There are lots of statistics, which as usual varies somewhat according to their remove from events, and as always it's illusory to think there will ever be a final precise figure. We are always talking about approximation. But the variation isn't very large, and it is probably safe to assume that we are in the territory of 850,000 casualties thereabouts, or a bit more if the arctic front losses are added.
My own consolidated figure for the main army field commands, based on their own casualty reporting and what information I have on Nachmeldungen etc, is 826,000. Geb. AOK20 not included, which adds another roughly 30,000, if you want to include them. The WFSt summaries produced by WVW list 2,005 combat losses for the KM by the end of the year, and 3723K/9876W/2099M= 15,698 for the LW as of 20 December. These (like the figures quoted in Halder) being a running tally that is certainly somewhat understated.
'Then there are the hospitalised sick and frostbitten, for which there are really no figures. But from what can be pieced together from bits and pieces, they must certainly have amounted to something like 6-700,000 cases.