Advanced Demand Planning Calculator for Engineering

Calculator inputs

This page uses a single-column section flow. The form below switches to three columns on large screens, two on smaller screens, and one on mobile.

Historical demand series

Enter comma or new-line values, oldest to newest.

Weighted moving average weights

Use oldest-to-newest weights, such as 1, 2, 3.

Exponential smoothing alpha

Higher alpha reacts faster to recent changes.

Active planning method

The chosen method drives the stock and capacity plan.

Days per historical period

Examples: 7 for weekly, 30 for monthly.

Forecast horizon periods

Number of periods to cover in the plan.

Service level (%)

Used to convert fill reliability into a z-score.

Lead time (days)

Time from order release to supply receipt.

Review period (days)

Extra coverage between replenishment checks.

Current inventory

On-hand stock available today.

Incoming confirmed supply

Released purchase or production already due in.

Scrap rate (%)

Used to gross up required production output.

Unit cost

Used for the safety-stock carrying-cost estimate.

Annual holding rate (%)

Inventory cost percentage for the buffer estimate.

Capacity per shift

Usable units a single shift can complete.

Shifts per day

Average staffed shifts available each day.

OEE (%)

Availability, performance, and quality adjustment.

Demand standard deviation override

Leave blank to use the historical sample deviation.

Actual next-period demand

Optional. Used for error, MAPE, and accuracy checks.

Example data table

Period	Demand	Comment
P1	120	Baseline weekly engineering demand.
P2	132	Small rise after a maintenance release.
P3	128	Normal pullback after replenishment.
P4	141	Demand improved with field usage growth.
P5	150	Higher consumption from project ramp-up.
P6	158	Recent period confirms upward demand pressure.

You can replace the sample history with spare-parts demand, consumables usage, assemblies, or engineering work-order pulls.

Formula used

Weighted Moving Average: Forecast = Σ(Weight × Demand) / Σ(Weight)

Exponential Smoothing: Next Forecast = α × Current Demand + (1 − α) × Previous Forecast

Linear Regression Forecast: Forecast = a + b × x, where a is intercept and b is slope.

Daily Demand: Daily Demand = Forecast per Period / Days per Period

Safety Stock: Safety Stock = z × σ_d × √(Lead Time + Review Period)

Reorder Point: ROP = Daily Demand × Lead Time + z × σ_d × √Lead Time

Target Stock: Target Stock = Daily Demand × (Lead Time + Review Period) + Safety Stock

Planned Production: Planned Production = (Forecast Horizon Demand + Safety Stock − Available Stock) / (1 − Scrap Rate)

Available Capacity: Capacity = Capacity per Shift × Shifts per Day × Planning Days × OEE

MAPE: MAPE = |Actual − Forecast| / Actual × 100

How to use this calculator

Enter at least three historical demand values in chronological order.
Add recent-weight values for the weighted moving average method.
Choose alpha, service level, lead time, and review period.
Enter current stock, incoming supply, scrap rate, and capacity inputs.
Select the active planning method that should drive the plan.
Optionally add actual next-period demand to measure forecast error.
Press Calculate Demand Plan to show the result above the form.
Use the CSV or PDF buttons to export the summary.

FAQs

1) What does this demand planning calculator estimate?

It estimates next-period demand, safety stock, reorder point, target inventory, planned production, capacity usage, forecast error, and projected ending inventory from historical engineering demand.

2) Which forecast method should I choose?

Use weighted moving average for short, recent patterns. Use exponential smoothing for stable recurring updates. Use regression when demand clearly trends upward or downward. Use the blended method when you want balance.

3) Why is safety stock included?

Safety stock protects availability during uncertain demand and lead times. A higher service level or higher demand variability raises the required buffer.

4) What is the reorder point telling me?

It marks the inventory position where replenishment should begin. When available inventory falls near that level, you should release supply to avoid shortages during lead time.

5) How does scrap rate affect planning?

Scrap reduces usable output, so the calculator increases planned production to compensate. Higher scrap means more units must be built or purchased to satisfy the same demand.

6) What does capacity gap mean?

A positive gap means required production exceeds available capacity. A negative value means the current shifts and OEE can cover the plan with spare room.

7) Why can forecast accuracy show N/A?

Accuracy needs an actual next-period demand value. If you leave that field empty, the calculator still plans stock and capacity but skips MAPE and forecast accuracy.

8) Can I use weekly or monthly data?

Yes. Set the days per period to match your history. Use 7 for weekly planning, around 30 for monthly planning, or any custom engineering review cycle.