Dilute a skincare active.

Worked example

You have a 15% serum and want 5% in 30 g. Serum needed = 30 x 5 / 15 = 10 g; base = 30 - 10 = 20 g, a 1:2 ratio. Check: (10 x 15 + 20 x 0) / 30 = 5%.

How it works

Percent by weight is mass-balanced: final % = (m1 x s1 + m2 x s2) / (m1 + m2). To dilute with a base that has none of the active: serum grams = total x target % / strength %, base grams = total minus serum grams.

What this is not. It is arithmetic, not a formulation check. Diluting a finished product also dilutes its preservative, can shift its pH (important for L-ascorbic acid and acids) and can make it unstable or let bacteria grow. Mix small batches, use clean tools, keep them cold and use them within days. Do not dilute prescription products unless your prescriber says to. Strength of the base should be 0% active; if your base already has some, use the blend mode.

Sources and basis: mass balance on percent-by-weight (w/w) concentration, standard chemistry. Preservative and pH cautions as in cosmetic-formulation practice; for formulated recipes see the DIY cosmetic formula scaler.

FAQ

How do I dilute a 20% vitamin C serum to 10%?

Mix equal weights: for 30 g, 15 g of the serum and 15 g of base. Use a base that is compatible with an acidic serum and use it quickly.

Can I just mix my serum into my moisturizer?

Many people do. The arithmetic here tells you the final percent, but the cream’s pH and preservative may not suit the mix, so dilute only what you will use soon.

Is 1 ml equal to 1 gram?

Only for water-like liquids. Oils are about 0.9 g/ml and thick creams vary, so weigh in grams where you can.

Is a lower strength better for beginners?

Often. For retinol and acids it is usual to start low and build up, but follow the schedule your product or prescriber gives.

How long does a diluted serum last?

Treat it as short-lived: days, not months, unless it was made with a proper preservative system in a lab or by a formulator.