Short answer
Perfume is formulated by weight because fragrance materials have different densities: a milliliter of benzyl benzoate weighs about 1.12 g, a milliliter of ethanol about 0.79 g. Measuring by volume therefore changes the proportions of a formula depending on which materials it contains, and drops are less reliable still. Weigh everything in grams, and convert to milliliters (or US fluid ounces) only at the end, to fill a bottle labeled by volume, using the finished perfume's measured density.
Density is the whole story
Density is mass per unit volume. Water is close to 1.00 g/ml at room temperature. Fragrance materials range on both sides of it:
| Material | Approx. density | 1 ml weighs about |
|---|---|---|
| Ethanol | 0.79 g/ml | 0.79 g |
| Citrus oils (e.g. sweet orange) | 0.84–0.85 g/ml | 0.85 g |
| Dipropylene glycol (DPG) | 1.02 g/ml | 1.02 g |
| Benzyl benzoate | 1.12 g/ml | 1.12 g |
Measure 1 ml of each and you have not used equal amounts: the benzyl benzoate is about 42% heavier than the ethanol. A formula measured in milliliters has different proportions from the same formula weighed in grams, and the difference depends on what is in it. The figures above are typical values to illustrate the point; your supplier's specification sheet gives the density (or specific gravity) of the lot you bought.
Why drops are worse
A drop is not a unit. Its size depends on the liquid's surface tension and viscosity, the dropper or pipette tip, the angle, the temperature and how fast you squeeze. A thin citrus oil and a thick resinoid produce very different drops from the same pipette, and the same person will not produce identical drops twice. Drops are fine for a first exploratory sniff; a formula you intend to repeat must be weighed.
Where volume legitimately comes in
Bottles are sold by volume. In the US, labels state net contents in both metric and US customary units (for example 50 ml and 1.7 fl oz; 1 US fl oz is 29.57 ml). So at the end of the process you need to know how many grams fill a bottle:
fill weight (g) = bottle volume (ml) × density of finished perfume (g/ml)
A hydroalcoholic perfume is mostly ethanol, so its density is usually a little above ethanol's, depending on the concentrate and any water. Measure it rather than assuming: tare a small graduated cylinder or volumetric flask, fill it to 10.0 ml with the finished perfume at room temperature, and weigh. If 10.0 ml weighs 8.2 g, the density is 0.82 g/ml and a 50 ml bottle takes 41.0 g.
Filling by weight on a balance is then more consistent than filling to a line by eye, and it is how you check that bottles hold what the label says.
Converting a volume recipe to weight
If you have an old recipe written in milliliters or drops, convert it once and work by weight from then on:
- For milliliters: multiply each by that material's density to get grams.
- For drops: remake the recipe once, weighing each material as you add the drops, and record the grams.
- Convert the grams to percentages (how to convert a perfume formula from percentages to grams covers the reverse direction too).
- Make the weighed version and compare it with the original. It may be noticeably different — which tells you how imprecise the original was.
The mixed-unit trap
The most common error is half-converting: weighing the concentrate in grams but measuring the alcohol in milliliters, then calculating concentration as if both were grams. Say you weigh 8.78 g of concentrate and add "40" of alcohol, intending 18% (8.78 ÷ 48.78). If that 40 was measured in milliliters, at about 0.8 g/ml it is only 32 g of alcohol, and the perfume is actually 8.78 ÷ 40.78 = about 21.5% concentrate. How to calculate perfume concentration shows the correct calculation, and perfume batch calculation errors lists this alongside the other common mistakes.
Temperature
Density changes slightly with temperature, which is another reason weighing is better: a gram is a gram regardless. If you do measure volumes — for a fill check, for example — do it at a consistent room temperature.
What you need
A balance you trust is the single most important piece of perfumery equipment. For small trials, a resolution of 0.01 g is a common minimum and 0.001 g is better; how to weigh perfume ingredients accurately explains how to choose and use one. The rest of the arithmetic is collected in the perfume dilution and calculations guide.
Measuring a density yourself takes a balance and a volumetric flask; see how to measure a fragrance oil's density.
Frequently asked questions
How many grams is 1 ml of perfume?
For a typical alcohol-based perfume, roughly 0.8–0.85 g, but measure your own: it varies with the concentrate, the alcohol and any water.
How many drops are in a milliliter?
There is no fixed answer. Figures like "20 drops per ml" refer to water from a particular dropper; fragrance materials vary widely. Don't formulate in drops.
Can I use a kitchen scale?
Kitchen scales usually read to 1 g or 0.1 g, which is too coarse for small perfume trials. Use a balance with at least 0.01 g resolution.
Written and reviewed by the RUŌOD Lab team. This article is general education about perfume formulation and record-keeping; it is not legal, regulatory or safety advice, and the examples are illustrations, not validated commercial formulas. How we write and check these guides.