Why the Goldilocks Principle matters in South African logistics
For distributors and transport operators in South Africa—whether running a Gauteng-based FMCG depot or managing imports through the Port of Durban—the right inventory level is "just right": enough stock to meet demand, but not so much that cash is tied up in pallets sitting in a warehouse. Below are 10 practical formulas with local examples you can apply today.
10 inventory planning formulas (with examples)
1. Economic Order Quantity (EOQ)
Formula: EOQ = sqrt(2DS / H) Where D = annual demand, S = ordering cost per order, H = annual holding cost per unit.
Example: Annual demand 12,000 units; ordering cost R500; holding cost R20/unit/year. EOQ = sqrt(2*12,000*500 / 20) ≈ 775 units. Order size of ~775 minimises ordering + holding costs.
2. Reorder Point (deterministic)
Formula: ROP = daily demand × lead time (days)
Example: Monthly demand 1,000 units → daily demand ≈ 33.3. If supplier lead time is 30 days, ROP ≈ 1,000 units. When on-hand stock falls to ~1,000, place an order.
3. Safety Stock (service level & variability)
Formula (practical): SS = z × sqrt(L×σd² + d²×σL²) Where z = service-level z-score (≈1.65 for 95%), L = lead time (days), σd = std dev of daily demand, σL = std dev of lead time.
Example: d=33.3, L=30, σd=20 units/day, σL=5 days, z=1.65 → SS ≈ 330 units. This protects against variable demand and variable lead times—important when port congestion increases σL.
4. Reorder Point with Safety Stock
Formula: ROP = d×L + SS
Example: Using above numbers, ROP ≈ 1,000 + 330 = 1,330 units.
5. Average Inventory
Formula: Average Inventory = Q/2 + SS
Example: Q=775, SS=330 → Average inventory ≈ 388 + 330 = 718 units. Use this to estimate space and working capital requirements.
6. Orders Per Year
Formula: Orders/year = D / Q
Example: 12,000 / 775 ≈ 15.5 → roughly 16 orders/year. Useful for scheduling transport and negotiating freight rates.
7. Total Annual Cost (excluding purchase)
Formula: Total Cost = (D/Q)×S + (Q/2)×H
At EOQ this cost is minimised—useful when comparing different suppliers or warehousing options in Cape Town vs Gauteng.
8. Inventory Turnover
Formula: Turnover = Annual demand / Average inventory
Example: 12,000 / 718 ≈ 16.7 turns per year. Higher turnover often reduces holding costs but may raise transport frequency.
9. Days of Inventory
Formula: Days on hand = 365 / Turnover
Example: 365 / 16.7 ≈ 22 days. Shows how long stock will cover sales—vital for seasonal peaks like December trading.
10. Fill Rate
Formula: Fill rate (%) = (Units supplied on time / Units demanded) × 100
Example: If 11,900 of 12,000 units were supplied on time → fill rate ≈ 99.2%. Targets help decide if safety stock or faster freight is needed.
Local tips for applying formulas
- Adjust H for rand volatility: carrying cost should include insurance, storage and opportunity cost (use higher H when currency risk pushes up landed costs).
- Factor port and road variability: Durban congestion and cross-border clearance delays increase σL—raise SS before peak seasons.
- Use EOQ as a guide: round to pallet quantities and consider supplier minimums—EOQ may suggest 775, but you might order 800 or 1,000 to match pallets.
- Review monthly: run these formulas monthly during high season (Nov–Jan) and quarterly otherwise.
Conclusion
These 10 formulas give South African transport and logistics businesses practical tools to balance service and cost. Start by calculating EOQ and ROP with safety stock for your top SKUs, then monitor fill rate and turnover to fine-tune. Small changes in lead-time variability or ordering frequency can save significant rand in freight and working capital.
Actionable first step: pick five high-value SKUs, compute EOQ, ROP and SS this week, and compare current stock policies—then adjust orders and freight accordingly.