Our circular economy expert Simon Hann reflects on why global brands need to understand the concept of ‘mass balance’ following the EU’s recent decision on how recycled content in single-use PET bottles will be measured under the EU’s Single-Use Plastics Directive (SUPD).
What is mass balance?
“Mass balance” is an accounting approach that allocates recycled content across outputs based on the recycled inputs entering a system, rather than physically tracking recycled material into each product. It sounds technical, but the basic idea is simple: what goes into a system must come out of it. Inputs and outputs have to balance.
Where things get complicated is how this idea is applied in practice to track recycled content (RC) through complex industrial processes, particularly in plastics and chemicals. In this context, “mass balance” isn’t just physics; it’s a chain-of-custody model, a set of accounting rules for how recycled material is tracked and allocated as it moves through a system. In practice, this isn’t about tracing individual molecules, but about accounting for quantities of recycled material through mixed production processes.
A common misunderstanding is that mass balance means “there is recycled content in every product.” In reality, it means recycled content is present within a system and allocated to specific outputs under defined rules. There are multiple ways to do “mass balancing”, and the rules you choose materially change the outcome.
The devil is in the details
In mechanical recycling, the recycled material going in can typically all count as recycled content (minus processing losses). But in chemical recycling and integrated petrochemical systems, material is mixed, split across multiple outputs, and lost at different stages of processing. You therefore need to decide how much recycled input can legitimately be claimed as recycled output, and to which products you allocate that recycled content.
Businesses have previously taken a different approach to allocation methods, each with its own pros and cons:
- Scenario 1, proportional approaches: if 100 tonnes of total material enters a system (containing 20 tonnes recycled content and 80 tonnes virgin material), and 40 tonnes becomes polymer output, then only 8 tonnes can be claimed as recycled polymer – representing 20% recycled content (Scenario A). This approach has high credibility but is harder to make commercially viable.
- Scenario 2, more permissive approaches: The full 20 tonnes of recycled input can be allocated to the 40 tonnes of polymer output, resulting in 50% recycled content – or concentrated through credits to produce 20 tonnes of polymer benefits as 100% recycled (Scenario B). This approach is easier to operationalise and helps early market development but weakens the claim’s environmental meaning.
In practice, most real-world schemes sit somewhere in between. The trade-off is always the same: Credibility versus operability. Too strict, and the system struggles to scale. Too loose, and the claims start to lose meaning.
Mass balance systems effectively create recycled content “credits” within a defined system (a plant, site, or group of facilities). These credits can then be allocated to specific outputs within that system.
Mass balance cannot be compared to a “book and claim” system, such as renewable electricity certificates, where physical electrons enter the grid but green attributes are traded separately. The crucial difference is that, under mass balance, the producer must embody the recycled content in the physical products they sell. You are allocating recycled content within your own material flows, rather than trading abstract certificates detached from production.
That said, if the system’s boundaries become very wide – for example, allowing credits to be transferred freely between company-owned plants in different countries – the distinction can start to blur. At that point, the credibility of the claim becomes harder to defend, even if the accounting remains technically compliant.
Why does it matter?
The EU has now embedded its own mass balance rules in legislation (via the SUPD and potentially PPWR), with stricter constraints than many existing voluntary certification schemes. For example, the EU limits how far recycled content credits can be transferred geographically and excludes certain uses (such as fuels) from being allocated towards packaging recycled content claims.
These changes create a real-world problem for global brands. The same product may carry a “recycled content” claim in the EU under a stricter allocation model, but be certified under looser rules elsewhere (for example, using global mass balance schemes such as ISCC PLUS).
A bottle that qualifies as “30% recycled content” under ISCC PLUS certification might not qualify as 30% recycled under EU SUPD rules, even though both are technically “mass balance” systems. The difference lies in the allocation rules, not the physical material.
The result is that the same claim can mean different things across markets, even when the wording looks identical. From a credibility and communications perspective, that is risky. A brand could find itself defending claims that are technically compliant in one region but would not meet regulatory expectations in another.
However, the real “line in the sand” is not about PET bottles, but about how mass balance and chemical recycling are likely to be treated under other future frameworks with broad impacts, such as the Packaging and Packaging Waste Regulation (PPWR).
One important shift under PPWR is that recycled content is not treated as an objective in isolation, but as part of system change towards circularity – designed to deliver wider environmental benefits, including climate mitigation and resource efficiency.
The criteria for measuring recycled content in frameworks such as the PPWR are not yet fully defined and developing them in a technically robust and enforceable way will be difficult.
Tips for brands
Firstly, get clear about which mass balance model you are using internally and understand how that differs across regions. Secondly, think carefully about whether consumer-facing claims remain defensible. Finally, audit your recycled content claims now, not when regulations tighten.
While the EU has come to a position on the SUPD, conversations are still very much alive in other jurisdictions including the US as they consider the role of chemical recycling within EPR.