Quick Answer: Blazers are drafted from a shoulder measurement outward, so every other dimension scales from it. On a curvy frame the bust needs more room than that scaling provides, which is why buying up to close the button gives you a shoulder that is two sizes too big. The correct approach is to buy for the shoulder and have the body altered — shoulder alterations are difficult and expensive, everything below the shoulder is straightforward and cheap.

Buy the shoulder. Alter the rest. That sentence solves most of the problem.

Why does the button gap happen at all?

Because of how a blazer is drafted, not because of anything about your body.

A tailored jacket is drafted from the shoulder. The shoulder seam length and armhole shape are the structural foundation, and the chest, waist, and hip measurements are scaled proportionally outward from a standard ratio. That ratio assumes a specific bust to shoulder relationship.

If your bust to shoulder ratio is larger than the draft assumes, the front panels do not have enough width to meet and lie flat. The button pulls, a horizontal fold appears across the bust, and a gap opens at the button.

The horizontal drag lines are the diagnostic. Fabric pulling sideways from the button means the front does not have enough width. This is a pattern issue, not a size issue, which is why going up a size only partially helps.

Going up a size adds width everywhere, including the shoulders, the armholes, and the sleeve length. You gain three inches at the bust and also gain a shoulder seam sitting an inch down your arm, which is the fit failure that cannot be corrected.

Why the shoulder cannot be fixed: altering a shoulder means detaching the sleeve, reshaping the armhole, and resetting the sleeve. It is the most expensive alteration a tailor performs and it changes the balance of the whole garment. Most tailors will do it reluctantly and charge accordingly.

Why everything else can: taking in a side seam, a back seam, or a waist dart is routine, fast, and cheap. A tailor can remove four inches from the waist of a jacket without difficulty.

The asymmetry between those two facts is the entire strategy.

How do I know which size to buy?

Ignore the button entirely at the first stage and check three things.

Check the shoulder seam. Find the bony point at the outer edge of your shoulder. The seam should sit directly on it. Half an inch either way is acceptable; an inch down the arm is not, and this is the check that determines your size.

Check the armhole. Raise your arm forward to shoulder height. The jacket should move with you without the whole garment lifting. A too small armhole restricts; a too large one makes the jacket ride up and creates bulk under the arm.

Check the back. Reach both arms forward as if driving. Fabric should pull slightly across the upper back and release when you relax. If it feels like a straitjacket, the back is too narrow, and a back that is loose and wrinkled is too wide.

Then look at the button, and expect it not to close. That is now an alteration question, not a size question.

What the alterations actually are, in ascending cost:

Moving the button. The cheapest possible fix, and it buys you up to an inch. Worth asking for first since some jackets have enough overlap to allow it.

Letting out the side seams. Only possible if the jacket has seam allowance, which better made jackets do and fast fashion often does not. Check the inside seam allowance before buying — a half inch or more on each side means the jacket can grow.

Adding or deepening bust darts. This reshapes the front panel to make room for the bust without adding width elsewhere, and it is the alteration that genuinely solves the problem rather than working around it.

Taking in the waist and back. Standard, cheap, and it is what turns a shoulder correct jacket that swims at the waist into a fitted one.

What if the shoulder fits but the waist swims?

That is the good problem, and it is the outcome you are aiming for.

A jacket that fits the shoulder and bust and is loose everywhere else is a fifteen minute alteration. Side seams and back seam taken in, waist suppression added, and the jacket is fitted.

Bring it to a tailor with the top you intend to wear under it. Blazer fit changes noticeably over a thin shell versus a knit, and being fitted over the wrong layer is the most common alteration mistake.

Ask for the waist to be taken in at the side seams and the center back seam, distributed rather than all at one point. Taking four inches out of two side seams alone distorts the pockets and the hem line.

Expect to lose some length flexibility. Suppressing the waist on a jacket makes the hem flare slightly, which is usually desirable on a curvy frame since it accommodates the hip.

Do not let a tailor take in the shoulders as a shortcut. If they suggest narrowing the shoulder to fix a fit issue elsewhere, that is a warning sign about the whole approach.

Things that are worth spending on and things that are not:

Worth it: a jacket you will wear weekly, in a neutral color, in a natural fiber. Alterations typically cost a fraction of the jacket and roughly double how often you wear it.

Not worth it: a trend cut, a synthetic fabric that will pill, or anything where the shoulder is already wrong. You cannot alter your way out of a bad shoulder, and money spent trying is wasted.

One shortcut worth knowing: an open blazer designed with no closure at all removes the button gap problem entirely. Many current cuts are drafted to be worn open, and they solve this by design rather than by alteration.

Which blazer styles work best on a curvy frame?

Four structural features, and they matter more than the size on the label.

A single button at the natural waist. One button creates a V shaped opening that narrows toward the waist, which is the most flattering line available. Two and three button jackets close higher and create horizontal emphasis exactly at the bust.

Princess seams or bust darts. These are vertical seams running from the shoulder or armhole down through the bust. They allow shape to be built into the panel rather than added as width, which is precisely the problem being solved. A jacket with princess seams will almost always fit a curvy frame better than one with only side seams.

A notch lapel of moderate width, not a shawl or a very narrow lapel. The lapel creates a vertical line down the chest, and a moderate notch lapel gives the strongest vertical without competing.

Slight waist suppression built in, with a hem that clears the hip. A completely straight boxy cut adds width; an aggressively nipped one pulls. The hem should end below the widest part of the hip, because a hem landing exactly on it creates a horizontal at the widest point.

What to be careful with: patch pockets sitting at hip level, heavy shoulder padding, double breasted cuts that add fabric layers across the bust, and very cropped lengths that end at the natural waist.

Seasonal buying notes, since blazer season is starting:

Autumn is peak blazer availability, which means the widest range of cuts to test the shoulder rule against. September and early October have the most stock, and this is the moment to find the one that works.

Buy for the layer you will wear underneath in October, not August. A blazer fitted over a sleeveless top in warm weather will be tight over a knit in November, and that is a mistake made every year.

Wool and wool blends alter better than synthetics. A tailor can press and shape wool; a polyester blend resists shaping and shows old seam lines where alterations were made.

FAQ: Curvy Blazer Fit Questions, Answered

Should I size up to close the button?

No. Sizing up moves the shoulder seam down your arm, and the shoulder is the one thing that cannot be economically corrected. Buy the shoulder size and have the front altered or wear it open.

How much does a blazer alteration cost?

Waist and side seam work is usually a modest fraction of the jacket’s price. Bust darts cost somewhat more. Shoulder alteration is several times the cost of either and is the one to avoid by buying correctly in the first place.

Can any jacket be let out?

Only if it has seam allowance inside. Check the inside of the side seam before buying — a half inch or more of spare fabric on each side means the jacket can be let out. Fast fashion jackets are frequently cut with almost none.

Is wearing a blazer open a compromise?

Not at all. Many current blazer cuts are drafted specifically to hang open, and an open blazer creates two strong vertical lines down the front, which is the most flattering configuration available. The button gap only exists if you insist on the button.

What about stretch blazers?

A blazer with elastane in the fabric accommodates the bust without gapping and is genuinely useful. The tradeoff is that stretch fabrics hold shape less well over time and cannot be pressed and reshaped by a tailor the way wool can.

TL;DR:

• Blazers are drafted outward from the shoulder, so a bust that needs more room creates a button gap that sizing up cannot fix.
• Buy for the shoulder seam sitting on your shoulder point, then alter everything below it — that is cheap and routine.
• Look for a single button at the waist, princess seams or bust darts, and a hem that clears the hip.
• Check for seam allowance inside before buying, and get fitted over the top you will actually wear in October.

Try the shoulder check on the blazer you already own. If the seam is down your arm, that is why nothing else about it works.