Protein Concentration Calculator
Calculate protein concentration using A280 (Beer–Lambert), a standard curve (Bradford/BCA/Lowry-style), or a quick dilution workflow. Includes blank correction, pathlength, dilution factor, unit conversions (mg/mL, µg/µL, g/L, µM), quick picks, and optional step-by-step.
Background
“Protein concentration” usually means mass concentration (like mg/mL). In many labs you measure either: (1) UV absorbance (often at 280 nm), or (2) an assay readout (Bradford/BCA/Lowry) that you convert to concentration using a standard curve. This tool does both — and also helps you handle dilution math cleanly.
How to use this calculator
- Pick a method at the top: A280, Standard curve, or Dilution.
- Enter your measurements (and a blank if you have one).
- If you diluted before measuring, set DF so the calculator reports the original concentration.
- Click Calculate. Use Step-by-step to see the exact math.
Quick tip: For A280, you’ll get the most useful outputs (mg/mL and µM) if you provide the protein’s molecular weight.
How this calculator works
- A280 (Beer–Lambert): A = ε·c·l → c = A/(ε·l)
- Standard curve: y = m·C + b → C = (signal − b)/m
- Dilution: C₁V₁ = C₂V₂
Best practice: use blank-corrected signals and apply dilution factor at the end to recover the original sample concentration.
Formula & Equation Used
Blank correction: ycorr = y − yblank
A280: c (M) = Acorr / (ε·l)
Convert M → mg/mL: mg/mL = M · (MW g/mol)
Standard curve: C = (ycorr − b)/m
Dilution correction: Coriginal = DF · Cmeasured
Dilution equation: C₁V₁ = C₂V₂
Example Problems & Step-by-Step Solutions
Example 1 — A280 (Beer–Lambert)
A280=0.84, blank=0.05, ε=43824 M⁻¹cm⁻¹, l=1 cm, DF=10.
- Correct: Acorr = 0.84 − 0.05 = 0.79
- Compute: c = 0.79/(43824·1) M
- Apply DF: coriginal = 10·c
Example 2 — Standard curve (Bradford)
signal=0.61, blank=0.10, m=0.85, b=0.05, DF=5 (curve unit mg/mL).
- ycorr = 0.61 − 0.10 = 0.51
- C = (0.51 − 0.05)/0.85 mg/mL
- Coriginal = 5·C
Example 3 — Dilution (find V₁)
Make 500 µL of 0.25 mg/mL from 2.0 mg/mL stock.
- V₁ = (C₂V₂)/C₁ = (0.25·500)/2.0 = 62.5 µL
- Diluent = V₂ − V₁ = 437.5 µL
Frequently Asked Questions
Q: Why do I need a blank?
A blank removes signal from the buffer/reagents/cuvette so your concentration is based on the protein signal only.
Q: My A280 gives concentration in M. I want mg/mL — how?
Multiply molar concentration by molecular weight: mg/mL = M · MW. (Because 1 g/L = 1 mg/mL.)
Q: What dilution factor should I enter?
If you diluted your sample before measuring, DF is the total fold dilution (e.g., 1:10 → DF=10). If you didn’t dilute, DF=1.
Q: My standard curve isn’t perfectly linear — can I still use this?
This calculator uses the common linear region model. If your curve is strongly non-linear, you’ll want a 4PL/5PL fit. (We can add that later if you want.)
