Mode selection is arithmetic plus judgement. The arithmetic gets you to two candidates; the judgement picks between them.
01The four modes at a glance
| Mode | Typical Bangkok → Europe | Relative cost | Best for |
|---|---|---|---|
| Sea FCL | 28–35 days | 1× | Volume, low value density, stable demand |
| Sea LCL | 32–42 days | 2–4× per CBM | Small volumes, sample runs, mixed SKUs |
| Air | 3–7 days | 8–15× | High value, perishable, urgent, light |
| Rail (via China) | 18–25 days | 2–3× | Middle ground on Asia–Europe lanes |
02The value-density test
The single most useful number is value per kilogram. Divide the invoice value by the gross weight.
- Under ~USD 5/kg — sea, almost always. Air freight will eat the margin.
- USD 5–20/kg — it depends. Run the inventory-carrying cost calculation below.
- Over ~USD 20/kg — air is often cheaper in total cost, because capital tied up in a 30-day sea transit costs real money.
Inventory carrying cost during transit is roughly: goods value × annual cost of capital × (transit days ÷ 365). On USD 200,000 of cargo at 8% capital cost, 30 days of sea transit costs about USD 1,315 in carrying cost alone — before you count the safety stock the long lead time forces the client to hold.
03Chargeable weight — the air freight trap
Airlines bill on the greater of actual weight and volumetric weight.
Volumetric weight (kg) = length × width × height in cm ÷ 6000.
A carton 100 × 60 × 50 cm weighing 40 kg has a volumetric weight of 50 kg. You bill 50 kg, not 40. Quote on actual weight and you have given away the difference.
04Sea LCL vs FCL — the break-even
LCL is sold per CBM (or per 1,000 kg, whichever is greater — the revenue ton). FCL is a flat rate per box.
A 20' container holds roughly 28 CBM usable; a 40' about 58 CBM; a 40'HC about 68 CBM.
Divide the FCL all-in rate by usable CBM to get an effective per-CBM cost, then compare with the LCL rate. In practice the break-even sits around 13–15 CBM for a 20', because LCL carries heavy destination handling charges that FCL does not. Above that, take the box even if it ships half empty.
05When judgement overrides the arithmetic
- Cut-off risk. If a client's production finishes two days before the vessel cut-off, one delay means an extra week. Air the first shipment, sea the rest.
- Destination congestion. A cheap ocean rate into a congested port is not cheap once demurrage runs.
- Cargo character. Lithium batteries, aerosols, and temperature-controlled goods narrow the choice before cost ever enters.
- Client cash flow. Sea freight ties up working capital for a month. Some clients would rather pay air and turn stock faster.
Recommend a mode, give the number behind it, and name the risk. That is what separates a quote from advice.