A jacket is the only thing in your wardrobe you buy to fit over other things. That single fact is why sizing one is harder than sizing a shirt, and why the question of whether you should size up or down for a jacket returns every autumn. The honest answer is that it depends on two things: where the shoulder seam lands, and what you intend to wear underneath. Get those right and the rest follows. Get them wrong and no amount of tailoring will rescue the garment.
Should a jacket be tight or loose?
Neither, and the test is physical rather than visual. Close the jacket, then cross both arms in front of you as if reaching for the opposite shoulders. If the back pulls hard across the shoulder blades, it is too tight. Now open your arms and try to slide a flat fist between your chest and the closed front. If it goes in easily with room to spare, it is too loose. Correct sits between those two.
A second check costs nothing. Raise your arms to shoulder height. The whole jacket should lift slightly with them. If the hem climbs above your waistband, the body is too short for your torso, which is a length problem rather than a size problem, and going up a size will usually make the shoulders wrong while barely fixing the hem.
The Bomber Jacket is cut slim through the body deliberately. It is designed to close over a crewneck, not over a hooded sweatshirt, and it fits best when you buy it with that in mind.
Should my jacket be a size bigger?
Only when the shoulder seam says so. Stand square to a mirror and find the seam where the sleeve joins the body. It should land within roughly a centimetre of the bony point at the outside of your shoulder. If it sits inboard of that point, digging into the muscle, size up. If it hangs out over your upper arm, size down.
The second error is the more common one, and the more visible. A dropped shoulder reads as ill-fitting from across a room in a way a snug chest never does. It is also the mistake that a sale price encourages, because the larger size is the one still in stock.
There is a practical reason to let this seam decide. Chest, waist and sleeve length can all be taken in by a tailor for modest money. The shoulder cannot be moved without effectively rebuilding the jacket. So when two sizes are genuinely arguable, choose the one whose shoulder is right and have the body adjusted.
How do I know my size for a jacket?
Measure rather than guess. Run a tape around the fullest part of your chest, under the arms, keeping it level front to back, and leave one finger of slack. That number is the one to read against a size chart — not the size you take in T-shirts, which are cut with far more ease.
Sleeve length is the second number worth checking. With your arms hanging at your sides, the cuff should finish at the wrist bone. A centimetre past it is fine on a casual shell. Three centimetres past it makes the whole jacket look borrowed.
If you land between two sizes on the chart, the tiebreaker is what goes underneath. Planning to wear a mid-weight knit? Take the larger. Wearing it over a shirt, or over a fine-gauge knit like The Alpine Crewneck, take the smaller. The Monroe Blazer is a Tailored Fit and runs true, so most people take their usual size and only go up if they want it to close over something substantial.
How big should your winter jacket be?
Bigger than an autumn one, and by a measurable amount rather than a vague one. A mid-weight knit adds roughly three to five centimetres to your chest measurement. That is the room a winter jacket needs and a transitional one does not.
The way to settle it is to stop guessing and try the jacket on over the actual knit you plan to wear beneath it — not a T-shirt, and not the thickest sweater you own either. If the front closes without strain and you can still cross your arms, it is right. The Solis Jacket sits in the lighter half of this range: it is built to close over a sweater in October, not over a parka layer in January.
When the size chart and the mirror disagree
Trust the mirror, then trust the shoulder. Size charts are drafted against an average body, and shoulders vary more between people than chests do. Someone with a broad back and a narrow waist will read as two different sizes depending on which number they measure, and the shoulder is the one that cannot be fixed later.
One more thing worth doing before you commit: sit down in it. A jacket that fits perfectly standing and rides up around the armholes when you sit is a jacket you will stop wearing by November. Knitwear with a half-zip such as The Quarter-Zip Knit gives you a way to shed warmth without shedding the jacket, which makes a snugger outer layer more livable.
A jacket you have sized properly is one you will own for years, which makes how you look after it the next question — see how to care for a faux leather jacket, and how to wash a nylon jacket if yours is a shell. More short answers on fit and fabric live on Style Answers.