# Safety Stock Calculator for Imported Parts with Long and Variable Lead Times

> Source: https://netray.co/tools/safety-stock-calculator-imported-parts-long-lead-times
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Calculate safety stock and reorder point when both demand and supplier lead time vary, as with sea freight into African ports.

## Category

supply-chain

## Erp

SYSPRO

## Industry

Discrete Manufacturing

## Region

Africa

## Audience

- Production planner
- Buyer
- Supply chain manager

## Tool

**Kind:** calculator

**Currency:** R 

**Inputs:**

#### Average daily demand

**Key:** d

**Type:** number

**Default:** 40

**Min:** 0

**Step:** 1

**Unit:** units per day

#### Standard deviation of daily demand

**Key:** sd_d

**Type:** number

**Default:** 14

**Min:** 0

**Step:** 0.5

**Unit:** units

#### Average replenishment lead time

**Key:** lt

**Type:** slider

**Default:** 70

**Min:** 1

**Max:** 150

**Step:** 1

**Unit:** days

**Help:** Order to receipt, including port clearance and inland transport.

#### Standard deviation of lead time

**Key:** sd_lt

**Type:** number

**Default:** 12

**Min:** 0

**Step:** 0.5

**Unit:** days

#### Target cycle service level

**Key:** service

**Type:** select

**Default:** 0.95

**Options:**

##### 90%

**Value:** 0.9

##### 95%

**Value:** 0.95

##### 97.5%

**Value:** 0.975

##### 99%

**Value:** 0.99

#### Unit cost

**Key:** cost

**Type:** number

**Default:** 185

**Min:** 0

**Step:** 5

**Unit:** ZAR

#### Annual carrying cost rate

**Key:** carry

**Type:** number

**Default:** 24

**Min:** 0

**Max:** 80

**Step:** 1

**Unit:** %

**Help:** Include interest, storage, insurance and obsolescence.

**Outputs:**

#### Z score

**Key:** z

**Expr:** normsinv(service)

**Format:** decimal

#### Demand std deviation over lead time

**Key:** sigma

**Expr:** sqrt(lt * sd_d * sd_d + d * d * sd_lt * sd_lt)

**Format:** decimal

**Description:** Combines demand variability and lead time variability.

#### Safety stock

**Key:** ss

**Expr:** z * sigma

**Format:** integer

**Primary:** true

#### Reorder point

**Key:** rop

**Expr:** d * lt + ss

**Format:** integer

**Primary:** true

#### Safety stock value

**Key:** ss_value

**Expr:** ss * cost

**Format:** currency

#### Annual cost of holding safety stock

**Key:** carry_cost

**Expr:** ss_value * carry / 100

**Format:** currency

#### Safety stock cover

**Key:** cover

**Expr:** if(gt(d, 0), ss / d, 0)

**Format:** days

**Chart:**

#### Safety stock vs average lead time

**Type:** line

**X:** lt

**From:** 5

**To:** 150

**Steps:** 30

**Series:**

- ss

**Disclaimer:** Assumes normally distributed demand and lead time that are independent. Intermittent or lumpy demand needs a different method.

## Content

**Intro:** Imports by sea into Durban, Cape Town or Mombasa have long and uneven lead times, and a simple demand-only safety stock formula underestimates the buffer you need. This tool includes lead time variability.

**Sections:**

#### How to use it

**Body:** Take daily demand statistics from the last 6 to 12 months of issues, not sales orders, so the figures reflect actual consumption. Take lead time statistics from purchase order receipts: order date to goods received date. Choose a service level that fits the part's criticality and enter the unit cost and carrying rate to see what the buffer costs.

**Bullets:**

- Compute lead time variability from at least 10 receipts.
- Use a higher service level only for true line-stoppers.
- Rerun after a change of forwarder or route.

#### The formula

**Body:** Safety stock equals the Z score for the service level times the standard deviation of demand during lead time. That standard deviation is the square root of lead time times demand variance, plus demand mean squared times lead time variance. The first term captures demand noise accumulating over the lead time; the second captures the uncertainty of when stock actually arrives. Reorder point is mean demand over lead time plus safety stock.

#### Reading the result

**Body:** Look at the sweep chart. Safety stock grows with the square root of lead time for the demand term but linearly with lead time variability, so reducing the variability, for example by changing the clearing process, often reduces stock more than shaving average days. The carrying cost line shows what each point of service level is costing in rand per year.

#### ERP notes

**Body:** Load the result as safety stock in the item warehouse record so MRP nets it. Set the planning lead time to the realistic figure, not the supplier's quoted ex-works time. Review monthly for A items and quarterly for the rest, and mark parts with a single source and long lead time for extra review.

**Cta:** Netray can help calculate and load safety stock for thousands of parts from your ERP history. Reach out if a spreadsheet is no longer enough.

**Faq:**

#### Why include lead time variability?

**Answer:** Because late containers create stockouts that demand-only formulas never see. If lead time ranges from 55 to 90 days, the uncertainty in arrival date often dominates the buffer, especially for steady-demand parts.

#### What service level should I use?

**Answer:** Ninety five percent suits most items, 97.5 to 99 percent suits true line-stoppers with no substitute, and 90 percent suits cheap C items. Higher levels cost more than they appear: the Z score rises quickly above 97 percent.

#### Is cycle service level the same as fill rate?

**Answer:** No. Cycle service level is the chance of no stockout in a replenishment cycle. Fill rate is the share of demand met from stock. Fill rate is usually higher for the same buffer, so this tool is conservative on that front.

#### What about lumpy demand?

**Answer:** The formula assumes roughly normal demand. For intermittent spare parts, use a Poisson or empirical approach, or a min-max with judgement. Check the distribution before trusting the number.

## Search terms

safety stock calculator, safety stock formula lead time variability, reorder point calculator, mrp safety stock imported parts, sea freight lead time variability, syspro safety stock, inventory planning africa, service level z score

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