How we calculate carbon savings
The carbon savings presented across our pledges in the Sustainable Chain programme are based on UK Government greenhouse gas conversion factors, alongside the HM Treasury Green Book appraisal framework and widely used industry carbon calculators and academic methodologies.
All figures are expressed in kilograms of carbon dioxide equivalent (kgCO₂e), which is the standard unit for measuring greenhouse gas emissions and includes carbon dioxide and other gases adjusted for their climate impact.
Methodology
Carbon savings are calculated using a consistent approach:
Carbon saving = baseline activity emissions – low-carbon alternative emissions
Where possible, emissions factors are taken from:
- UK Government DESNZ/DEFRA conversion factors
- Transport and energy datasets aligned with national reporting standards (GOV.UK)
These factors provide kgCO₂e per unit of activity (e.g. per kilometre travelled, per kWh of electricity etc), ensuring consistency with national carbon accounting.
Annual and lifetime savings
Each pledge generates an annual saving based on the behaviour change made. To reflect the long-term impact of a sustained pledge, we also calculate a lifetime saving using HM Treasury Green Book guidance, which is the UK Government’s standard framework for appraising policy and programme impacts.
Under the Green Book, behavioural and administrative changes are assigned a lifetime horizon of 10 years. This is the most defensible UK Government-aligned assumption for individual pledge commitments, and reflects the approach used in public sector carbon appraisal.
Annual saving: the emissions avoided in a single year by making this behaviour change.
Lifetime saving (10 years): the cumulative emissions avoided if the pledge is sustained, calculated as annual saving × 10.
How we reach 8 million tonnes
Our target of 8 million tonnes CO₂e avoided is calculated as follows:
- Average annual saving per pledge across five categories: 1,120 kgCO₂e
- Lifetime saving per pledge (10 years): 11,200 kgCO₂e
- Average number of pledges per person: 2.5
- Lifetime saving per pledger: 11,200 × 2.5 = 28,000 kgCO₂e (28 tCO₂e)
- Target pledgers: 300,000
- Total lifetime impact: 300,000 × 28 = 8,400,000 tCO₂e
We have set a headline commitment of 8 million tonnes as a robust, rounded target grounded in this methodology.
Assumptions by pledge
#1. Recycling
Annual saving: ~200 kgCO₂e per year
Lifetime saving (10 years): ~2,000 kgCO₂e
Assumptions:
- Based on a typical UK household recycling mixed materials (paper, plastic, glass, metals)
- Savings come from avoided ‘virgin’ material production, which is significantly more carbon-intensive than recycling
- Savings vary significantly depending on material type (e.g. aluminium vs plastic), so this figure represents a conservative blended annual estimate
Sources:
- UK conversion factors include emissions from waste disposal vs recycling pathways (GOV.UK)
- Recycling reduces lifecycle emissions by avoiding extraction, processing, and manufacturing of raw materials
#2. Commute via public transport
Annual saving: ~900 kgCO₂e per year
Lifetime saving (10 years): ~9,000 kgCO₂e
Assumptions:
- Daily commute: ~30 km round trip
- 5 days/week, ~48 weeks/year (~7,200 km annually)
- Switching from petrol car to train or public transport
Emissions factors:
- Petrol car: ~0.16 kgCO₂e/km
- Train: ~0.035 kgCO₂e/km
Calculation:
- Baseline (car): ~1,150 kgCO₂e/year
- Alternative (train): ~250 kgCO₂e/year
- Annual saving ≈ 900 kgCO₂e
Cars emit several times more CO₂ per passenger kilometre than rail, making commuting behaviour a major emissions driver.
#3. Switching to an electric vehicle (EV)
Annual saving: ~1,200 kgCO₂e per year
Lifetime saving (10 years): ~12,000 kgCO₂e
Assumptions:
- Annual mileage: ~10,000 km
- Comparison: petrol vehicle vs battery electric vehicle (UK electricity mix)
Emissions factors:
- Petrol car: ~0.16 kgCO₂e/km
- EV: ~0.04 kgCO₂e/km
Calculation:
- Petrol: ~1,600 kgCO₂e/year
- EV: ~400 kgCO₂e/year
- Annual saving ≈ 1,200 kgCO₂e
This includes operational emissions only. Manufacturing emissions are excluded, consistent with most annual footprint calculations.
#4. Switching to a renewable energy tariff
Annual saving: ~1,500 kgCO₂e per year
Lifetime saving (10 years): ~15,000 kgCO₂e
Assumptions:
- Average UK household electricity consumption
- Switching from grid-average electricity to 100% renewable tariff
Sources:
- UK grid electricity includes fossil fuel generation
- Renewable tariffs significantly reduce associated emissions per kWh
- Carbon calculators and UK datasets show savings of ~500–1,500 kgCO₂e/year, depending on household usage
Actual savings depend on household energy consumption and supplier sourcing and certification model.
#5. Skipping one long-haul flight
Annual saving: ~1,800 kgCO₂e per flight avoided
Lifetime saving (10 years): ~18,000 kgCO₂e
Assumptions:
- Return long-haul flight (e.g. UK to North America)
- Economy class
Emissions factors:
- Long-haul flight: ~0.11–0.15 kgCO₂e per passenger-km
Typical outcome:
- Long-haul return flight ≈ 1,500–2,000 kgCO₂e
Flight emissions vary based on distance, aircraft type and seating class (premium seats have a higher per-passenger impact). The lifetime figure assumes one long-haul flight avoided per year for 10 years.
Summary: annual and lifetime savings by pledge
Pledge | Annual saving | Lifetime saving (10 years)
Recycling | ~200 kgCO₂e | ~2,000 kgCO₂e
Commute by public transport | ~900 kgCO₂e | ~9,000 kgCO₂e
Switch to EV | ~1,200 kgCO₂e | ~12,000 kgCO₂e
Switch to renewable energy | ~1,500 kgCO₂e | ~15,000 kgCO₂e
Skip one long-haul flight | ~1,800 kgCO₂e | ~18,000 kgCO₂e
Average across five pledges | ~1,120 kgCO₂e | ~11,200 kgCO₂e
Key considerations
- These are average estimates
All figures represent typical UK scenarios, not exact individual outcomes. Real-world emissions vary depending on behaviour, geography, and technology.
- Scope of emissions
The calculations primarily reflect direct (operational) emissions, rather than full lifecycle impacts (e.g. manufacturing of vehicles or infrastructure).
- Consistency over precision
For comparability, all pledges are calculated using the same emissions datasets and the same annualised approach. This ensures consistency across actions, even where exact real-world values may differ.
- Lifetime horizon
The 10-year lifetime figure is applied consistently across all pledges, in line with HM Treasury Green Book guidance for behavioural interventions. This is deliberately conservative: a pledge sustained for longer would generate proportionally greater savings.
Why this approach is credible
We’ve chosen to use UK Government-backed emissions factors (DEFRA/DESNZ) and the HM Treasury Green Book appraisal framework. This aligns with national greenhouse gas accounting standards, and reflects the methodology used in corporate carbon reporting, sustainability disclosures, and carbon footprint calculators. The Green Book is the same framework used by the UK public sector to appraise the long-term impact of policy interventions — applying it to individual pledges ensures our target is held to the same standard of rigour.
Summary
These pledges represent practical, evidence-based actions individuals can take to reduce their carbon footprint, with savings calculated using recognised UK methodologies and conservative assumptions.
While individual results may vary, the figures provide a robust and credible benchmark for understanding real-world climate impact. Our 8 million tonne target is built from the bottom up — pledge by pledge — using the same emissions factors that underpin UK corporate carbon reporting.
Climate change can feel abstract, so we’ve focused on what people can control: everyday decisions. These figures turn those choices into measurable impact, using consistent and credible data to show how small actions can add up to meaningful carbon savings.
References
UK Government DESNZ/DEFRA Greenhouse Gas Conversion Factors: GOV.UK
Journey emissions comparisons methodology and guidance: GOV.UK
HM Treasury Green Book — appraisal and evaluation in central government: GOV.UK
How we calculate travel emissions — Sustainable Climate Impact Fund: SCIF
UK travel carbon footprint by transport mode — Statista
