Quick Answer: The first jacket of the season should cover 10 to 18C with one layer underneath - which rules out both a summer jacket and a winter coat. In practice that is a trench, an unlined or lightly lined blazer, a denim or leather jacket, or a quilted jacket, and the right one depends on whether your days are wet, formal, or long. The mistake is skipping this jacket entirely and going from bare arms to a wool coat, which is why late September feels like it has nothing to wear. This layer gets more wear than any coat you own.

Here is how to choose between them.

What temperature range does the first jacket need to cover?

Roughly 10 to 18C, which is most of September and October. That is a wide band, and it is why this jacket has to work over a tee at the top of the range and over a knit at the bottom.

It has to fit over a mid layer. The single most common sizing error is buying this jacket to fit over a tee in August, which makes it unwearable once you add a knit.

It should be windproof more than it is thick. At these temperatures wind is the main variable, which is why a thin trench outperforms a thicker but more porous jacket.

It needs to be carryable. You will take it off most afternoons, so weight and how it folds matter as much as warmth.

And it should work in rain. September rain is the most common weather event of the season, and a jacket that cannot handle a shower gets left at home on exactly the days you needed it.

How do the main options compare?

Trench: the widest range and the best in rain. Works over a tee or a knit, belted or open, and handles both a work setting and a weekend. It is the most versatile option and the most expensive to buy well.

Blazer: best for formal days, worst in rain. An unlined or lightly lined blazer covers about 14 to 20C, dresses up an outfit instantly, and is useless once it is wet.

Denim jacket: casual, durable, narrow range. Comfortable 15 to 20C, but it is not windproof and it is the least warm of the group. Its strength is that it goes with everything.

Leather jacket: warmest of the four and fully windproof. Covers 8 to 16C with a knit, blocks wind entirely, and improves with age - but it is the least formal and the heaviest to carry.

Quilted jacket: the most practical and least versatile. Genuinely warm for its weight, water resistant, packs down, and it reads as country or sport rather than city.

If you only buy one, buy the trench. It covers the widest band, the most settings, and the most weather - which is why it is the one that survives every trend cycle.

What should you check before buying?

Try it over the knit you actually wear. Bring one, or buy where you can return and check it at home. This one test prevents most first-jacket regret.

Check the shoulder seam. It should sit at or just past your shoulder point. Too far in restricts movement over a layer; far too far out reads as borrowed.

Raise your arms. A jacket that pulls across the back when you reach will be uncomfortable on a commute, which is where it spends most of its life.

Check the sleeve length over a cuff. A sleeve that covers a shirt cuff entirely will look wrong every time you wear it with one - and it is a cheap alteration.

Look for a lining and a proper hem. An unlined jacket in a rough fabric is uncomfortable over bare arms, which is half of the temperature range it is meant to cover.

And be sceptical of belted waists that only work belted. A trench should also hang well open, because that is how it will be worn on the warmer half of its range.

5 Mistakes Everyone Makes Buying the First Jacket

1. Skipping it entirely. Going straight from summer to a wool coat leaves a six-week gap that is exactly when the weather is most changeable.

2. Sizing it over a tee. The jacket has to close over a knit. Sizing it in August against bare arms guarantees it is too small in October.

3. Buying something not rain capable. September rain is routine. A jacket that cannot take a shower gets left at home on the days it was most needed.

4. Choosing warmth over wind resistance. At 12 to 16C wind is what makes you cold, not air temperature. A thin windproof layer beats a thicker porous one.

5. Buying a colour that only works with half the wardrobe. This jacket is worn five days a week for three months - it needs to go over almost everything, which usually means a neutral.

FAQ: Choosing Your First Autumn Jacket, Answered

What jacket do I need in September?

One that covers roughly 10 to 18C over a single layer - a trench, an unlined blazer, a denim or leather jacket, or a quilted jacket. Not a summer jacket and not a winter coat.

If I can only buy one, which?

A trench. It covers the widest temperature band, works in rain, and moves between formal and casual settings more easily than any of the alternatives.

What size should I buy?

One that closes comfortably over the knit you actually wear. Sizing it over a tee in August is the most common and most expensive mistake in this category.

Is a leather jacket warm enough for autumn?

Yes - it is the warmest of the common options because it blocks wind entirely. With a knit underneath it works down to about 8C.

Do I still need a wool coat?

Yes, but later. The wool coat covers November onward; this jacket covers September and October, which is a longer stretch than most people expect.

How do I know it fits properly?

Raise your arms. If it pulls across the back, it will be uncomfortable on a commute, which is where the jacket spends most of its working life.

TL;DR:

• The first jacket covers 10 to 18C over one layer - roughly all of September and October.
• Buy the trench if you buy one. Widest range, best in rain, most settings.
• Size it over the knit you actually wear, not over a summer tee.
• Wind resistance beats thickness at these temperatures.
• It must handle rain. September showers are routine.
• Raise your arms in the fitting room - commute comfort is the real test.
• Neutral colour, because it goes over nearly everything for three months.

This jacket outwears the coat you spend more on.