Understanding how freight is calculated helps you estimate costs accurately and optimise your packaging to avoid overpaying. This guide explains the calculation methods used for air, sea, and road freight.
The Golden Rule: Weight vs Volume
Carriers charge based on whichever is greater: actual weight or volumetric (dimensional) weight. This is because a large, light package takes up more space than a small, heavy one — and the carrier needs to cover that space cost.
Sea Freight Calculations
CBM (Cubic Metres) for LCL
For LCL shipments, freight is charged per CBM:
CBM = Length (m) × Width (m) × Height (m)
Example: 10 cartons, each 60cm × 40cm × 50cm
Per carton: 0.60 × 0.40 × 0.50 = 0.12 CBM
Total: 0.12 × 10 = 1.2 CBM
At R1,000/CBM, freight would be R1,200 (most carriers have a minimum of 1 CBM).
Weight/Measurement Ratio
Sea freight uses a 1:1000 ratio — 1 CBM = 1,000 kg.
If your 1.2 CBM shipment weighs 2,000 kg, the carrier charges on 2 CBM (weight-based) because:
Weight-based: 2,000 kg ÷ 1,000 = 2.0 CBM
Volume-based: 1.2 CBM
Chargeable: 2.0 CBM (higher value)
FCL Pricing
FCL is charged per container, regardless of how much you fill it:
- 20ft container — ~33 CBM, max ~28 tonnes
- 40ft container — ~67 CBM, max ~26.5 tonnes
- 40ft HC — ~76 CBM, max ~26.3 tonnes
Tip: FCL becomes cheaper than LCL at approximately 15+ CBM. Compare LCL vs FCL →
Air Freight Calculations
Volumetric Weight Formula
Air freight uses a 1:6000 ratio (IATA standard):
Volumetric weight (kg) = Length (cm) × Width (cm) × Height (cm) ÷ 6,000
Example: Carton 60cm × 40cm × 50cm, actual weight 8 kg
Volumetric: (60 × 40 × 50) ÷ 6,000 = 20 kg
Actual: 8 kg
Chargeable: 20 kg (volumetric is higher)
At R50/kg, the freight cost is R1,000 (charged on 20 kg, not 8 kg).
Multiple Cartons
Calculate total volumetric and total actual, then compare:
| Carton 1 | Carton 2 | Carton 3 | Total | |
|---|---|---|---|---|
| Dimensions (cm) | 60×40×50 | 80×60×40 | 50×50×30 | |
| Volumetric (kg) | 20.0 | 32.0 | 12.5 | 64.5 |
| Actual weight (kg) | 25 | 15 | 18 | 58 |
| Chargeable | 64.5 kg |
Road Freight Calculations
Domestic SA Road Freight
Road freight pricing depends on:
- Distance — Durban to Johannesburg vs Durban to Cape Town
- Weight — Charged per tonne or per load
- Vehicle type — 1-tonne bakkie vs 34-tonne interlink
Palletised cargo is often charged per pallet for part-loads.
Cross-Border Road Freight
Cross-border adds:
- Border clearance time
- Customs documentation at borders
- Transit bonds for goods moving through countries
Optimise Your Freight Costs
- Pack efficiently — Reduce carton sizes to minimise volumetric weight
- Consolidate shipments — Combine smaller orders into one
- Choose the right mode — Air vs sea comparison →
- Ship FCL when possible — Better per-unit cost above 15 CBM
- Avoid peak season — Aug-Nov sees highest sea freight rates