Concentration math intimidates people more than it should. This is a plain-English guide to the three ideas you’ll use most in in-vitro research: molarity, stock solutions, and dilutions.
> Scope note: this is laboratory calculation guidance for research solutions only. It is not dosing, medical, or administration advice.
1. Concentration, simply
Concentration is how much stuff is in how much liquid. Two common ways to express it:
- Mass per volume — e.g., milligrams per millilitre (mg/mL). Easiest for everyday bench work.
- Molarity (moles per litre, M) — counts *molecules*, useful when a protocol specifies molar concentrations.
To convert mass/volume to molarity you need the molecular weight (MW) of the compound:
> molarity (mol/L) = concentration (g/L) ÷ molecular weight (g/mol)
Because 1 mg/mL = 1 g/L, you can go straight from mg/mL and MW to molarity.
2. Making a stock solution
A stock is a concentrated solution you’ll later dilute. The relationship is simple:
> mass = concentration × volume
So if you dissolve a known mass of powder in a known volume of solvent, the concentration is mass ÷ volume. Want a specific concentration? Rearrange to find the volume of solvent or the mass of powder you need. (Our on-site in-vitro Lab Tools calculator does this for you.)
Accuracy tip: for precise work, base the mass on the net peptide content from the COA rather than the gross labeled mass.
3. Diluting a stock: C₁V₁ = C₂V₂
To make a weaker working solution from a stronger stock, use the dilution equation:
> C₁V₁ = C₂V₂
- C₁ = stock concentration
- C₂ = target (final) concentration
- V₂ = final volume you want
- V₁ = volume of stock to use = (C₂ × V₂) ÷ C₁
Then add solvent equal to V₂ − V₁ to reach the final volume.
Example: You have a 5 mg/mL stock and want 2 mL at 1 mg/mL. V₁ = (1 × 2) ÷ 5 = 0.4 mL of stock, then add 1.6 mL of solvent. Done.
A target can never be *stronger* than the stock — if your math says V₁ > V₂, your target is too high for that stock.
Common units and conversions
- 1 mg/mL = 1000 µg/mL = 1 g/L
- 1 M = 1000 mM = 1,000,000 µM
- 1 mL = 1000 µL; 1 L = 1000 mL
Key takeaways
- Concentration = amount ÷ volume; molarity counts molecules and needs MW.
- Stocks: mass = concentration × volume.
- Dilutions: C₁V₁ = C₂V₂, then top up with solvent.
FAQ
When do I need molecular weight? Only when working in molar units (M/mM/µM). For mg/mL you don’t need MW.
Why use net peptide content? Because a vial isn’t 100% peptide. Using net mass gives a more accurate concentration.
Is this dosing guidance? No. This is in-vitro laboratory math for research solutions only.
Browse verified research peptides
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*For laboratory and research use only. Not for human or veterinary use, consumption, or administration. Nothing in this article is medical advice or a claim that any compound treats, cures, or prevents any condition.*