How this calculator works
Every bag has a fixed cost of materials, manufacturing and transport before it is used once. A single-use high-density polyethylene bag weighs only about 5.5 g, so its cost is small. A cotton tote can weigh 140 g and its cotton growing, spinning, weaving and shipping make it far more emissions intensive, even though it is reused for years. The UK Environment Agency compared the global warming potential of supermarket bags and found that a paper bag needs 3 uses, a low-density polyethylene bag for life 4 uses, a non-woven polypropylene bag 11 uses and a cotton bag 131 uses before each matches one single-use bag. Those figures assume the disposable bag is thrown away after one use. If your plastic bags get a second life as bin liners, the break-even counts rise to 7 for paper, 9 for the bag for life, 26 for polypropylene and 173 for cotton.
Formula
lifetime footprint (kg CO2e) = 0.010 kg CO2e x reuses to break even for that bag type. footprint per use = lifetime footprint / times you reuse it. bags per year = bags per week x 52 / times you reuse each bag. waste per year = bags per year x grams per bag / 1000. The annual footprint is bags per year x lifetime footprint / reuses, which equals the per-use footprint times the bags you need in a year.
Worked example
A cotton bag reused 50 times, with 8 bags a week going out of the house. Its lifetime footprint is 0.010 x 131 = 1.31 kg CO2e, so each use costs 0.0262 kg CO2e. At 50 uses per bag you need 8 x 52 / 50 = 8.32 bags a year, which is 1.165 kg of cotton and 10.9 kg CO2e. Fifty uses is below the 131 break-even, so the tote is still worse than taking 416 single-use bags a year.
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Frequently asked questions
What happens if I reuse plastic shopping bags as bin liners?
That second use is real and it raises the bar for reusables. A disposable bag that replaces a purchased bin liner has already avoided one other bag, so the cotton break-even rises from 131 to about 173 uses. If you already buy bin liners separately, the lower figure is the fair one.
Is paper always better than plastic?
Not on global warming potential. Paper bags are heavier and their pulping takes energy, so a paper bag needs 3 uses to match a single-use plastic bag and still does worse on many other indicators. It does break down more readily, which matters for litter.
Which reusable bag is the best choice?
The one you already own and will keep using. If you are buying new, a non-woven polypropylene bag breaks even after 11 uses and is light, while cotton needs 131 uses. A bag that sits in a cupboard has no benefit at all.
Do the figures apply outside the United Kingdom?
The break-even counts depend on the electricity mix and transport distances. In a province with very low-carbon electricity, manufacturing the reusable bag emits less and the break-even comes down; the ranking of the bag types is unchanged in most studies.