Start with the spray count
Every size decision comes back to one number: how many sprays you get. A standard small atomiser delivers roughly 0.09 to 0.1 ml per spray, which gives you approximately 20 to 25 sprays from 2 ml, 55 to 65 from 5 ml, 110 to 130 from 10 ml and 340 or more from 30 ml.
Now divide by your actual wear rate. Most people use two sprays for an ordinary day and three or four for something they want noticed, or for a fragrance with modest projection. At two sprays a wear, a 2 ml decant gives you about ten wears, a 5 ml about thirty, a 10 ml about sixty and a 30 ml around a hundred and seventy.
2 ml — for ruling things out
Two millilitres is a decision-making tool, not a wearing quantity. Ten wears sounds like more than it is: by the time you have worn a fragrance to work twice, on an evening out once and around the house a few times, it is gone. What it does brilliantly is settle a specific question cheaply — is the cumin in Spicebomb Extreme a problem on my skin, can I tolerate Viking's mint, am I one of the people who cannot smell Molecule 01.
Buy 2 ml when you suspect you might dislike something, when you are comparing two similar fragrances side by side, or when you want to check a much-hyped release without spending real money. It is also the right size for a clone-versus-original comparison, which is one of the more instructive things you can do with twenty dollars.
5 ml — the honest trial
This is the size we recommend more than any other, and it is the one most people should default to for a fragrance they have never worn. Thirty wears is enough to wear something in genuinely different circumstances — a hot afternoon, a cold evening, an office, a restaurant, a long day where you find out what it smells like at hour nine.
That range is the whole point. Fragrances behave differently in heat, on tired skin, under a coat, and after eight hours. A verdict formed from three wears in the same week is not a verdict, it is a first impression. Five millilitres buys you enough wears to be confident, and it is still a fraction of a bottle.
10 ml — for something you already like
Ten millilitres is the volume-seller and the best value on the ladder for most people, but it is money spent slightly ahead of certainty if you have never worn the fragrance. Sixty wears is not a trial, it is a season or two of ownership.
Buy 10 ml when you already know you like the fragrance — you have had a 5 ml, or worn a friend's bottle, or owned it before. Also buy 10 ml when the fragrance is genuinely seasonal: a tropical summer citrus or a heavy winter tobacco gets worn perhaps fifteen times a year, so 10 ml is a multi-year supply and a bottle would be absurd.
It is also the right size for building a rotation. Four or five 10 ml decants across different characters — one fresh, one woody, one sweet, one formal — is a complete fragrance wardrobe for less than one niche bottle.
30 ml — when it has earned it
Thirty millilitres is for a fragrance that is already part of how you dress. A hundred and seventy wears is a year or more of regular use, and at this size the per-millilitre price approaches what you would pay for a bottle without the commitment of owning a hundred millilitres of anything.
There is one particularly good case for 30 ml: a boutique-exclusive or very expensive fragrance you love and wear often. If a bottle is six hundred dollars and only sold in 100 ml, a 30 ml decant is a genuinely rational purchase rather than a compromise.
A practical rule
If you cannot decide, buy 5 ml. It is the size that most often turns out to have been the right call, because it is large enough to produce a real answer and small enough that a wrong answer costs you the price of a dinner rather than the price of a bottle.
And if you are choosing between one 10 ml of something you are unsure about and five 2 ml decants of five different fragrances, take the five. Discovering that one of them is a fragrance you love is worth far more than sixty wears of something you were ambivalent about.
Frequently asked
Roughly 110 to 130 sprays. At two sprays per wear that is around 60 wears, which is about eight weeks of near-daily use or a full season for something worn weekly.
Enough to rule one out, not enough to be sure about one. Two millilitres gives about ten wears, which will not cover the range of weather and settings that actually determine whether you keep reaching for something.
Ten millilitres has the best price per millilitre for most people, and 30 ml is better still. But value only counts if you like the fragrance — a 5 ml you love beats a 30 ml you tolerate.
Several small ones, if you are still exploring. The chance of finding a fragrance you genuinely love across five 2 ml decants is worth more than a large quantity of something you were unsure about.