So being a 39 inch chest, the jacket would likely fit me with a sweater on? I don’t see myself ever wearing just a t shirt. I’d like to wear the jacket as a winter coat. I am 5’6, 150 lbs, 39 chest. Just wondering what size would be correct for me? I am thinking either 40 or 42? ThoughtsWith sheepskin you have to take account of the thickness of the shearling. It's a matter of personal preference how tight or loose you want it. But for me an unlined leather jacket would be more like 3" over chest size for a slim fit. More like 4" to get a jumper on under it and 6" to get a very heavy sweater or C3 or something like that under. It's not so easy to work out how thick shearling is though and Irvins had very thick fleece. However it does flatten with age and use, and that makes more room.
This one is said to be 42" all round chest outside so is probably 39.5" to 40" or so inside taking the shearling into account. If you normally wear a 38 it should fit you fine I would have thought. You don't really need to get anything too heavy under it unless you live in the frozen north.