Protein Molecular Mass Calculator
Paste a protein sequence using one-letter amino acid codes (FASTA format is supported). The calculator returns molecular mass in Daltons (Da) and kilodaltons (kDa).
What is protein molecular mass?
Protein molecular mass (often called molecular weight) is the sum of the masses of all amino acid residues in a protein chain, plus the mass of terminal groups. In practical lab work, this value helps with SDS-PAGE interpretation, mass spectrometry planning, protein concentration conversions, and buffer preparation.
If you have ever needed to convert mg/mL to micromolar concentration, the molecular mass is the missing piece. A reliable protein molecular weight calculator can save time and reduce mistakes before an experiment even starts.
How this calculator works
This tool parses your amino acid sequence, removes whitespace, ignores FASTA header lines that start with >, and sums residue masses from standard amino acid mass tables.
It then adds one water molecule to represent a complete peptide chain and optionally subtracts hydrogen mass for disulfide bond formation.
Supported sequence format
- Standard one-letter amino acid sequences (A, R, N, D, C, etc.)
- Multi-line sequences
- FASTA input with header lines
Mass options
- Average isotopic mass: Best for most routine molecular biology and biochemistry calculations
- Monoisotopic mass: Useful for high-resolution mass spectrometry workflows
Why molecular mass matters in the lab
- Concentration conversions: Convert mass concentration to molar concentration accurately
- Mass spectrometry checks: Compare expected and observed masses
- Construct validation: Confirm approximate size after tag additions or truncations
- Protein engineering: Track how mutations alter theoretical mass
Example use case
Suppose your recombinant protein has a calculated molecular mass of 42,350 Da. If your sample concentration is 1 mg/mL, that corresponds to roughly:
1 mg/mL ÷ 42,350 g/mol ≈ 23.6 µM
This is often the number you need when setting enzyme assays, binding studies, or SEC injections.
Important notes and limitations
- This calculator gives a theoretical mass from sequence alone.
- It does not automatically account for post-translational modifications (glycosylation, phosphorylation, acetylation, etc.).
- Signal peptide cleavage, initiator methionine removal, and proteolysis can shift observed mass.
- For disulfides, enter the number of formed bonds if known.
Quick troubleshooting tips
Why are my results different from SDS-PAGE?
SDS-PAGE estimates apparent size based on migration behavior, which can differ from true mass due to charge, shape, and detergent interactions.
Why are some characters ignored?
Any non-standard symbols are excluded from mass calculation. The output reports ignored characters so you can clean your sequence.
Should I use average or monoisotopic mode?
Use average mass for everyday protein calculations and monoisotopic mass for high-resolution MS peak matching.
Bottom line
A protein molecular mass calculator is one of the most practical bioinformatics tools you can keep handy. Whether you are cloning a fusion protein, designing purification steps, or interpreting analytical data, a fast theoretical mass estimate helps you make better experimental decisions.