Calculate Molarity and Molar Absorptivity
Provide the mass concentration and molecular weight. Absorbance is optional, but it is required for the extinction coefficient result.
Example Data Table
These examples show the relationship between mass concentration, molarity, dilution, and molar absorptivity.
| mg/mL | Molecular Weight | Dilution | Absorbance | Path | Original Molarity | Extinction Coefficient |
|---|---|---|---|---|---|---|
| 2.00 | 50,000 g/mol | 1× | 0.80 | 1 cm | 4.00e-5 M | 20,000 L mol⁻¹ cm⁻¹ |
| 1.50 | 180.16 g/mol | 1× | 0.60 | 1 cm | 0.008326 M | 72.06 L mol⁻¹ cm⁻¹ |
| 0.75 | 66,000 g/mol | 5× | 0.15 | 1 cm | 1.136e-5 M | 66,000 L mol⁻¹ cm⁻¹ |
| 3.00 | 300 g/mol | 2× | 0.45 | 0.5 cm | 0.010000 M | 180 L mol⁻¹ cm⁻¹ |
Formula Used
The mass concentration conversion works because 1 mg/mL is numerically equal to 1 g/L.
The extinction coefficient result is expressed as L mol⁻¹ cm⁻¹. Use the diluted concentration whenever absorbance was measured after dilution.
How to Use This Calculator
- Enter the known concentration in mg/mL.
- Enter molecular weight in grams per mole.
- Enter the total dilution factor. Use 1 for no dilution.
- Enter your corrected absorbance and path length to calculate ε.
- Add wavelength when you want it included in CSV output.
- Review the result above the form, then download CSV or save a PDF.
Reliable Molarity and Absorbance Calculations
Why Molarity Matters
Concentration describes how much substance exists in a known volume. Laboratories often report protein, dye, or chemical concentrations in milligrams per milliliter. This unit is useful for preparation. It does not directly show the number of molecules or moles present. Molarity solves that problem. It expresses moles of solute in each liter of solution.
Convert Mass Concentration
The conversion from mg/mL to molarity requires molecular weight. A value of one mg/mL equals one g/L. Divide the mass concentration by molecular weight in g/mol. The result is mol/L, also called molarity. For example, a solution containing 2 mg/mL of a 50,000 g/mol protein has a concentration of 0.00004 M. That equals 40 micromolar.
Use the Beer–Lambert Relationship
Extinction coefficient connects concentration with light absorption. The Beer–Lambert law is A = εcl. In this equation, A is absorbance, ε is molar extinction coefficient, c is molar concentration, and l is optical path length. Most standard cuvettes have a one centimeter path length. Confirm the actual path length when using microvolume instruments.
Correct the Absorbance Reading
Absorbance must represent the corrected sample value. Subtract an appropriate blank before entering it. The blank should contain the same buffer, solvent, and additives without the analyte. Blank correction reduces interference from tubes, reagents, or solvent color. A poor blank can create a misleading extinction coefficient.
Account for Dilution
Dilution changes the concentration present during absorbance measurement. Enter the total dilution factor used before reading the sample. A tenfold dilution means the measured concentration is one tenth of the original concentration. The calculator uses diluted molarity for the extinction coefficient. This approach matches the concentration that passed through the optical path.
Choose the Correct Molecular Weight
Molecular weight accuracy matters. Use the molecular weight of the actual analyte form. Salt forms, hydrates, conjugates, and protein tags may increase the value. For proteins, use the sequence-specific molecular mass when available. A wrong molecular weight produces a proportionally wrong molarity result.
Work Within Instrument Limits
Choose absorbance values within the instrument’s reliable range. Very high absorbance can reduce linearity. Many instruments perform best with absorbance values near 0.1 to 1.0. Dilute concentrated samples when necessary. Recalculate the dilution factor before estimating molar absorptivity.
Interpret the Extinction Coefficient
Molar extinction coefficients are often reported in L mol⁻¹ cm⁻¹. This calculator returns that unit when absorbance, path length, and valid concentration are supplied. The result helps estimate unknown concentrations later. Rearranging the Beer–Lambert law gives c = A divided by εl.
Keep Units Consistent
Use consistent units throughout every calculation. Mass concentration stays in mg/mL. Molecular weight stays in g/mol. Path length stays in centimeters. Absorbance has no unit. Enter positive values and review the result before recording it. Precision is useful, but realistic input data remains more important.
Record Experimental Conditions
Replicate readings improve confidence. Measure the same prepared sample at least twice. Record wavelength and temperature with the result. Some analytes change absorption with pH, binding, or temperature. Use a wavelength appropriate for the analyte. Compare results only when experimental conditions remain comparable. Document the instrument model and measurement wavelength in records.
Frequently Asked Questions
What does mg/mL mean?
It means milligrams of solute in each milliliter of solution. It is a mass-per-volume concentration. Numerically, one mg/mL is equal to one g/L, which makes the molarity conversion straightforward when molecular weight is known.
How is mg/mL converted to molarity?
Divide the mg/mL concentration by molecular weight in g/mol. Because mg/mL equals g/L numerically, the result is mol/L. For instance, 1 mg/mL divided by 100 g/mol equals 0.01 M.
Why is molecular weight required?
Mass alone cannot show how many moles are present. Molecular weight links mass to moles. A heavier molecule produces fewer moles at the same mg/mL concentration than a lighter molecule.
Does one mg/mL always equal one g/L?
Yes. Both units describe the same mass-to-volume ratio. One milligram per milliliter converts directly to one gram per liter. This equality supports the calculator’s direct molarity formula.
When is the extinction coefficient calculated?
The calculator determines molar extinction coefficient when corrected absorbance, path length, and valid concentration are available. It applies the Beer–Lambert relationship, ε = A divided by c times l.
Do I need absorbance to calculate molarity?
No. Molarity only needs mg/mL concentration and molecular weight. Absorbance is optional. Add it only when you also want the molar extinction coefficient.
What path length should I enter?
Enter the effective optical path length used for the absorbance measurement. A standard cuvette is often 1 cm. Microvolume devices may use shorter path lengths, so follow the instrument documentation.
How does dilution affect the result?
Dilution does not change the original molarity. It changes the concentration present while you measure absorbance. The calculator divides original molarity by the dilution factor before calculating the extinction coefficient.
Can this calculator be used for proteins?
Yes. Enter the protein concentration, sequence-specific molecular weight, corrected absorbance, and path length. Ensure the molecular weight includes relevant tags, conjugates, or modifications when they are present.
What units are used for molar extinction coefficient?
The result is reported in L mol⁻¹ cm⁻¹. This is equivalent to M⁻¹ cm⁻¹. It describes how strongly one molar solution absorbs light through a one-centimeter optical path.
Why might my result be unreliable?
Check blank correction, dilution, molecular weight, and instrument linearity. Use these values carefully for dependable laboratory concentration calculations.