Per-package calculations: why tkm eliminates double counting

When the same truck carries several packages, a question arises: how do you correctly assign the trip’s emissions to each package? The answer lies in the tonne-kilometre.

The double-counting problem

Example:

• 1 shipment from Madrid to Seville (500 km)
• 10 packages inside
• All 10 travel together in the same truck

If the software adds up kilometres per package:

Package 1: 500 km
Package 2: 500 km
Package 3: 500 km
...
Total: 5,000 km  ← ERROR! The truck only travelled 500 km

How the correct per-tkm calculation works

Formula:

tkm = weight (tonnes) × distance (km)

Example with a 10-package shipment:

Package	Weight	Distance	tkm
Package 1	10 kg (0.01 t)	500 km	5 tkm
Package 2	10 kg (0.01 t)	500 km	5 tkm
…	…	…	…
Total 10 packages	100 kg (0.1 t)	500 km	50 tkm

Why is there no double counting?

Because kilometres are not added up — they are weighted by mass.

Correct:
10 packages × 0.01 t × 500 km = 50 tkm

This is identical to calculating the full shipment:
0.1 t × 500 km = 50 tkm

The result is the same — no duplication.

The key is weight

• Sum the tkm of all packages = tkm of the total shipment
• Emissions are calculated on the tkm
• Each package “carries” only its proportional share

Method	Risk of double counting?
Adding kilometres per package	Incorrect — yes
Calculating by tonne-km	Correct — none

The tonne-kilometre is precisely the unit GLEC uses to avoid this problem.

Complete visual example

┌─────────────────────────────────────────────────────────┐
│                    PACKAGE                              │
│                                                         │
│   ┌─────────┐      ┌─────────┐      ┌─────────┐        │
│   │  Leg 1  │ ───► │  Leg 2  │ ───► │  Leg 3  │        │
│   │  500km  │      │  200km  │      │  50km   │        │
│   │ 2t load │      │ 2t load │      │ 2t load │        │
│   └─────────┘      └─────────┘      └─────────┘        │
│                                                         │
│   Total tkm = (500×2) + (200×2) + (50×2) = 1,500 tkm   │
│                                                         │
│   Emissions = Σ (tkm × factor for each vehicle)        │
└─────────────────────────────────────────────────────────┘

Each leg applies the emission factor of its vehicle type, accumulating into the package total.