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IFRA limit calculator
Every IFRA limit is a percentage of the finished product. Almost nobody works in the finished product. This is the one conversion between them, and getting it backwards is the most common expensive mistake in the subject.
The sum, in words
An IFRA limit is a maximum percentage of the finished product, at a stated category. Your concentrate is diluted on its way into that finished product. So the ceiling for your concentrate is the limit divided by the concentrate's own share, or equivalently the limit multiplied by a hundred and divided by the concentrate percentage.
Eugenol is restricted to 2.5 percent of the finished product at category 4, which is fine fragrance. If your eau de parfum is 20 percent concentrate, the concentrate may carry 2.5 multiplied by 100, divided by 20, which is 12.5 percent eugenol. That arrives in the bottle at exactly 2.5 percent.
The other direction is the same sum rearranged. A concentrate holding 8 percent of a material, used at 20 percent, puts 8 multiplied by 20, divided by 100, which is 1.6 percent, into the finished product.
The error this exists to prevent
Reading the limit against the concentrate. A perfumer sees a limit of 2.5 percent, looks at a concentrate holding 8 percent, and reformulates a blend that was never over anything: at 20 percent strength it was sitting at 1.6 percent of the finished product, well inside the limit.
The same error in the other direction is worse. A concentrate assessed for a 20 percent eau de parfum, then sold neat as a perfume oil, has every figure multiplied by five, and materials that were comfortably inside are now over.
Get the category right first
None of this arithmetic helps if the limit you started with was the wrong one. The category is decided by the finished product a customer buys, not by the blend: the same material is limited to 2.5 percent in fine fragrance, 0.64 percent in a body lotion, 4.9 percent in a shower gel and not at all in a candle. A fragrance oil moved from one product type to another has to be reassessed rather than assumed.
What this calculator does not do
It assumes the material arrives on your formula as itself, and that is very often not the case. A restricted material frequently reaches a blend as a constituent of a natural: clove bud oil is mostly eugenol, so a formula with no eugenol line on it can still be far over the limit. Every route the material arrives by has to be summed before this arithmetic means anything.
It also does not know whether your material is prohibited outright, whether a specification applies to it, whether the amendment you are working to is the current one, or what else on your formula is restricted. It converts one number. Doing that across 263 Standards, every time a formula changes, is a different job.