A cardigan has two jobs when it is part of a layered outfit: it must fit over what is underneath and sit comfortably under anything worn on top. That sounds obvious until a roomy knit catches inside a coat or a neat front pulls open over a shirt. Try the whole sequence before deciding on a size.
Start with the layer closest to your body. A cardigan for sleeveless tops may need a different sleeve shape from one worn over a button-down. Check the upper arm and shoulder together. Generous fabric at the torso does not help if a shirt sleeve is tightly compressed above the elbow.
Open and closed are different fits
Button the cardigan if you intend to wear it fastened. Look at the spaces between buttons without pulling the hem down or adjusting your posture. Gaping can reflect size, button spacing, or how the front band behaves. If the body is already loose elsewhere, another cut may be more useful than simply sizing up.
Then wear it open. Some fronts hang straight; others curl or fold outward. Decide whether you like the result with your actual neckline underneath. A cardigan bought only because it looks good buttoned in a photograph may disappoint if you mostly wear it as an open layer.
The hem should work with your usual waistbands. A thick ribbed edge can create a ridge beneath a fitted coat. A short cardigan can leave a gap above lower-rise pants when you reach. A longer one can bunch in a chair. Test those positions rather than relying on the garment length printed beside a model's height.
Give the sleeves somewhere to go
Put your outer layer on and take it off again. Check whether the cardigan cuffs remain at your wrists or are dragged up the arms. If you normally push sleeves back while working, see whether they stay comfortably without squeezing. A delicate cuff may not be the right choice if that is how you wear it every day.
Before buying, inspect the care label and the attachment of buttons. A spare button is useful, but the main question is whether the garment's care suits your schedule. The best layering cardigan is not necessarily the finest or thickest knit; it is the one whose shape cooperates with the clothes around it.
