Which perfume size to buy.

Worked example

Wearing a fragrance 2 times a week at 3 sprays of 0.1 ml uses 0.6 ml a week, 2.61 ml a month. A 30 ml bottle at $70 lasts 11.5 months ($2.33 per ml). A 50 ml bottle at $100 lasts 19.2 months ($2.00 per ml). A 100 ml bottle at $150 lasts 38.3 months, 2.3 months past a 3-year keep time, so about 6.1 ml goes unused and the price per ml actually used is $150 / 93.9 ml = $1.60. Here the 100 ml bottle is still the best value, but the gap from $1.50 sticker to $1.60 used shows the cost of the unused part.

How it works

ml used per month = wears per week x sprays x ml per spray x 365.25 / 12 / 7. Months to finish = size / ml per month. ml used = the smaller of the bottle size and ml per month x your keep time in months. Effective price per ml = price / ml used. Price per wear = price / (size / (sprays x ml per spray)). The best size is the one with the lowest effective price per ml.

What this is not. Keep time is your own judgement. This page does not claim a published expiry date for perfume; how long you keep a bottle is a personal call. Cost is not the only factor: a smaller bottle also lets you rotate scents.

Sources and basis: pure arithmetic on the numbers you enter; no published figure is used, and any default marked as an assumption or rule of thumb is not a published standard.

FAQ

Is a 100 ml perfume better value than 50 ml?

Usually per ml, yes. It is only better in practice if you will finish it in a time you are comfortable with. Enter your wears per week to find out.

How many ml of perfume do I use a month?

wears per week x sprays x ml per spray x 4.35. At 2 wears a week and 3 sprays of 0.1 ml that is about 2.6 ml, so a 50 ml bottle lasts around 19 months.

How long does an open bottle of perfume last?

There is no single published date. Three years is a common rule of thumb and the default here, but you can change it. Judge by smell and by your own comfort level.

Why does the page show an effective price per ml?

Because ml you never use still cost money. The effective price divides the bottle price by only the ml you would use within your keep time.