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Spare Parts Inventory Calculator

Optimize Min/Max Stock Levels, Safety Stock & Reorder Points

Balance availability vs. carrying cost to determine optimal inventory levels

Part Information
$
Purchase price per unit
Average time from order to receipt
Standard deviation of lead time (±)
Historical consumption per year
Demand fluctuation
How many are currently in use
Probability part is in stock when needed
Criticality & Cost Factors

Equipment Criticality Assessment

Cost Parameters
$
Cost per hour when equipment down
$
Admin cost per purchase order
% of part cost to store/year (typical: 20-30%)
$
Extra cost for rush delivery
Run-to-Failure

Order when needed. Low value, non-critical.

Stock Item

Maintain min/max. Calculated levels.

Insurance Spare

Always have 1+ on hand. Critical items.

Recommended Stock Levels
Minimum Stock
2
units
Reorder trigger point
Reorder Point
3
units
Place order at this level
Safety Stock
1
units
Buffer against variability
Maximum Stock
5
units
Upper inventory limit
Order Quantity
3
units
Optimal order size (EOQ)

Inventory Level Visualization

Target Service Level 95%
80% 90% 95% 99% 99.9%
Annual Inventory Value
$75
Based on avg stock level
Annual Holding Cost
$19
Cost to store inventory
Expected Stockout Cost
$125
Risk-weighted annual cost

📋 Stocking Recommendation

Based on the inputs provided, this part should be stocked with calculated min/max levels.
  • Min Stock (2): Ensures coverage during lead time with 95% confidence
  • Reorder Point (3): Trigger purchase order when stock reaches this level
  • Max Stock (5): Prevents overstocking while maintaining availability
  • Order Quantity (3): Economic order quantity balances ordering vs. holding costs
Formulas & Methodology

Safety Stock Formula

SS = Z × σd × √LT
Z = Service factor (1.28 for 90%, 1.65 for 95%, 2.33 for 99%)
σd = Standard deviation of demand
LT = Lead time in same units as demand

Reorder Point Formula

ROP = (Daily Demand × Lead Time) + Safety Stock
The inventory level at which a new order should be placed to avoid stockout during lead time.

Economic Order Quantity (EOQ)

EOQ = √(2 × D × S / H)
D = Annual demand
S = Ordering cost per order
H = Annual holding cost per unit

Min/Max Method

MIN = Daily Demand × (Lead Time + Safety Days)
MAX = MIN + EOQ (or MIN × 1.5 to 2.5)
Min triggers reorder, Max prevents overstocking.

📊 Z-Score Reference (Service Factors)

80%
0.84
90%
1.28
95%
1.65
97.5%
1.96
99%
2.33
99.9%
3.09
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