The Amino Acid MW Calculator is a complete reference tool for the molecular weights and physicochemical properties of all 20 standard amino acids. Search by name or code, filter by group, and click any card for instant access to free-form mass, residue mass, monoisotopic mass, pKₐ, hydropathy index, molecular formula, and codon usage — everything needed at the bench or in a bioinformatics workflow.
Scenario: You have synthesized a short peptide, MTEYK, and need its expected molecular weight before HPLC-MS analysis to confirm successful synthesis.
Inputs: Paste MTEYK into Sequence Mode and select "Residue MW (peptide)".
Calculation: Sum the residue masses — Met (131.193) + Thr (101.105) + Glu (129.115) + Tyr (163.176) + Lys (128.175) = 652.764 Da, then add one water molecule for the free termini: 652.764 + 18.015 = 670.779 Da.
This theoretical mass is what you would compare against the observed [M+H]⁺ peak from mass spectrometry (671.79 for the singly protonated ion) to confirm the peptide was synthesized correctly.
| 1-Letter | 3-Letter | Name | Free MW (Da) | Residue MW (Da) | Monoisotopic (Da) | pKa (side chain) | Group |
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How to Use the Amino Acid MW Calculator
Step-by-Step Instructions
The tool has two interactive modes. In Browse Mode, all 20 amino acid cards are displayed on load. Use the search box to filter by name, one-letter code, or three-letter code in real time. Use the group filter chips (Hydrophobic, Polar, Positive, Negative, Special) to restrict the display to a single physicochemical category. Clicking any card opens a detail panel immediately below showing five key mass values and a full properties table including molecular formula, charge, polarity, essential amino acid status, and mRNA codon usage.
In Sequence Mode, paste or type a peptide or protein sequence in single-letter code and select whether to sum residue MWs or free-form MWs. Click Calculate Sequence MW to get the total mass and sequence length instantly. Any non-standard characters are excluded and reported. Scroll further to the Complete Reference Table for a side-by-side view of all 20 amino acids alphabetically with all key values on one screen.
Free Form MW vs Residue MW — The Core Distinction
Free form MW is the molecular weight of an amino acid as an isolated chemical compound, with both its free amino group (−NH₂) and carboxyl group (−COOH) intact. This is the value listed in chemical catalogues and used when working with amino acids in solution, such as when preparing standards for HPLC amino acid analysis or supplementing cell culture media.
Residue MW is the mass contribution of an amino acid when incorporated into a polypeptide chain. During peptide bond formation, the −OH from the carboxyl group of one amino acid condenses with the −H from the amino group of the next, eliminating one water molecule (18.015 Da). Therefore the relationship is always: Residue MW = Free Form MW − 18.015 Da. When calculating the total MW of a peptide or protein from sequence, you sum the residue MWs of all amino acids and add back one water molecule (18.015 Da) to account for the free N- and C-termini.
Average Mass vs Monoisotopic Mass
The MW values shown in the main cards and reference table are average masses, calculated using the weighted average atomic weights of all naturally occurring isotopes of each element. This reflects the actual distribution of isotopes in a bulk sample and is the value most relevant to protein biochemistry, column chromatography, and SDS-PAGE molecular weight estimation.
Monoisotopic mass, listed in the detail panel, is calculated using only the most abundant (lightest) isotope of each element: ¹H, ¹²C, ¹⁴N, ¹⁶O, and ³²S. This value is used in mass spectrometry of small peptides, where the mass spectrometer can resolve individual isotope peaks. For peptides and proteins larger than approximately 2 kDa, the monoisotopic peak is no longer the tallest isotope peak in the distribution, so average mass becomes more appropriate for MS interpretation of larger molecules.
When to Use This Reference Tool
Common laboratory scenarios where this tool is valuable include: calculating the expected MW of a synthetic peptide before HPLC-MS analysis; verifying the theoretical mass of a recombinant protein fragment; estimating the molar extinction coefficient contribution of Trp and Tyr residues (which requires knowing their residue MWs); selecting buffer components when working with charged amino acids near their pKa values; and predicting membrane-spanning segments by scanning hydropathy scores. The codon usage data in the detail panel is also useful when designing site-directed mutagenesis experiments, as it shows which codons encode each amino acid and can guide codon selection for optimal expression in a target organism.
Common Mistakes to Avoid
A very common error is summing free-form MWs to calculate a peptide MW without subtracting water for each peptide bond. For a peptide of n amino acids, (n−1) water molecules are lost during synthesis, and the free terminus adds back one. Always use residue MWs and add one water molecule at the end. Another frequent mistake is using monoisotopic masses when average masses are needed (or vice versa), which produces MW predictions that do not match measured values. For SDS-PAGE and most spectrophotometric work, use average masses. For MS of peptides below 2 kDa, use monoisotopic. A third mistake is neglecting post-translational modifications (PTMs) — the masses given here are for unmodified amino acids; phosphorylation adds +79.966 Da, glycosylation adds variable mass, and disulfide bonds subtract 2.016 Da per pair.
Understanding Hydropathy Index and pKa Values
The hydropathy index values shown are from the Kyte-Doolittle scale (1982), which assigns each amino acid a score from −4.5 (most hydrophilic) to +4.5 (most hydrophobic). Positive values (Ile, Val, Leu, Phe, Cys, Met, Ala, Trp) indicate residues that preferentially partition into hydrophobic environments such as membrane bilayers or protein cores. Negative values indicate residues that interact favourably with water and are typically found on protein surfaces. Sliding-window hydropathy plots across a sequence are the classical method for predicting transmembrane helices, where a window of 19 hydrophobic residues suggests a membrane-spanning segment.
The side-chain pKa values listed represent the intrinsic pKa of each ionisable group in a model peptide context. In a folded protein, pKa values can shift dramatically — by 2–5 pH units in some cases — due to the local electrostatic environment and burial in the hydrophobic core. This is why catalytic residues such as His in serine proteases or Asp in aspartic proteases can function at different pH optima than would be predicted from their free amino acid pKa values.
Frequently Asked Questions
What is the difference between free form MW and residue MW?
Free form MW is the molecular weight of an amino acid as a standalone molecule with intact amino and carboxyl groups. Residue MW is the mass when the amino acid is incorporated into a peptide chain; during peptide bond formation a water molecule (18.015 Da) is lost, so Residue MW = Free MW − 18.015 Da. When calculating protein MW from sequence, sum residue MWs then add 18.015 Da for the free termini.
What is the difference between average mass and monoisotopic mass?
Average mass uses weighted average atomic weights reflecting natural isotope abundances and is the standard for most biochemical work and SDS-PAGE estimation. Monoisotopic mass uses only the most abundant isotope of each element (¹H, ¹²C, ¹⁴N, ¹⁶O, ³²S) and is used in mass spectrometry of small peptides. For proteins above approximately 2 kDa, average mass is more relevant for MS interpretation because the monoisotopic peak is no longer the tallest peak in the isotope envelope.
How do I calculate the molecular weight of a peptide from sequence?
Sum the residue MWs of each amino acid in the sequence, then add 18.015 Da for the single water molecule needed to cap the N- and C-termini. The Sequence Mode on this page does this automatically. For a free amino acid mixture rather than a peptide, sum the free-form MWs without adding water.
Which amino acids have ionisable side chains at physiological pH?
Six amino acids have ionisable side chains: Aspartic acid (pKa 3.9) and Glutamic acid (pKa 4.1) are negatively charged at pH 7; Histidine (pKa 6.5) is partially charged near physiological pH; Cysteine (pKa 8.3), Tyrosine (pKa 10.1), Lysine (pKa 10.5), and Arginine (pKa 12.5) are uncharged at neutral pH but titrate at higher values. These pKa values can shift significantly when residues are buried inside a folded protein.
What is the Kyte-Doolittle hydropathy index used for?
The Kyte-Doolittle hydropathy index assigns each amino acid a score from −4.5 (most hydrophilic) to +4.5 (most hydrophobic). Positive values indicate residues that prefer non-aqueous environments such as membrane bilayers or protein cores; negative values indicate water-favouring residues typically found on protein surfaces. Sliding-window plots of hydropathy along a protein sequence are widely used to predict transmembrane helices, signal peptides, and surface-exposed regions.