year of the snape

They aren't worn with trousers

I previously established that Hogwarts robes don't seem to be worn with trousers. Snape, Harry, and Ron are all implied to not be wearing trousers under their robes, along with everyone else:

James whirled about; a second flash of light later, Snape was hanging upside down in the air, his robes falling over his head to reveal skinny, pallid legs and a pair of graying underpants.

(OotP)

He got up and pulled on his jeans because he didn’t want to walk into the station in his wizard’s robes — he’d change on the train.

(PS)

Everybody except Harry was riotously happy and talkative as they changed out of their robes into jeans and sweatshirts

(OotP)

Below the robes, there will be a long undershirt/shift and modern underwear (explained in more detail in the post linked above).

The length

Hogwarts robes are long. They billow around the student's feet:

He [Percy] had already changed into his billowing black Hogwarts robes

(PS)

Their robes billowed and swirled around them as they splashed across the flooded vegetable patch to double Herbology

(OotP)

And cover your shoes, and are floor length:

Ron’s showing much too much ankle in his school robes

(HBP) - said by Molly.

Ron’s were a bit short for him, you could see his sneakers underneath them.

(PS)

Harry got to his feet, trod on the hem of his robes, and stumbled slightly.

(GoF)

It means students need to get used to moving in a floor-length garment and not step over the hems constantly — which is probably a struggle for first-year muggleborns.

(The scene where Harry stepped on his robes is when he's called up as a Triwizard Champion — so in a moment of high stress, he might step on his robes)

(Another note, not all robes are floor length. Dumbledore is often described as wearing "full-length wizard robes", meaning they are floor-length, but some characters have robes that leave their shoes visible for Harry to describe)

The collar

Hogwarts robes are cut similarly to some men's dress robes. This makes sense, the way school uniforms are formal-looking and a person unfamiliar with the culture could think a dress suit and a school uniform look similar.

In some trepidation, Harry opened the last parcel on his camp bed. It wasn’t as bad as he had expected, however; his dress robes didn’t have any lace on them at all — in fact, they were more or less the same as his school ones, except that they were bottle green instead of black.

(GoF)

We know robes come in different cuts and not all are considered "dress robes" and it isn't just a matter of color & fabric (though it is that too. Dress robes tend to come in nicer fabrics such as velvet. (Dumbledore is practically always wearing dress robes)):

Like her father, Luna was wearing bright yellow robes, which she had accessorized with a large sunflower in her hair. Once you got over the brightness of it all, the general effect was quite pleasant. At least there were no radishes dangling from her ears. […] “You look smart. I told Daddy most people would probably wear dress robes, but he believes you ought to wear sun colors to a wedding, for luck, you know.”

(DH)

Pansy Parkinson in very frilly robes of pale pink was clutching Malfoy’s arm.

(GoF)

There was just no getting around the fact that his robes looked more like a dress than anything else. In a desperate attempt to make them look more manly, he used a Severing Charm on the ruff and cuffs. It worked fairly well; at least he was now lace-free, although he hadn’t done a very neat job, and the edges still looked depressingly frayed as the boys set off downstairs.

(GoF)

“Yes, my tiara sets off the whole thing nicely,” said Auntie Muriel in a rather carrying whisper. “But I must say, Ginevra’s dress is far too low-cut.”

(DH)

And we know, thanks to Draco, a high collar on a black robe is considered formal dress robes cut for men:

Malfoy was in front; he was wearing dress robes of black velvet with a high collar, which in Harry’s opinion made him look like a vicar.

(GoF)

And Hogwarts robes have a similarly high collar that keeps it's shape:

Turning up the collars of their robes against the chilly September air

(OotP)

This collar is wide enough to be pulled on like a dress over their heads:

Madam Malkin stood Harry on a stool next to him, slipped a long robe over his head, and began to pin it to the right length.

(PS)

And for Hermione and Marriatta to hide their faces in:

They heard the lock slide back and Hermione emerged, sobbing, her robes pulled up over her head

(CoS)

and Marietta gave a wail and pulled the neck of her robes right up to her eyes

(OotP)

But it still needs to keep its form (as mentioned before).

We know there are no buttons, since Harry mentions buttons on other outfits, but not on his school robes:

“Now then,” said Slughorn, returning to the front of the class and inflating his already bulging chest so that the buttons on his waistcoat threatened to burst off

(HBP)

The beaded bag containing all of their possessions (apart from the Horcrux, which Harry was wearing around his neck) was tucked into an inside pocket of Hermione’s buttoned-up coat. 

(DH)

Nor do they have laces, since Harry would probably mention fiddling with them or tugging on them if they were there.

So I concluded the collar is held in place by a single small metal hook/clasp like this:

That is swen onto the robes themselves.

The belt

The billowing, loose-fitted robe is held in place with a sturdy belt:

Harry tucked the sword and the Sorting Hat into his belt, Ron took hold of the back of Harry’s robes

(CoS)

That can hold wands, swords, etc, easily.

This belt is the only thing that gives the robes their shape, as the only things that Madam Malkin seems to be tailoring/adjusting are the sleeves' hems and the length of the garment:

Madam Malkin stood Harry on a stool next to him, slipped a long robe over his head, and began to pin it to the right length.

(PS)

So, there is a lot of access fabric, and the robes aren't tailored to the body. They are barely tailored at all, which explains why all students are expected to get them; they are very simple, very simple garments.

The Pockets

Their school robes come with multiple large pockets. They are either actually big, or they are enchanted to make them big, as when Harry is wearing his school robes, he puts everything in his robes' pockets.

His wand:

Harry took off his black school robes, removed his wand from his pocket

(GoF)

Choclate Frogs:

Harry felt in the pocket of his robes and pulled out a Chocolate Frog, the very last one from the box Hermione had given him for Christmas. He gave it to Neville, who looked as though he might cry.

(PS)

His knife:

“Sirius’s knife!” said Harry, and he pulled it out from inside his robes and slid it into the crack between the door and the wall.

(OotP)

The Mauraders Map:

“Mischief managed!” The map went blank at once. He folded it carefully, tucked it inside his robes

(PoA)

He folded the map and tucked it inside his robes. 

(PoA)

And even the Invisibility Cloak on occasion:

There was a knock on the door. Harry hastily stuffed the Marauder’s Map and the Invisibility Cloak into his pocket.

(PoA)

The pockets have a lot of space.

And there are multiple pockets per robe (it seems):

“No idea,” said Harry. But Ron was examining Harry’s robes. All the pockets were hanging out.

(CoS)

The Sleeves

Contrary to popular interpretations, the sleeves are not wide, as they can be rolled up or pushed up to their elbows, and they stay there:

Harry felt a lurch of fear. He wasn’t ready. How could you make a dementor less frightening? But he didn’t want to ask for more time; everyone else was nodding and rolling up their sleeves. […] They all retreated, backed against the walls, leaving Neville alone beside the wardrobe. He looked pale and frightened, but he had pushed up the sleeves of his robes and was holding his wand ready.

(PoA)

Which implies they are tighter around the wrist at least, but not overall tight. As they are large enough for Ron to slip his wand into:

“Now, don’t forget, it’s Locomotor Mortis,” Hermione muttered as Ron slipped his wand up his sleeve.

(PS)

If the sleeves were very tight, Harry would mention it. He would also mention if the sleeves were particularly wide, which is why I think the sleeves look something like this:

Tighter around the wrist to allow pushing them up to the elbows or be rolled up, but wider above to make them look like a robe, but not super billowy, so they don't get in the way.

The color

Hogwarts robes are black:

pointed face was standing on a footstool while a second witch pinned up his long black robes.

(PS)

Harry took off his black school robes

(GoF)

Harry noticed that Ron kept moving his prefect’s badge around, first placing it on his bedside table, then putting it into his jeans pocket, then taking it out and laying it on his folded robes, as though to see the effect of the red on the black.

(OotP)

And they look the same for all houses. In the books, there is no house indicator on a person's robes beside their prefect badge if they have one or a scarf/rosette.

In PoA, when all the students are in identical sleeping bags, Harry can still tell the house and year of the speaker:

“It’s very lucky he picked tonight, you know,” said Hermione as they climbed fully dressed into their sleeping bags and propped themselves on their elbows to talk. “The one night we weren’t in the tower. . . .” “I reckon he’s lost track of time, being on the run,” said Ron. “Didn’t realize it was Halloween. Otherwise he’d have come burst- ing in here.” Hermione shuddered. All around them, people were asking one another the same question: “How did he get in?” “Maybe he knows how to Apparate,” said a Ravenclaw a few feet away. “Just appear out of thin air, you know.” “Disguised himself, probably,” said a Hufflepuff fifth year.

(PoA)

In CoS, Harry doesn't know Myrtle is a Ravenclaw; that information is from tertiary canon and not the books. This implies there is nothing on their robes to give them away.

Harry can tell who's from what house because he recognises students' faces/voices and knows what house they are in, not because their robes have a marking of their house — they don't. The fact that he tends to know other students years often, supports this:

The following day, two more girls asked him, a second year and (to his horror) a fifth year

(GoF)

“Hold it!” said Hermione, throwing out an arm and halting a passing fourth year, who was attempting to push past her with a lime-green disk clutched tightly in his hand.

(HBP)

He knows random students' year, not just house. He just recognises them because Hogwarts is a small school.

Shoes

They don't have school shoes. Shoes are not mentioned in the school supply list before first year. They wear socks with their shoes of choice:

He and Ron took off their jackets and pulled on their long black robes. Ron’s were a bit short for him, you could see his sneakers underneath them.

(PS)

narrowly missing Hermione, it burst at Harry’s feet, sending a wave of cold water over his sneakers into his socks. 

(GoF)

and laced up Goyle’s boatlike shoes.

(CoS)

Fabric

Since Hogwarts is in Scotland, where it is cold, the material is kind of important. We know the material is sturdy enough to hold the high collar in place (as mentioned above).

And we know it isn't silk and is warmer than that:

They were shivering, which was unsurprising, given that their robes seemed to be made of fine silk, and none of them were wearing cloaks. 

(GoF)

Though, not that warm:

The temperature dropped with every step they took. As Harry shivered and drew his robes tightly around him

(CoS)

(Nor does it seem to have warmign charms)

As Harry always puts on a cloak when he's cold.

My assumption is that Hogwarts robes are made of wool. Wool is a natural fibre that has been common for clothes since humans started making them, basically. It's great at insulation and regulating body temperature (both in warm and cold climates), breathable, and durable. It should be washed by hand, though, as they wash it with magic, it's probably alright. In addition, I believe they wear an undershirt/shift under their robes, which means there isn't skin contact and won't need to be washed often.

The cloak

The robes come with a black winter cloak:

4. One winter cloak (black, silver fastenings)

(PS)

They have a high collar fastened with a silver clasp/pin:

their cloak collars turned up against the wind.

(OotP)

Yeah, Hogwarts cloaks have a high collar that can be turned up, not a hood. Harry mentions hooded cloaks a lot — Hogwarts robes/cloaks do not have hoods.

It has pockets like the robes:

"Hermione looked very cross indeed and stuffed her box of badges back into her cloak pocket."

(GoF)

It is long (floor-length) and billowy:

their socks and the hems of their cloaks soaked and freezing

(PoA)

Drawing their cloaks more closely around themselves

(GoF)

Hermione bundled up Crookshanks in her cloak

(GoF)

As Hermione keeps Crookshanks in it, and the hems reach the floor like the robes.

Other Accessories

They have Hogwarts house-themed accessories such as scarves, gloves and rosettes:

Three-quarters of the crowd was wearing scarlet rosettes, waving scarlet flags with the Gryffindor lion upon them

(PoA)

Hermione and Ginny sat down opposite them wearing red-and-gold scarves, gloves, and rosettes.

(OotP)

She [McGonagall] was wearing a Gryffindor scarf, but tore it from her throat with shaking hands as she strode toward them, looking livid.

(OotP)

walking down to the Quidditch pitch with Ron, Ginny, and the others. It was almost unbearable to turn away from the mass of students streaming out into the sunshine, all of them wearing rosettes and hats and brandishing banners and scarves

(HBP)

Students and professors both own these items. Maybe there is a shop in Hogsmead for them, though, I'm pretty sure some of them are homemade by students.

Prefect badges:

and Harry noticed a shiny red and gold badge on his chest with the letter P on it

(PS)

A large P was superimposed on the Gryffindor lion. He had seen a badge just like this on Percy’s chest on his very first day at Hogwarts.

(OotP)

A boy [Tom] of about sixteen entered, taking off his pointed hat. A silver prefect’s badge was glinting on his chest.

(CoS)

And Quidditch captain badges:

“Wow, I remember when Charlie wore one of these,” said Ron, examining the badge with glee. “Harry, this is so cool, you’re my Captain — if you let me back on the team, I suppose, ha ha. ...”

(HBP)

Along with any other jewellery or accessories, students make or buy for themselves. (Such as Luna's radish earrings or Parvati's butterfly hairpin)

The Hat

They have a pointed black wizard hat that comes with their uniform:

2. One plain pointed hat (black) for day wear

(PS)

They seem to be wearing it in the opening feast:

Through the forest of pointed black Hogwarts hats, Harry saw a long line of scared-looking first years filing into the Hall.

(CoS)

It was a sea of pointed black hats; each of the long House tables was lined with students, their faces glimmering by the light of thousands of candles, which were floating over the tables in midair.

(PoA)

But it is optional most of the time. Though we do see them wear it:

A boy [Tom] of about sixteen entered, taking off his pointed hat. A silver prefect’s badge was glinting on his chest.

(CoS)

“Maybe I’ll skive off Divination,” he said glumly as they stood again in the courtyard after lunch, the wind whipping at the hems of robes and brims of hats. 

(OotP)

So the hats are black, pointy, and have a brim (unlike the movie version we see). I headcanon that in British public school fashion, the hats are mandatory for the start of term and end of term feasts, along with a few other occasions and optional the rest of the time.

Aesthetics

AKA, the style - how does all of this come together?

Since Harry doesn't think the robes look like anything other than robes (not vicar robes like Draco's dress robes, nor a dress like Ron's dress robes) it means they look unmistaknly like something Harry would think of as wizard robes — which means they look like 1980s Hollywood wizard robes since this would be Harry's frame of reference. In general, the Wizarding World fashion seems to be 19th century, doing a Renaissance fair, if we want to give the style a name. Which leads to a garment that look like this:

For what Hogwarts robes probably actually look like in the books. Sleeves that widen out (though, not as much as the image, probably), a lot of fabric only kept in a shape thanks to a belt, reach the floor, collar held up by a single small metal hook — and wouldn't look like a dress to 11-year-old Harry, but like wizard robes.

And because I couldn't help myself, I took some old artwork of the golden trio and drew the robes and cloaks according to the aforementioned evidence on them like dress-up dolls: