Molarity Calculator¶
Convert between concentration units for peptide solutions. Calculate molarity from mass and molecular weight, or determine the mass needed to achieve a target molarity.
Calculator¶
Mode 1: Mass → Molarity
Calculate molarity from peptide mass and molecular weight.
Concentration Results
| Molarity | — |
| Amount (nmol) | — |
| Concentration (mg/mL) | — |
| Concentration (µM) | — |
Mode 2: Target Molarity → Required Mass
Calculate the peptide mass needed to reach a target molarity.
Required Mass
| Peptide Mass Needed | — |
| Total nmol | — |
How Molarity Calculations Work¶
Molarity (M) is the number of moles of solute per liter of solution. It is the standard concentration unit in chemistry and molecular biology because it directly describes the number of molecules — not just the mass — present in solution.
The fundamental relationship is:
C = n / V = m / (MW × V)
Where C = concentration (M), n = amount (mol), V = volume (L), m = mass (g), and MW = molecular weight (Da, equivalent to g/mol).
Why molarity over mass concentration? Molarity tells you the actual number of molecules in solution. Two peptides at 1 mg/mL may have very different molar concentrations if their molecular weights differ. For reactions, binding studies, and dose-response experiments, it is the number of molecules — not the mass — that matters.
Unit relationships:
- mg/mL is a mass concentration — it does not depend on MW
- mM (millimolar) = 10⁻³ M — a molar concentration that depends on MW
- µM (micromolar) = 10⁻⁶ M — standard for most peptide assays
- Converting between them: mM = (mg/mL) × 1000 / MW
Core Formulas¶
Molarity from mass and volume:
C (M) = m (g) / (MW × V (L))
Molarity in mM (practical lab form):
C (mM) = [m (mg) / (MW × V (mL))] × 1000
Mass needed for target molarity:
m (mg) = C (mM) × MW × V (mL) / 1000
Variable definitions:
- C = concentration (M — mol/L; mM — mmol/L)
- m = mass of peptide (g or mg)
- MW = molecular weight (Da, g/mol)
- V = volume of solution (L or mL)
- n = amount of substance (mol or mmol)
Worked Examples
Example 1: Peptide mass → Molarity
Given: 10 mg of a peptide (MW = 1,000 Da) dissolved in 1 mL of buffer.
Step 1: Convert mass to grams → 10 mg = 0.010 g
Step 2: Convert volume to liters → 1 mL = 0.001 L
Step 3: Apply C = m / (MW × V)
C = 0.010 g / (1,000 g/mol × 0.001 L) = 0.01 M
Step 4: Convert to convenient units
- 0.01 M = 10 mM
- 0.01 M = 10,000 µM
- Amount = 0.01 M × 0.001 L × 10⁹ = 10,000 nmol
Alternatively, using the practical formula: C(mM) = [10 mg / (1,000 × 1 mL)] × 1000 = 10 mM ✓
Example 2: Target Molarity → Peptide mass
Given: Need 500 µL of a 100 µM peptide solution. MW = 500 Da.
Step 1: Convert target concentration to M → 100 µM = 100 × 10⁻⁶ M = 10⁻⁴ M
Step 2: Convert volume to liters → 500 µL = 500 × 10⁻⁶ L = 5 × 10⁻⁴ L
Step 3: Apply m = C × MW × V
m = (100 × 10⁻⁶ M) × (500 g/mol) × (500 × 10⁻⁶ L)
m = 2.5 × 10⁻⁵ g = 0.025 mg (25 µg)
Step 4: Using the practical formula in common units:
m(mg) = 0.1 mM × 500 × 0.5 mL / 1000 = 0.025 mg ✓
Result: Weigh 25 µg of peptide and dissolve in 500 µL of buffer to obtain a 100 µM solution.
Unit Conversion Reference
Molar concentration unit relationships:
- 1 M = 1,000 mM = 1,000,000 µM
- 1 mM = 0.001 M = 1,000 µM
- 1 µM = 0.001 mM = 10⁻⁶ M
Mass-to-molar conversion for a given MW:
1 mg/mL = (1000 / MW) mM | 1 mM = (MW / 1000) mg/mL
Quick lookup table — mg/mL to mM for common peptide MWs:
| MW (Da) | 1 mg/mL = | 1 mM = | 10 mg/mL = |
|---|---|---|---|
| 500 | 2.000 mM | 0.500 mg/mL | 20.00 mM |
| 1,000 | 1.000 mM | 1.000 mg/mL | 10.00 mM |
| 1,500 | 0.667 mM | 1.500 mg/mL | 6.67 mM |
| 2,000 | 0.500 mM | 2.000 mg/mL | 5.00 mM |
| 3,000 | 0.333 mM | 3.000 mg/mL | 3.33 mM |
| 5,000 | 0.200 mM | 5.000 mg/mL | 2.00 mM |
| 10,000 | 0.100 mM | 10.000 mg/mL | 1.00 mM |
Frequently Asked Questions
What's the difference between molarity and molality?
Molarity (M) is moles of solute per liter of solution. It depends on volume, which changes with temperature.
Molality (m) is moles of solute per kilogram of solvent. It is temperature-independent because it is based on mass, not volume.
In peptide research, molarity is far more common because volumes are convenient to measure with pipettes and volumetric flasks. Molality is typically used in physical chemistry studies involving colligative properties (boiling point elevation, freezing point depression).
Why do I need the molecular weight for molarity but not mg/mL?
mg/mL is a mass concentration — it simply expresses how much mass is dissolved per volume. It requires no information about the substance itself.
Molarity expresses number of molecules per volume. To convert between mass and number of molecules, you need the molecular weight, which acts as a "molecular exchange rate" — telling you how many grams correspond to one mole (6.022 × 10²³ molecules).
How do I make a precise molar solution from a small peptide amount?
- Determine the exact peptide mass (accounting for purity and counterion content if applicable).
- Choose a target concentration appropriate for your intended use (typically 1–10 mM for stock solutions).
- Calculate the required volume using: V(mL) = m(mg) × 1000 / (C(mM) × MW)
- For very small peptide amounts (<1 mg), use a microbalance and prepare a larger volume of a more dilute stock, or prepare a small volume in a microcentrifuge tube.
- Always vortex and sonicate to ensure complete dissolution, especially for hydrophobic peptides.
What's the relationship between mM and mg/mL?
The two are directly linked by molecular weight:
mM = (mg/mL) × 1000 / MW
mg/mL = (mM × MW) / 1000
For example, a peptide with MW = 1,000 Da at 1 mg/mL is 1 mM. The same mass concentration of a peptide with MW = 500 Da would be 2 mM — twice as many molecules per mL because each molecule weighs half as much.
Can I use this calculator for stock solution preparation?
Yes. The calculator is ideal for stock solution preparation. Use Mode 1 (Mass → Molarity) to check the concentration of a stock you have already prepared, or Mode 2 (Target Molarity → Mass) to calculate exactly how much peptide to weigh for a desired stock concentration.
For best results:
- Prepare stocks at 1–10 mM for most peptides
- Use our Dilution Calculator to plan serial dilutions from your stock
- Account for peptide purity and salt/counterion content — use our Purity Calculator for precise adjustments
- Store peptide stocks aliquoted at −20°C or −80°C to avoid freeze-thaw degradation
Quick Conversion Reference¶
| Mass (mg) | MW (Da) | Vol (mL) | Molarity |
|---|---|---|---|
| 1 | 1,000 | 1 | 1.0 mM |
| 5 | 1,000 | 1 | 5.0 mM |
| 10 | 1,000 | 1 | 10.0 mM |
| 5 | 500 | 1 | 10.0 mM |
| 10 | 5,000 | 1 | 2.0 mM |
Formula¶
Molarity (M) = mass (g) / (MW (Da) × volume (L))
To convert to common units: - mM = M × 1,000 - µM = M × 1,000,000 - nmol = molarity (M) × volume (L) × 10⁹
Related Tools¶
- Dilution Calculator — Calculate reconstitution volumes
- Purity Calculator — Account for purity in mass calculations
- Molecular Weight Calculator — Calculate MW from sequence