What size bottles to buy?

If you've got a decent collection (20+), what size bottles are you buying?

I'm planning on picking up a few where my decants have run out, and debating on how big to go. 

Do you take the best cost-per-ml and buy the 150ml?  Or do you go with the lowest price and buy the 50ml?

For me, 50-75ml is the way to go (in other words, the smallest full-size bottle a brand offers). It's very rare that I finish a botte, and ones I am buying today I probably will never finish. Unless you're decanting or wanting to sell the bottles themselves as partials, there's no reason to pay more for juice you're never going to use.

Think about it this way: a single wearing is about 0.5ml, and that's if you spray a lot. Meaning it would take you more than half a year to get through a single 100ml bottle, if that's all you wear. If you have 20 50ml bottles, that's 1000ml total, which would take about 5 1/2 years to get through, and that's if you stop buying bottles/decants entirely at 20.

I love Guerlains in their Les Legendaires line and they only come in 75 ml. bottles. I like that size, but still have to find out how long it will take me to finish them. Liu Eau de Parfum will be finished more easily than Mitsouko Eau de Toilette I can imagine because the latter is more for special occasions and for colder seasons whereas Liu can be worn all year and for any occasion in my opinion.

I go for the smallest possible, 10-15ml often hit the sweetspot to me. If i love something enough to empty a 10ml even though i have a lot of bigger bottles then i would consider getting a bigger format but i tend when not available to go for 30 or 50ml. 

I used to always buy 100 ml thinking that I needed the biggest bottle. But it soon dawned on me that it was almost impossible to finish a bottle this size when you have 100 of them. Now, I love and appreciate 30 and 50 ml. I think they're cute on my vanity, and I can realistically finish bottles of this size(s).

I always go for 30 or 50mL if I can, and I adore perfumers who make spray mini replicas of their full size bottles. 

I used to buy 100ml but lately I started buying decants and will opt for 30-50ml of scents I grown to love. 

I like smelling new things and that will allow me to have an opportunity to go through a bottle quicker.

I usually stick to 50ml unless it’s a scent I know I’ll wear a lot. With a decent collection, it’s hard to finish big bottles before they start to turn, so I’d rather have more variety than huge backups.

That said, if the 150ml has a crazy good cost-per-ml and it’s a favorite, I might go for it—especially if it's something super versatile. But in most cases, I’d rather finish a 50ml and keep things fresh.

I used to buy 100 ml perfumes casually, I actually didn't use more than 10-20 ml of them, I got bored of them, now I only buy smaller perfumes or decants.

Although I have 26 bottles of perfumes in my cabinet drawer I have had a number of them for well over 10 years.  The Carons are still lovely, the Goutal, the old Yardley.  So I think I don't actually buy many perfumes and each bottle lasts me.  I try quite a few carded and decanted samples so like to ring the changes and try new things.

Aromatics Elixir I always buy the 100 ml as despite its many tweaks and it being quite dissimilar to the very first bottle I had, it is still recognisably Aromatics Elixir and is my favourite.

By the time I buy a perfume I know I am going to wear it so would buy the smaller bottle if it were still good value i.e. not ludicrously expensive per ml compared to the next size up but the next size up if it were what I wanted to spend.  A case in point is Phul Nana by Grossmith.  I've wanted this one for an age but the 50 ml is £230 which I think excessive and the 100 ml is £330 i.e. much better value but I don't want to buy a £330 bottle of perfume so have kept my money to myself.  

Its rare i finish a bottle, though my 50ml xerjoff tony iommi and cruz del sur both have less than 20 ml left . But im  one of those who has a perfume addiction so I end up buying new fragrances all the time. Those two are probably the only ones I would restock because I use it so often.

Like everyone else I struggle to finish a bottle so I stick to 50ml, but 30ml is my perfect size x

I like switching up my fragrances and experimenting with combining and layering them so I prefer buying decants and sample sizes, if I like one enough to buy a bigger size I'll usually buy it in the smallest size available so I have more on hand but I'm not buying way more than I'll ever use (unless it's something I REALLY love and could see myself reaching for multiple times a week).

I'd actually say 50ml or smaller.

My ADHD means I'm often tired of a fragrance long before I'm through a full bottle. I'm very grateful more houses/companies are selling 10ml sprays.

For a large collection (20+ bottles), 50ml is best for variety, 100ml offers the best balance of price and longevity, and 150ml+ is only worth it for daily go-to scents. Stick to 50ml unless it’s a signature fragrance you wear often.