Tl;dr: 7:1 raw wood to charcoal conversion. And a lot of charcoal needed for each iron batch. Or "To put that in some perspective, a Roman legion (roughly 5,000 men) in the Late Republic might have carried into battle around 44,000kg (c. 48.5 tons) of iron – not counting pots, fittings, picks, shovels and other tools we know they used. That iron equipment in turn might represent the mining of around 541,200kg (c. 600 tons) of ore, smelted with 642,400kg (c. 710 tons) of charcoal, made from 4,620,000kg (c. 5,100 tons) of wood."
His sources seem to mostly focus on timber for makong things, but he does link to coppacing and pollarding, which would have been used for traditional charcoal.
https://acoup.blog/2020/09/25/collections-iron-how-did-they-...
Tl;dr: 7:1 raw wood to charcoal conversion. And a lot of charcoal needed for each iron batch. Or "To put that in some perspective, a Roman legion (roughly 5,000 men) in the Late Republic might have carried into battle around 44,000kg (c. 48.5 tons) of iron – not counting pots, fittings, picks, shovels and other tools we know they used. That iron equipment in turn might represent the mining of around 541,200kg (c. 600 tons) of ore, smelted with 642,400kg (c. 710 tons) of charcoal, made from 4,620,000kg (c. 5,100 tons) of wood."