akar.earthSerial number: Definition
What is a carbon credit serial number? How unique serials track each tonne from issuance to retirement, what they encode, and why they prevent double counting.
A serial number is the unique identifier a registry assigns to each carbon credit when it is issued. Credits are usually issued in batches with a serial range, so every individual tonne of CO2 equivalent can be traced from issuance to retirement.
Why it matters
Serial numbers are the basic defence against double counting. Because each tonne has one identifier in one registry, it can only be sold and retired once. Buyers use serials in contracts and due diligence to confirm exactly which credits they are getting. Auditors, rating agencies and NGOs use them to trace retirements back to projects.
How serial numbers work
- Format: each registry has its own format. Serials typically encode the programme, project, vintage, batch and unit range. ART, for example, issues serialised 1 tCO2e jurisdictional REDD+ credits through the ART registry.
- Transfers: when credits move between accounts, the serials move with them. A block can be split into smaller ranges.
- Retirement and cancellation: retired serials are moved to a retirement account and cannot be used again.
- Cross-registry moves: when units move between registries, the source must cancel them so they do not exist in two places.
Indonesia angle
Indonesia now runs its own registry, SRUK, which replaces SRN-PPI for recording carbon units traded on IDXCarbon. Projects that issue on an international registry and want to sell domestically, or vice versa, must make sure serials are cancelled in one system before units appear in the other. FCPF and ISFL jurisdictional credits can move into the ART registry from the World Bank CATS registry, which is one model of how this works.
Common mistakes
- Writing offtake contracts without specifying serial ranges or vintage.
- Assuming a serial number alone proves quality; it only proves identity.
- Not checking whether a serial has already been retired before paying.
Related terms
See registry, issuance and double counting, or explore the climate finance hub.
Frequently asked questions
What information does a carbon credit serial number contain?
It varies by registry, but usually encodes the programme, project ID, vintage, issuance batch and a range of unit numbers. Registries show further attributes alongside it.
Can I check a credit's serial number?
Yes. Most international registries have public search tools where you can look up issuance and retirement records by project or serial number.