Footprint Widget

Footprint (kg CO₂e) = Σ amount × emission factor

Footprint per unit = total footprint ÷ units (the “functional unit”: one guest, one bag of chips, one use).

Share of a stage = stage footprint ÷ total footprint × 100 %. The biggest stage is the hotspot.

CO₂e = CO₂ equivalent: all greenhouse gases counted as if they were CO₂. Amount in the factor’s unit: kWh, litres, kg, tonne-km.

Example:

Rows: amount × factor

Reuse break-even (uses) = extra footprint to make the reusable item ÷ footprint saved per use. Round up.

Footprint saved per use = footprint of the single-use items one use replaces − washing per use.

Extra footprint to make = what the reusable item “costs” in CO₂e before its first use. Footprint to make = mass × factor.

Example:

Reusable vs single-use

Reusable item

Single-use item

Washing and use

Puzzle: the cotton bag (Danish EPA study)

How often must you use a cotton shopping bag before it beats a simple plastic (LDPE) bag? It depends on what you count.

Bag	Climate change only	All impact categories
Conventional cotton bag	52 uses	7,100 uses
Organic cotton bag	149 uses	20,000 uses

Reference: one simple LDPE carrier bag. Lesson: the answer depends on the impact category and the reference (Bisinella et al., 2018).

Life cycle cost = purchase price + fuel cost per km × km per year × years + other operating cost per km × km per year × years

Crossover (km) = (price B − price A) ÷ (variable cost per km A − variable cost per km B); in years = crossover km ÷ km per year.

GHG break-even (km) = extra GHG to build B ÷ GHG saved per km.

Variable cost per km = energy (fuel or electricity) cost per km + other operating cost per km. Life cycle cost: Heizer et al. (2023), Supplement 5, Equation S5-2.

Example:

Two vehicles

A

B

Use

Greenhouse gas (GHG)

Cost per tonne avoided (€/t CO₂e) = extra cost per year ÷ tonnes CO₂e saved per year. Negative = the measure saves money.

Cheapest-first plan: sort the measures by € per tonne, add them from the cheapest until the target is reached. The picture is a cost curve (“marginal abatement cost curve”): bar width = tonnes saved, bar height = € per tonne.

Compare with the carbon price: EU ETS allowance 82.40 € per tonne (5 Oct 2026).

Example:

Measures for a flat of four

Each measure alone, against today. € per year: + = extra cost, − = saves money. Illustrative numbers.