How to Formulate a Face Serum: The Base, the Actives and What Breaks It

One thing to know before we start: Genie is the AI formulator for indie brands. It develops the formula. A contract manufacturer produces it. Those are two different jobs, and this post is about the first one.

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What Makes a Serum Different From a Moisturizer

A serum is a delivery vehicle. Its job is to carry a concentrated payload of actives into skin efficiently, which means it has to be light enough to absorb fast and formulated at the right pH to keep those actives stable and functional.

Most face serums are water-based. Some are anhydrous (oil-based, no water at all). A few are two-phase or emulsion serums. This guide focuses on water-based serum formulation, which is where most indie founders start and where the most interesting formulation decisions happen.

Compared to a moisturizer, a serum typically has:

• A thinner, lower-viscosity texture (though gel serums can be quite thick)
• A higher concentration of key actives
• A more tightly controlled pH, because many actives only work within a narrow window
• A simpler emollient layer, since occlusion is usually left to the moisturizer that follows

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Step 1: Choose Your Serum Type and Skin Story

Before you touch a single ingredient, decide what the serum does for the person using it. Not in clinical terms. In sensory and experiential terms.

Does it leave skin looking more even-toned after four weeks of use? Does it make skin feel plumped and bouncy right after application? Does it help skin feel calm and less reactive over time?

This matters for formulation because your skin story determines your active selection, your pH target, and your texture. A brightening serum built around vitamin C will have completely different formulation constraints than a hydration serum built around hyaluronic acid and peptides.

Common serum archetypes and their primary actives:

• Brightening: L-ascorbic acid (vitamin C), niacinamide, alpha arbutin, tranexamic acid
• Hydration and plumping: Hyaluronic acid (multiple molecular weights), sodium PCA, glycerin, panthenol
• Barrier support: Ceramides, cholesterol, fatty acids, beta-glucan
• Smoothing and resurfacing: AHAs (glycolic, lactic, mandelic), PHAs, BHAs
• Firming and anti-aging: Peptides (matrixyl, argireline, copper peptides), retinol, bakuchiol
• Soothing and sensitivity: Centella asiatica, bisabolol, allantoin, azelaic acid

Pick a primary active or a complementary pair. A serum trying to do everything usually does nothing well.

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Step 2: Build Your Water Phase

In a water-based serum, the water phase is the foundation. It carries your humectants, water-soluble actives, and anything that needs to be dissolved before the formula comes together.

Distilled or deionized water is your base. Tap water introduces minerals that can destabilize actives and feed microbial growth. The water percentage in a typical serum runs anywhere from 60% to 90% of the formula, depending on how concentrated and textured you want the final product.

Humectants pull moisture into skin and give a serum its immediate sensory payoff. The workhorses:

• Glycerin: typically 2% to 10%. Below 2% you lose the humectant effect. Above 10% it can feel sticky on some skin types. The right level depends on what else is in the formula and the texture you're after.
• Hyaluronic acid (sodium hyaluronate): typically 0.1% to 2%. Higher molecular weight HA sits on the surface and creates a plumping, film-forming effect. Lower molecular weight penetrates deeper. Many formulas use two or three molecular weights together. The grade and supplier matter here: HA quality varies significantly, and a low-grade material at 1% will not perform like a high-grade material at 0.5%.
• Sodium PCA and panthenol: both excellent supporting humectants, typically 0.5% to 3% each.

Pro tip: Dissolve your humectants in water before adding anything else. Undissolved glycerin sitting on top of an active can create hot spots in the formula.

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Step 3: Select and Dose Your Actives

This is where serum formula percentages get specific, and where most indie founders either over-formulate or under-dose.

Every active has a functional range. Below it, the ingredient is decoration. Above it, you risk irritation, instability, or regulatory issues. The right dose within that range depends on the grade of the material, what else is in the formula, and the pH you're targeting.

Vitamin C (L-Ascorbic Acid)

L-ascorbic acid is the most studied form of vitamin C for skin, and the most demanding to formulate. It works best at a pH of 2.5 to 3.5. Above pH 4, it starts to lose efficacy. Above pH 5, it's largely inactive.

Typical use level: 5% to 20%. Lower percentages are gentler and easier to stabilize. Higher percentages are more potent but require tighter pH control and better packaging (airless, opaque, or nitrogen-flushed) to prevent oxidation.

How you'll know it's failing: the formula turns yellow, then orange, then brown. That's oxidized ascorbic acid. It's not dangerous, but it's no longer working, and your customer will notice.

Niacinamide

Niacinamide is one of the more forgiving actives to formulate. It's water-soluble, stable across a wide pH range (roughly 5 to 7), and plays well with most other ingredients.

Typical use level: 2% to 10%. Most of the published evidence clusters around 4% to 5% for visible results on uneven tone and texture.

The niacinamide-plus-vitamin-C question: the old concern that they react to form niacin (which causes flushing) has largely been debunked at typical use temperatures and concentrations. The real issue is pH compatibility. Niacinamide is happy at pH 5 to 7. Vitamin C wants pH 2.5 to 3.5. You can't satisfy both in one formula without compromising one of them. That's a formulation decision, not a chemistry myth.

AHAs and BHAs

Glycolic acid, lactic acid, mandelic acid, and salicylic acid all work by exfoliating dead skin cells, supporting a smoother, more even surface over time. They are pH-dependent: most need to be at pH 3 to 4 to be effective as exfoliants.

Typical use levels vary by acid. Salicylic acid has a legal limit in leave-on products in both the US and EU. The exact number is confirmed when a chemist reviews the formula. The same applies to AHAs at higher concentrations, which fall under specific guidelines in some markets.

How you'll know the pH is off: a properly acidic AHA formula should have a noticeably tart smell. If it smells neutral, check your pH.

Retinol

Retinol is light-sensitive, oxygen-sensitive, and pH-sensitive. It's most stable at pH 5 to 6 and needs to be protected from light and air. Encapsulated retinol is easier to formulate but behaves differently depending on the encapsulation technology and supplier.

Typical use level: 0.01% to 1%. In the EU, leave-on face products have a regulatory limit on retinol concentration. That limit is checked at chemist review.

Peptides

Peptides are expensive, sensitive to pH extremes, and sometimes incompatible with certain preservatives. Matrixyl 3000, for example, should be added at the end of the water phase at low temperatures to preserve activity. Copper peptides can interact with other actives and turn the formula an unexpected color.

Typical use level: 0.5% to 5%, depending on the peptide and its delivery system. Supplier recommendations are your starting point, not your ceiling.

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Step 4: Add Your Texture Builders

Most water-based serums need a thickener or gelling agent to get the right slip and skin feel. The most common options:

• Carbomers (carbopol): Create a clear, elegant gel. They need to be neutralized with a base (sodium hydroxide solution or triethanolamine) to reach their working viscosity. Neutralization also raises the pH, so you plan your pH-sensitive actives around this step.
• Hyaluronic acid at higher percentages: At 1.5% to 2%, high-molecular-weight HA creates a gel-like texture on its own.
• Hydroxyethylcellulose (HEC): A natural-derived thickener that works well in lower-pH formulas where carbomers struggle.
• Xanthan gum: Effective but can create a slightly stringy texture at higher levels. Usually combined with other gellants.

Pro tip: The order of addition matters. Disperse your gelling agent in water before adding actives. Adding a carbomer to a formula that already contains electrolytes (like niacinamide at high levels) can prevent it from hydrating properly, and you'll end up with a lumpy, under-thickened serum.

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Step 5: Preserve It Properly

A water-based serum without a preservative system is a petri dish. Contamination doesn't always look like contamination: some bacteria and mold grow invisibly before they reach a level that's visible or smells off. By then, the product is compromised.

Common preservative systems and their considerations:

• Phenoxyethanol: Widely used, globally approved, effective against a broad spectrum. Typically 0.5% to 1%. Has a legal maximum in the EU (1%). Works best paired with a co-preservative.
• Ethylhexylglycerin: Often combined with phenoxyethanol. Boosts efficacy and improves skin feel.
• Sodium benzoate and potassium sorbate: Popular in "clean" formulas. pH-dependent: they work best below pH 5, which makes them a good fit for AHA serums but a poor fit for peptide serums at neutral pH.
• Caprylyl glycol and other multifunctional ingredients: Provide some antimicrobial activity while also acting as skin-conditioning agents.

Preservative efficacy testing (PET or challenge testing) is required before a product goes to market. This is done by your contract manufacturer or an independent lab. It is not optional.

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Step 6: Control Your pH

pH is the variable that makes or breaks a serum. It determines whether your actives work, whether your preservative works, and whether the formula stays stable on the shelf.

General targets by serum type:

• Vitamin C (L-ascorbic acid) serums: pH 2.5 to 3.5
• AHA/BHA exfoliant serums: pH 3 to 4
• Niacinamide, peptide, and hydration serums: pH 5 to 6.5
• Retinol serums: pH 5 to 6

Adjust pH with a dilute sodium hydroxide solution (to raise) or citric acid solution (to lower). Always adjust in small increments. Always measure after each addition. A calibrated digital pH meter is non-negotiable.

What breaks it: Adding a high-pH ingredient (like a carbomer that's been pre-neutralized) to a low-pH vitamin C formula will crash your vitamin C's efficacy. Plan the order of addition with pH in mind from the start.

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Step 7: Account for What Breaks the Formula

Some failure modes are obvious. Others show up three months into stability testing. Here are the ones worth knowing before you start:

Separation

An emulsion serum that separates has an emulsification problem: either the wrong emulsifier, the wrong ratio, or a processing temperature issue. You'll see a clear layer forming at the top or bottom of the bottle. In a true water-based serum (no oils), separation usually means an incompatible ingredient pair or a gelling agent that wasn't properly hydrated.

Discoloration

Vitamin C oxidation is the most common culprit. Copper peptides reacting with other actives is another. If your formula changes color between manufacture and the end of its shelf life, you have a stability or packaging problem.

pH Drift

Some formulas drift in pH over time, especially those with AHAs or vitamin C. A formula that's at pH 3.2 on day one and pH 4.5 at month three has lost most of its vitamin C efficacy before it reaches the customer. Stability testing catches this.

Microbial Contamination

A failed PET result means your preservative system isn't doing its job. This can happen because the preservative is at the wrong pH, incompatible with another ingredient, or simply underdosed. Some natural-origin preservatives have narrower effective ranges than their conventional counterparts.

Incompatible Actives

Retinol and AHAs in the same formula at low pH will degrade each other. Vitamin C and copper peptides can react. High concentrations of niacinamide with zinc can cause pilling. These interactions are formula-specific, not universal rules, but they're worth checking before you finalize a formula.

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From Formula to Manufactured Product

Building the formula is the first half of the job. Getting it made is the second.

Genie develops the formula: you pitch the concept, the AI builds it from a 180,000-row ingredient database, and an experienced chemist reviews it and prepares the final spec documents your manufacturer works from. Genie also matches you with a contract manufacturer from its network.

Starting production requires a $500 refundable deposit, to Genie. After the chemist review, you pay your manufacturer's sample round at cost, collected by Genie and paid to your manufacturer once their sample pricing is on your order. When you approve your run, the rest of what it costs is shown in full on your order before you approve: your manufacturer invoices their price on their own payment terms, and the rest is paid on your order. Your deposit is refunded in full any time before you approve your run, or if Genie cannot make your product.

For founders who want to take the formula to their own manufacturer, Genie offers a chemist-reviewed, manufacturing-ready tech pack. See /pricing for details by product type.

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Frequently Asked Questions

What percentage of active ingredients should a face serum contain?

There is no single right answer. Each active has a functional range, and the right level within that range depends on the grade of the material, the rest of the formula, and the pH. A vitamin C serum might use 5% to 20% L-ascorbic acid. A niacinamide serum typically works at 2% to 10%. Dosing to the bottom of a range is not always weaker: a high-grade material at a lower percentage often outperforms a low-grade material at a higher one.

Can I combine vitamin C and niacinamide in one serum?

The chemistry concern (that they react to form niacin and cause flushing) is largely a non-issue at typical use temperatures and concentrations. The real formulation challenge is pH: vitamin C as L-ascorbic acid needs pH 2.5 to 3.5, while niacinamide is most stable and effective at pH 5 to 7. Combining them means compromising one or both. Most formulators choose one as the lead active and build the pH around it.

How do I know if my serum formula is stable?

Stability testing is the only way to know. A basic accelerated stability study puts the formula through temperature cycling and elevated heat (typically 40°C for 8 to 12 weeks) and checks for pH drift, color change, separation, viscosity change, and microbial growth. This is done before you go to market, usually at your contract manufacturer or an independent lab.

What preservative should I use in a water-based serum?

It depends on your formula's pH, your target market's regulatory requirements, and your brand's ingredient positioning. Phenoxyethanol paired with ethylhexylglycerin is a widely used, globally approved combination. Sodium benzoate and potassium sorbate work well in low-pH formulas. All preservative systems need to be validated with a preservative efficacy test before the product launches.

Do I need a chemist to formulate a face serum?

You need chemist review before the formula goes to manufacturing. A chemist confirms that actives are dosed within safe and effective ranges, checks regulatory limits for your target markets, and prepares the spec documents your manufacturer works from. On Genie, that review is part of the production process: the AI builds the formula, and a chemist reviews it before it goes to your manufacturer.

What is the difference between a serum and an essence?

The line is blurry and often marketing-driven. In general, an essence is lighter and more watery than a serum, with a lower concentration of actives. It's applied earlier in a routine, often right after cleansing, to prep skin for the products that follow. A serum is typically more concentrated and targeted. Formulation-wise, both are water-based, but a serum usually has more sophisticated thickening and a higher active payload.

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Key Takeaways

• A water-based serum is built in layers: water phase, humectants, actives, texture builders, preservative, and pH adjustment. Order of addition matters.
• Every active has a functional range. The right dose within that range depends on the grade of the material, the rest of the formula, and your pH target. Never treat a single percentage as the answer.
• pH is the variable that controls whether your actives work, whether your preservative works, and whether the formula holds on the shelf. Know your target before you start.
• Some active combinations are genuinely incompatible at certain pH levels. Vitamin C and niacinamide is the most discussed example. Plan your pH around your lead active.
• Stability testing and preservative efficacy testing are not optional. They happen before the product reaches a customer.
• Chemist review is part of the path from formula to manufactured product. It's where regulatory limits are confirmed and spec documents are prepared.

Ready to build your serum formula? Get started free on Genie