Cosmetic Calculator
Scale a cosmetic formula from percentages to batch weights, and check expiry from shelf life and the open-jar symbol.
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What is this?
A Cosmetic Calculator does two jobs. It converts a formulation written in percentages into the actual ingredient weights for a batch of any size, and it works out when a cosmetic should be discarded from its manufacture date, shelf life and the Period After Opening symbol on the packaging.
How to Use the Cosmetic Calculator
Two tools for anyone making or using cosmetics. The Formulation tab turns a recipe written in percentages into the actual weights for a batch of any size. The Shelf life tab works out when a product should be thrown away, from its manufacture date, its shelf life and the open-jar symbol on the packaging.
Cosmetic recipes are always written as percentages by weight, because that is what makes a formula scalable - the same percentages produce a 30 g sample or a 5 kg batch. The percentages must total exactly 100, and water is normally the ingredient adjusted to make them do so.
1. Enter Your Batch Size
Choose how much you want to make, in grams, kilograms, ounces or pounds. Everything is calculated by weight rather than volume.
2. List the Ingredients and Percentages
Add each ingredient with its percentage. A running total shows whether the formula is balanced, over or short, so you can see the problem before you calculate.
3. Read the Weights
Each ingredient's weight is its percentage of the batch. The table also shows grams alongside your chosen unit, since most cosmetic scales read in grams.
4. Check a Product's Shelf Life
Switch tabs and enter the manufacture date and shelf life, normally 36 months. If the product has been opened, add that date and the number in the open-jar symbol.
5. Use the Earlier Date
Two clocks run on any cosmetic: the shelf life from manufacture, and the Period After Opening once the seal is broken. The calculator reports whichever runs out first.
Key Formulas Used in the Calculator
Ingredient Weight
Weight=100Percentage×Batch Size
An ingredient at 70% of a 500 g batch is 0.70 × 500 = 350 g. Because every ingredient scales the same way, the weights always add back up to the batch size when the percentages total 100.
The Formula Must Total 100%
∑All Percentages=100%
A formula summing to 98% is missing something. Water is conventionally the ingredient adjusted to bring the total to 100, which is why it usually appears as an awkward number rather than a round one.
Shelf Life from Manufacture
Expiry=Manufacture Date+Shelf Life in Months
Unopened cosmetics typically last 36 months. Adding months keeps the day of the month where possible - 31 January plus one month is 28 February, or 29 in a leap year.
Period After Opening
Use By=Date Opened+PAO Months,whichever comes first
The open-jar symbol showing 12M means twelve months after opening. Once a product is opened, both clocks run at once and the earlier date is the one that counts.
Benefits
Scales any percentage recipe to any batch size
Warns when a formula does not total 100%
Shows a running percentage total as you type
Works in grams, kilograms, ounces and pounds
Tracks both the shelf-life and open-jar clocks and reports the earlier
Handles month-end dates correctly - 31 January plus a month is 28 February
Includes typical open-jar lifespans for common product types
When & Where to Use
Scaling a tested formula from a sample to a production batch
Converting a percentage recipe into weights for the scale
Checking that a formula you have been given adds up to 100%
Working out how much preservative a batch needs
Deciding whether an old product in the cupboard is still safe
Checking whether an unopened product is still within date
Teaching formulation percentages to students
Who Should Use This Calculator
The Cosmetic Calculator is for indie skincare and cosmetic makers, formulation students, small-batch producers scaling recipes, and anyone wanting to know whether a product they own is still safe to use.
Tips to Get the Best Deal
Weigh by mass, not volume - oils and water differ enough in density to change a formula
Percentages must total exactly 100, and water is normally what absorbs the difference
Write formulas in percentages so they scale to any batch size
Preservative percentages come from the supplier and should not be reduced
Note the date you open a product; the open-jar clock starts then
An expired product can look and smell fine - preservatives fail before that
Small batches are easier to weigh accurately with a 0.01 g scale
Frequently Asked Questions (FAQs)
How do I convert a cosmetic formula from percentages to weights?
Multiply each percentage by the batch size and divide by 100. An ingredient at 6% of a 500 g batch is 0.06 x 500 = 30 g. Because every ingredient scales the same way, the weights add back up to the batch size.
Why must a cosmetic formula total 100%?
Because the percentages describe the whole product. If they sum to 98% then 2% of the product is unaccounted for, which usually means an ingredient is missing or a percentage was mistyped. Water is conventionally adjusted to bring the total to 100.
How long do cosmetics last?
Unopened products typically last 36 months from manufacture. After opening, the open-jar symbol applies - commonly 12 months, but mascara is 3 to 6 months, liquid eyeliner 3 to 4, skincare in jars 6 to 10, and natural products up to 6.
What does the open-jar symbol mean?
It is the Period After Opening. A symbol reading 12M means the product should be used within twelve months of being opened, regardless of how much shelf life remains from manufacture.
Which date applies if a product is both old and recently opened?
Whichever comes first. A product made three years ago and opened last week has already reached its shelf-life limit, so the open-jar period does not extend it.
Can this decode a batch code?
No. Batch codes are brand-specific and undocumented, and decoding them means reverse-engineering each brand's scheme from hundreds of collected samples. This calculator works from dates you supply rather than guessing at a code.
Should I measure ingredients by volume instead?
No. Cosmetic formulation is always by weight. Oils, waters and butters differ in density, so measuring by millilitres gives a different formula from the one you intended.
Pro Tips
Weigh by mass, not volume - oils and water differ enough in density to change a formula.
Percentages must total exactly 100; water normally absorbs the difference.
Write formulas in percentages so they scale to any batch size.
Note the date you open a product - the open-jar clock starts then.
An expired product can look and smell fine long after its preservatives have failed.
Batch codes are brand-specific and cannot be decoded from a general rule.