Insulation is rated in grams per square meter — typically shortened to "gram weight" or just "grams." 80g, 150g, 200g, 300g, 400g. The number is the weight of insulation in a square meter of the lining, and it correlates roughly with how warm the garment will keep you at a given activity level.
Here's the trap. The chart on the rack tells you 200g is good to about 20°F, 300g good to 10°F, and so on. Those numbers assume you're standing still or moving lightly. If you're working — actually moving, lifting, framing — you generate heat, and the numbers shift. A 200g coverall while doing roof tear-off in 15°F weather will have you dripping by hour two.
Activity-adjusted rule of thumb. Stationary work (security, parking attendant, equipment operator on a heated cab) — bump the gram weight up one tier from the rack chart. Light activity (electrical, light carpentry, supervision) — use the chart number. Moderate activity (framing, plumbing, most general construction) — drop one tier. Heavy activity (demo, roofing tear-off, snow removal, anything where you're sweating in summer) — drop two tiers and add layers underneath you can shed.
Layering beats over-insulating. Two 150g garments give you more flexibility than one 300g garment because you can shed one when you warm up. A heavy coverall over a t-shirt has you sweating once you start moving, and once you sweat, the sweat soaks the insulation, the insulation loses its loft, and now you're cold and wet at the same time. That's how guys end up with hypothermia at 25°F.
Carhartt's coveralls run from un-insulated quilted to 300g. The Yukon and the Arctic are their two heaviest lines. Walls and Berne run similar gradients at lower price points. We stock all three.
Things to check past the gram weight — knee articulation (can you kneel?), back length (does it cover the gap when you bend over?), zipper protection (is the zipper covered with a storm flap or exposed?), and ankle gussets (do snow and grit fall in?). A correctly-rated coverall that pulls up your back when you bend is a bad coverall.
Try them on with the layer you'll wear underneath. A coverall sized over a t-shirt is too tight when you put a hoodie under it.