A 20-foot shipping container is about 2.2 tonnes of steel. A 40-foot one is closer to 3.7. Making that steel the conventional way, from iron ore in a blast furnace, releases roughly two tonnes of CO2 for every tonne produced. So a retired 20-footer holds something like four tonnes of carbon that has already been spent. Give the box a second job and you don’t spend it again. That is the whole pitch for container buildings, and it holds up.