Protein Molecular Weight Estimator
By Santaji GadepatilinUpdated October 27, 2026
A protein's molecular weight can be estimated from its number of amino acids, at about 110 daltons per residue. It is a quick check when planning gels, purifications and assays. This free protein molecular weight estimator shows the formula and steps.
How to use the protein molecular weight estimator
Enter the number of amino acids. The estimated molecular weight is shown in kDa. Or enter the kDa (for example from a gel) to estimate the length.
Protein molecular weight formula
MW (kDa) ≈ amino acids × 110 ÷ 1000
Example
A 350-residue protein: 350 × 110 = 38,500 Da = 38.5 kDa.
Gene length
Each amino acid is coded by 3 nucleotides, so a 350-residue protein needs a coding sequence of 1,053 bp (including the stop codon).
Estimates for different lengths
| Amino acids | Approximate MW |
|---|---|
| 50 | 5.5 kDa |
| 100 | 11.0 kDa |
| 200 | 22.0 kDa |
| 300 | 33.0 kDa |
| 500 | 55.0 kDa |
| 1,000 | 110.0 kDa |
| 2,000 | 220.0 kDa |
From DNA to protein size
Each amino acid is coded by 3 nucleotides. A 1,500 bp open reading frame (including the stop codon) encodes 499 amino acids — about 55 kDa.
Why 110 Da?
The average mass of an amino acid in proteins is about 128 Da, but each peptide bond releases one water molecule (18 Da), giving an average residue mass of about 110 Da. Proteins rich in heavy residues (Trp, Tyr, Arg) are heavier than estimated.
Measured vs predicted size
On SDS-PAGE gels, proteins may run at a different apparent size than predicted because of:
- Post-translational modifications (glycosylation adds mass).
- Unusual charge or shape.
- Incomplete denaturation.
Mass spectrometry gives the most accurate molecular weight.
Daltons and kilodaltons
1 kDa = 1,000 Da. A dalton is about the mass of one hydrogen atom (1 g/mol).
Frequently asked questions
Why use 110 Da per amino acid?
It is the average mass of an amino acid residue in proteins, a widely used estimate.
How accurate is this?
Usually within about 10%. Exact values need the actual sequence; glycosylation and other modifications add more mass.
How many amino acids is a 50 kDa protein?
About 455 amino acids.
How heavy is a 500 amino acid protein?
About 55 kDa.
How many amino acids in a 30 kDa protein?
About 270 (30,000 ÷ 110).