Transparency is core to what we do at CarbonTrail. This post explains exactly how we calculate shipping emissions, the assumptions we make, and why we've chosen a conservative approach.
The Basic Formula
At its core, our calculation is straightforward:
Let's break down each component.
Distance Calculation
We use the Haversine formula to calculate great-circle distance between two points on Earth. This gives us the shortest distance between your warehouse and the customer's shipping address.
Important notes:
- We use postcode centroids for coordinates, not exact addresses
- We calculate direct distance, not actual routing
- This typically underestimates actual travel distance by 10-20%
Weight Determination
We determine shipment weight in this priority order:
- Shopify line item weights (most accurate)
- Cached product weights from your catalog
- Your configured default weight
- Global fallback of 500g
We recommend ensuring your Shopify products have accurate weights for the best calculations.
Emission Factors
Our emission factors are based on industry averages from EPA and DEFRA sources:
| Transport Mode | Factor (kgCO₂/km/kg) |
|---|---|
| Ground | 0.00012 |
| Air | 0.0006 |
| Ocean | 0.00002 |
Transport Mode Inference
Since we don't always know the actual transport mode, we infer it based on:
- Same country: Ground transport
- Adjacent countries with land border: Ground transport
- International + distance over 3000km: Air transport
- Default international: Air transport (conservative)
Why Conservative Estimates?
We deliberately err on the side of overestimation rather than underestimation. Here's why:
- Credibility: It's better to offset more than necessary than to under-offset
- Regulatory safety: Conservative claims are harder to challenge
- Unknown factors: We can't account for hub-and-spoke routing, returns, or failed deliveries
What We Don't Calculate
To be clear about our scope, we do NOT include:
- Product manufacturing emissions
- Packaging production
- Warehouse operations
- Last-mile delivery specifics
- Return shipments (tracked separately)
Versioning and Updates
All calculations are tagged with a methodology version number. When we update our factors or approach:
- Historical calculations retain their original version
- New calculations use the updated methodology
- We notify merchants of material changes
- Recalculation is available on request
For complete technical details, see our full Methodology Document.