Diluting a 10 Percent Active Down to a Target Strength

Somebody hands you a stock solution labeled "10% niacinamide" or "10% salicylic acid in propylene glycol" and asks you to land at 2% in the finished batch. It sounds like it should be obvious, and it mostly is, except the number of formulators who get this wrong on their first pass is higher than you'd think, usually because they forget which percentage they're diluting from.

The core equation

The relationship you need is the same one used in any dilution problem:

C1 × V1 = C2 × V2

Where C1 is the concentration of your stock, V1 is the volume or weight of stock you're using, C2 is your target finished concentration, and V2 is your total batch size. Rearranged to solve for how much stock you need:

V1 = (C2 × V2) / C1

So if you want a 500 gram batch at 2% active, and your stock is 10% active:

V1 = (2 × 500) / 10 = 100 grams of stock

That means 100 grams of your 10% stock, topped up with 400 grams of everything else, gets you to 2% active in a 500 gram batch. The active itself, in absolute terms, is 10 grams either way you slice it, since 100 grams of a 10% stock contains 10 grams of pure active, and 10 grams in a 500 gram batch is exactly 2%.

Where formulators trip up

The most common mistake is treating the stock's labeled percentage as if it were the target, and adding the whole stock amount without doing the ratio. Someone wants 2% final and just weighs out 2% of the batch weight in stock solution, forgetting the stock itself is only 10% active. That mistake underdelivers your active by a factor of five, silently, and nobody notices until efficacy claims don't hold up or a customer complains the product doesn't work like the last batch.

The opposite mistake happens with restricted actives. If you're diluting something that has a maximum permitted concentration on the Hotlist or a similar list, doing the math backward can push you over the limit instead of under it. Always double check which direction your rounding needs to go.

A quick reference table

Target finished %	Stock strength	Stock needed per 100 g batch
0.5%	10%	5.0 g
1.0%	10%	10.0 g
2.0%	10%	20.0 g
5.0%	10%	50.0 g

Notice the pattern: at a 10% stock, the grams of stock you need per 100 grams of batch is always ten times your target percentage. That shortcut works cleanly for a 10% stock specifically, which is part of why suppliers like round numbers like 10% for their premixes.

When the active itself is a blend

It gets more layered when your "10% active" stock is actually a supplier premix that already contains a carrier, a stabilizer, or a secondary active. In that case you're not just diluting one number, you're diluting a blend, and every component in that blend needs to be walked through the same multiplication before it lands in your finished percentage. A 10% vitamin C stock that's actually 10% ascorbic acid plus 2% ferulic acid as a co-antioxidant means your 20 gram addition per 100 gram batch delivers 2% ascorbic acid and 0.4% ferulic acid, not just the headline active.

Why this matters for the filing itself

When you submit a Cosmetic Notification Form, the concentration or concentration range you declare for each ingredient needs to reflect what's actually in the finished product, not what's in your raw material drum. Health Canada screens those percentages against the Cosmetic Ingredient Hotlist, and if a restricted ingredient's declared level is wrong because a dilution calculation was off, that's a problem that surfaces at the worst possible time, either as a request for clarification or, worse, after the product is already on shelves.

Keep your dilution math on the same page as your formula sheet, not in a separate spreadsheet you have to hunt for later. When Cosmetic Comply processes your ingredient list, it carries percentages through from your raw materials to the finished product and flags anything that lands near a restricted threshold, so a rounding slip in a dilution calculation doesn't quietly become a compliance problem down the line.

Whatever your target strength, do the C1V1 math explicitly, on paper or in your formula software, rather than eyeballing it. It takes thirty seconds and it's the difference between a product that performs as advertised and one that quietly doesn't.