Shopping Bag Footprint Calculator (plastic vs paper vs cotton, uses to break even)

Understanding the shopping bag footprint calculator — concept and principle

This calculator compares the climate footprint of different shopping bags, from thin plastic to cotton, based on how many times you reuse them. It uses the life cycle assessment of supermarket carrier bags by the UK Environment Agency and shows how many uses a reusable bag needs to match a single-use plastic bag.

Footprint spread over uses: with the default inputs the result is 62.7 kg CO₂e (3.31× single-use HDPE).

How it is calculated

Unit: carrying one month's shopping (483 items) home.

Yearly CO₂e = 12 × global warming potential (GWP) per month ÷ uses × (your items ÷ 483)

Single-use HDPE comparison = 12 × 1.578 × (your items ÷ 483)

Uses to match HDPE = GWP ÷ 1.578, rounded up

New bags per year = 12 × bags per month in the study × (items ÷ 483) ÷ uses

GWP per month (kg CO₂e): HDPE 1.578, paper 5.523, LDPE bag for life 6.924, non-woven polypropylene 21.51, cotton 271.533.

The principle

A life cycle assessment adds up emissions from making, transporting and disposing of a product. Heavier, more resource-intensive bags such as cotton have much higher footprints per bag, so they must be reused many times before they beat a thin bag used once. Spreading a bag's footprint over its uses gives the footprint per trip.

Worked example

Default inputs: cotton bag, 52 uses, 483 items a month.

Yearly footprint = 12 × 271.533 ÷ 52 = 62.7 kg CO₂e.
Single-use HDPE for the same shopping = 12 × 1.578 = 18.9 kg CO₂e.
Ratio = 62.66 ÷ 18.94 = 3.31×; uses to match = 271.533 ÷ 1.578 → 173.
New bags per year = 12 × 45.59 ÷ 52 → 11; bag material = 1.93 kg.

Each point is the calculator re-run with only “Times you use each bag” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Times you use each bag	Bag footprint per year (kg CO₂e)
13	250.6
26	125.3
39	83.5
52	62.7
65	50.1
78	41.8
104	31.3

How to read the result

The more times a reusable bag is used, the lower its footprint per trip. Paper needs about 4 uses, an LDPE bag for life about 5, polypropylene about 14 and cotton about 173 to match a thin plastic bag reused once as a bin liner. Climate is only one impact: litter, marine pollution and other effects are not counted.

Common mistakes

Assuming a reusable bag is better from its first use.
Ignoring that newer studies with different assumptions give different numbers.
Forgetting that the HDPE figure assumes 40% reuse as bin liners.

Keep in mind: This is an educational estimate based on one study, not a full environmental assessment.

Frequently asked questions

How many times must I use a cotton bag?

About 173 times in this study to match single-use HDPE on climate impact.

Is paper better than plastic?

Not for climate unless the paper bag is reused several times.

What is the best choice?

Whatever bag you already own, reused as many times as possible.

Sources: Environment Agency (UK), Life cycle assessment of supermarket carrier bags: a review of the bags available in 2006, Report SC030148, 2011

Checked 9 Nov 2026. UK Environment Agency (C. Edwards and J. M. Fry), “Life cycle assessment of supermarket carrier bags: a review of the bags available in 2006”, Report SC030148, February 2011 — Table 3.1 (reference flows and bag weights), Tables 5.1–5.9 (GWP) and the executive-summary break-even table.

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