A blazer that fits over a thin T-shirt may feel completely different over the sweater you actually wear to work. Extra room is needed in particular places: upper arms, upper back, and the closed front. Simply buying the next size can add length and shoulder width without fixing the uncomfortable area.
Wear or bring a representative sweater when you shop. Choose an ordinary one from your rotation, not your thinnest knit merely to make the blazer work. If most of your sweaters have dropped shoulders or wide sleeves, their fabric must fit somewhere inside the jacket.
Start with the sleeve intersection
Put the jacket on without dragging the sweater sleeve upward. Reach forward, bend your elbows, and cross your arms loosely. Notice whether fabric bunches high in the sleeve or the jacket pulls across your upper back. A wider jacket sleeve or different armhole shape may solve this better than extra room at the waist.
Next, let your arms rest. A blazer chosen only for reach can look and feel excessively loose when you stand normally. The useful compromise lets you perform your routine movements while retaining the silhouette you want. There is no need to demand the same unrestricted motion as a sweatshirt from a sharply tailored garment, but ordinary sitting and reaching should not require constant readjustment.
Fasten it the way you will wear it
Try the intended front button, even if you often leave the jacket open. Look for a strong pull across the closure or a collar that lifts away as the fronts draw together. A sweater with a thick ribbed hem may create bulk exactly where the blazer closes. Compare it with a finer hem before deciding the jacket itself is too small.
Sleeve lining can affect how easily the layers slide. An unlined body with lined sleeves may work differently from an entirely unlined jacket. Read the construction details rather than assuming that less lining always makes layering easier.
If alterations are part of the plan, ask a tailor about the specific change before purchasing. Shortening a sleeve and changing a shoulder are different jobs; their feasibility depends on seams, buttonholes, and available fabric. Do not price both as a simple adjustment.
Finally, take off the sweater layer and see whether the blazer still serves another outfit you need. It does not have to fit every season. A jacket that works over your most-used knit may be a better purchase than an all-purpose compromise that is uncomfortable with the clothes you wear most.
