Protein Concentration Calculator
Calculate protein concentration from mass and solution volume, UV absorbance at 280 nm, or a protein-assay standard curve. View results in mg/mL and g/L, with optional molar concentration when molecular weight is available.
Protein Concentration Calculator
Choose the calculation method that matches your experimental data. The calculator applies the selected formula and shows the calculation step by step.
Protein Concentration Result
Step-by-Step Calculation
What Is Protein Concentration?
Protein concentration describes the amount of protein present per unit volume of solution. Common laboratory units include mg/mL, g/L, µg/mL and molar units such as µM. The appropriate unit depends on the experiment and the information available about the protein.
A concentration calculation is only meaningful when the input measurement and calculation method match. A known protein mass divided by solution volume is a direct calculation, whereas A280 and colorimetric assays infer concentration from an optical response.
Why Measure Protein Concentration?
Protein concentration is used to normalize samples, prepare buffers and reactions, compare expression or purification fractions, calculate enzyme specific activity, and load comparable amounts of protein into downstream assays. Accurate concentration data can therefore affect many later calculations.
For assay-based measurements, the original sample concentration is usually the value of interest, not merely the concentration after mixing with assay reagent. A dilution performed before the assay must be accounted for when reporting the original sample concentration.
Method 1: Protein Mass ÷ Solution Volume
When the protein mass and final solution volume are known, concentration is calculated directly:
mg/mL = mg ÷ mL
For example, 2.5 mg of protein in 1.0 mL of solution gives 2.5 mg/mL. If the sample was previously diluted five-fold, the original concentration would be 12.5 mg/mL.
Method 2: A280 Protein Concentration
Purified proteins can often be quantified from absorbance at 280 nm using the Beer-Lambert relationship when a suitable extinction coefficient is known. The molar concentration is:
where A is absorbance, ε is the molar extinction coefficient and l is path length in cm.
To convert molar concentration to mg/mL, multiply mol/L by molecular weight in g/mol. The numerical result in g/L is equal to mg/mL.
Method 3: Standard-Curve Protein Assay
Bradford, BCA and other colorimetric protein assays generally use standards of known concentration to establish a response curve. If the fitted equation is y = mx + b, where y is assay response and x is protein concentration, the unknown is:
The resulting concentration must be corrected for any dilution made before the assay. The standards and unknown samples should be treated consistently and measured within the validated assay range.
Worked Example: Mass and Volume
Calculation: 4 mg ÷ 2 mL = 2 mg/mL.
If diluted 5-fold: original concentration = 2 × 5 = 10 mg/mL.
Worked Example: A280
Molar concentration: 0.876 ÷ (43,800 × 1) = 2.00 × 10⁻⁵ M.
Mass concentration: 2.00 × 10⁻⁵ × 50,000 = approximately 1.00 mg/mL.
Worked Example: Standard Curve
x = (0.780 − 0.050) ÷ 0.450 = 1.622 mg/mL.
If the sample was diluted 5-fold before measurement, the original concentration is approximately 8.11 mg/mL.
Protein Concentration Units
| Unit | Equivalent | Typical use |
|---|---|---|
| 1 mg/mL | 1 g/L | Protein stocks and purification fractions |
| 1 µg/mL | 0.001 mg/mL | Dilute assay samples |
| 1 M | MW g/L | Molar calculations when molecular weight is known |
| 1 µM | 0.001 mM | Biochemical reaction concentrations |
Why Molecular Weight Matters
Mass concentration and molar concentration describe the same solution in different ways. Converting between them requires molecular weight:
For proteins, the molecular weight should correspond to the actual molecular species being quantified, including relevant processing or tags when those materially affect the value.
Understanding A280 Measurements
Protein A280 measurements arise mainly from aromatic residues, particularly tryptophan and tyrosine, with contributions from other chromophores depending on the sample. The extinction coefficient is wavelength- and condition-dependent. A280 is most straightforward for purified proteins with a known or appropriately estimated extinction coefficient.
Buffers, nucleic acids, detergents and other absorbing substances can interfere with spectrophotometric measurements. For mixed lysates, a validated colorimetric assay may be more appropriate than assuming that all absorbance originates from protein.
Protein Assay Standard Curves
For Bradford, BCA and related assays, standards of known protein concentration are measured alongside unknown samples. Their responses are used to construct a calibration curve, and the unknown is interpolated from that curve. The relationship may be linear or nonlinear over different concentration ranges.
Dilution Factor and Original Concentration
If a sample was diluted before concentration measurement, multiply the measured concentration by the dilution factor to recover the estimated concentration of the original sample:
For a five-fold dilution, the dilution factor is 5. For a 1:10 dilution, it is 10. Be consistent about how your laboratory defines and records dilution factors.
When A280 Should Not Be Used Alone
A280 can be unsuitable when the sample contains substantial nucleic acid, other UV-absorbing compounds, multiple proteins with unknown extinction coefficients, or conditions that compromise the assumed relationship. A purified protein with a validated extinction coefficient is a much better fit for a direct A280 calculation.
For crude lysates and many formulation matrices, use the protein assay method validated for the sample type rather than treating A280 as universally specific for protein.
Common Protein Concentration Mistakes
- Forgetting to multiply by the pre-assay dilution factor.
- Mixing µg/mL, mg/mL and g/L without converting units.
- Using an extinction coefficient for the wrong protein or wavelength.
- Using a standard-curve slope from a different assay run or condition.
- Applying a linear standard-curve equation outside its validated range.
- Ignoring blank correction or matrix effects.
- Assuming a 1 cm path length when the instrument used a different effective path length.
Accuracy and Measurement Limitations
The calculator performs the mathematical conversion; it cannot determine whether the experimental measurement itself is valid. Accuracy depends on pipetting, weighing, instrument calibration, sample homogeneity, assay chemistry, standard preparation, blanking, optical path length and the validity of the extinction coefficient or standard curve.
Replicate measurements and an appropriate validated assay range help distinguish calculation errors from experimental variability.
How to Choose the Calculation Method
| Situation | Method | Key requirement |
|---|---|---|
| Known protein mass and final volume | Mass ÷ volume | Reliable mass and volume |
| Purified protein with known ε | A280 / Beer-Lambert | Valid ε, wavelength and path length |
| Bradford/BCA or similar assay | Standard curve | Standards and validated curve |
How to Use This Calculator
- Select the method matching your experimental data.
- Enter the relevant measurements and units exactly as shown.
- Enter the dilution factor used before measurement; leave it at 1 if undiluted.
- Click Calculate and review the concentration and calculation steps.
- Check that the result is within the appropriate assay or measurement range before using it downstream.
Calculation Methodology and Transparency
Mass/volume: concentration is mass divided by final solution volume, followed by dilution correction.
A280: molar concentration is calculated from A/(εl), converted to mg/mL using molecular weight, then corrected for dilution.
Standard curve: concentration is calculated as (absorbance − intercept)/slope, then corrected for dilution.
The calculator does not select an extinction coefficient, validate a standard curve, determine assay linearity, or correct experimental interference automatically.
The equations used here follow standard protein-quantification relationships: direct mass/volume concentration, Beer-Lambert A280 quantification, and algebraic inversion of a standard-curve equation. Experimental suitability depends on the validated method used for the specific sample.
Frequently Asked Questions
What is the formula for protein concentration?
For a known mass, concentration is mass divided by solution volume. For A280, concentration can be calculated from A/(εl). For a standard curve y = mx + b, concentration is (y − b)/m.
What is protein concentration in mg/mL?
It is the amount of protein in milligrams present in each milliliter of solution. Numerically, 1 mg/mL equals 1 g/L.
How do I calculate protein concentration from A280?
Use c = A280/(εl) to obtain molar concentration, then multiply by molecular weight to convert to g/L, which has the same numerical value as mg/mL.
Can I use this calculator for Bradford or BCA?
Yes, if you have the standard-curve equation from your assay. Enter the unknown absorbance, slope and intercept, then apply the dilution factor.
Why do I need a dilution factor?
A dilution performed before measurement lowers the measured concentration. Multiplying by the dilution factor estimates the concentration in the original sample.
What molecular weight should I use for a protein?
Use the molecular weight appropriate to the protein species being measured, including relevant processing or tags when applicable.
Can I use A280 for a crude cell lysate?
A280 may be affected by nucleic acids and other UV-absorbing components in crude lysates. A validated protein assay may be more appropriate.
What if my standard curve is nonlinear?
Use the validated curve model for your assay rather than forcing a linear equation. This calculator’s standard-curve mode assumes a linear y = mx + b relationship.
Does path length matter for A280?
Yes. Beer-Lambert concentration depends on optical path length. Enter the actual effective path length used for the measurement.
Why can two protein assays give different concentrations?
Different assays respond differently to protein composition, buffers, standards and interfering substances. Compare methods only when their analytical conditions and validation support the comparison.
References and Scientific Sources
The sources below provide guidance on protein quantitation, A280 calculations, Beer-Lambert relationships and standard-curve protein assays.
Thermo Fisher Scientific — Spectrophotometric Determination of Protein Concentrations
Technical note covering A280 protein concentration, Beer-Lambert law and extinction coefficients.
Thermo Fisher Scientific — Protein Assay FAQs
Guidance on standard curves, interpolation and accounting for dilution in protein assays.
Thermo Scientific NanoDrop One User Guide
Reference for Protein A280 calculations, path length, extinction coefficients and molecular concentration.
Merck/Sigma-Aldrich — Protein Quantitation
Overview of UV absorbance, reagent-based protein assays and factors affecting method suitability.
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Protein Concentration Calculator Disclaimer
This calculator is provided for educational and informational purposes. It performs mathematical calculations from user-entered mass, volume, absorbance, extinction coefficient or standard-curve values. It does not validate sample preparation, assay chemistry, instrument calibration, standard quality, linearity, extinction coefficients or analytical method performance. Follow validated laboratory procedures and appropriate scientific guidance.