Keep a couple of bags in your car, backpack or coat pocket so they are there when you shop. The best bag is one you already own, of any material, used until it wears out.
England's charge shows the scale: the main seven supermarkets sold 126 million single-use bags in 2025/26, down 98 percent from 7.6 billion in 2014, before the charge. A lightweight HDPE bag weighs about 8 g, so avoiding 100 bags a year saves roughly 800 g of plastic.
The catch: the Danish Environmental Protection Agency's 2018 life-cycle assessment (LCA) found a conventional cotton tote must be reused about 7,100 times to match an LDPE bag across every impact category (52 times on climate change alone), so do not buy new cotton bags as an eco statement; reuse what you have.