised.
3. If Current Stock plus Stock Promised has fallen to or below the Reorder Point, place a new order: add the Reorder Quantity to Stock Promised, to arrive after Lead Time days.
4. Record a result row if the day matches the Calculate Every interval, or if an order was placed or received on that day.

Because a new order is only triggered once per day and only when the stock position is at or below the reorder point, more than one order can be outstanding at a time if demand and lead time make that necessary.

Example

Using the default inputs — Demand of 1200 unit/yr, Current Stock of 150 units, Reorder Point of 200 units, Reorder Quantity of 500 units, Lead Time of 30 days, a Time Horizon of 1 year, and results calculated at least every week — the simulation behaves as follows:

• Stock starts at 150 units, already below the 200-unit reorder point, so an order for 500 units is placed on Day 1 (Stock Promised becomes 500).
• Stock continues to fall with daily demand until Day 31, when the order arrives (Current Stock jumps up by 500 units and Stock Promised returns to 0).
• This cycle of gradual decline, a new order near the 200-unit reorder point, and a jump on arrival roughly 30 days later repeats over the 1-year horizon.

This shows that with these inputs, the 500-unit reorder quantity and 30-day lead time keep stock from running out, since a new order is placed well before stock would be depleted.

Important Assumptions and Interpretation

• Demand is assumed constant and applied every day; the calculator does not model seasonal or random demand variation.
• Orders are placed at most once per day and always for the full Reorder Quantity — there is no minimum order quantity check or capacity limit. For the annual cost impact of a supplier-imposed minimum order quantity, see the moq calculator instead.
• The simulation does not report a stockout warning explicitly; if Current Stock falls below zero in the results, demand exceeded available stock before the next order arrived.
• Results are a projection based on the inputs given, not a guarantee of actual future stock levels, which will vary with real demand and supplier performance.