# What is the molecular weight of an antibody? > A typical IgG antibody weighs about 150 kDa, its Fc and Fab fragments about 50 kDa each, and pentameric IgM about 970 kDa. Compare every class, chain and fragment. A typical [antibody](/guides/what-are-antibodies) has a molecular weight of about **150 kilodaltons (kDa)**, or 150,000 daltons. That figure refers to immunoglobulin G (IgG), the class used for most research reagents and therapeutic [monoclonal antibodies](/guides/what-are-monoclonal-antibodies). Because a dalton and a gram per mole have the same numerical value, it is also a molar mass of about 150,000 g/mol. The pieces of an IgG are simple fractions of that total. Each heavy chain weighs about 50 kDa and each light chain about 25 kDa, and the Fab and Fc fragments that enzymes cut from the molecule weigh about 50 kDa apiece. Other antibody classes are larger: secreted IgM, a ring of five antibody units, reaches about 970 kDa. ## What is the molecular weight of an antibody? The molecular weight of a typical IgG antibody is approximately 150 kDa, built from two heavy chains of about 50 kDa and two light chains of about 25 kDa. Two times 50 plus two times 25 gives the familiar 150 kDa. The four chains are held together by [disulfide bonds](/guides/disulfide-bonds) and noncovalent contacts into a Y shape. Each arm of the Y pairs a whole light chain with the front half of a heavy chain, and the stem is the paired back halves of the two heavy chains. That layout explains why the fragments described below fall at round fractions of 150 kDa. The same four-chain plan runs through all five human [antibody classes](/guides/how-many-types-of-antibodies-are-there), but their heavy chains differ in length, and some classes assemble several four-chain units into one molecule. Those two differences account for nearly all the variation in the next table. ## What are the molecular weights of IgG, IgM, IgA, IgD and IgE? IgG1, IgG2 and IgG4 are about 146 kDa, IgG3 about 165 kDa, serum IgA about 160 kDa, IgD 184 kDa, IgE 188 kDa and secreted pentameric IgM about 970 kDa. ![Secreted IgM is the largest major antibody form at about 970 kDa, compared with 385 kDa for secretory IgA and 146 to 188 kDa for monomeric classes](/images/charts/antibody-molecular-weight-by-class.webp '**Figure 1. Human antibody molecular weight by class and form.** Typical values for each form; IgG shows the IgG1, IgG2 and IgG4 value. Sources: [Janeway et al. (2001)](https://www.ncbi.nlm.nih.gov/books/NBK27106/), [Patel et al. (2023)](https://www.ncbi.nlm.nih.gov/books/NBK513460/).') Table 1. Molecular weights of human antibody classes. Typical values in kilodaltons for the secreted forms; individual antibodies differ with sequence and glycosylation. Sources: Janeway et al. (2001), Patel et al. (2023). Antibody Form Molecular weight IgG1, IgG2, IgG4 Monomer 146 kDa IgG3 Monomer with a long hinge 165 kDa IgA1, IgA2 Serum monomer 160 kDa Secretory IgA Dimer with J chain and secretory component 385 to 390 kDa IgD Monomer 184 kDa IgE Monomer 188 kDa IgM Single four-chain unit 190 kDa IgM Secreted pentamer with J chain 970 kDa Within IgG, the outlier is IgG3, whose unusually long hinge adds about 19 kDa to the usual 146 kDa. IgM and IgE monomers weigh more than IgG for a different reason: their heavy chains carry an extra constant domain in place of the hinge. IgM's large number follows from its assembly. A single IgM unit weighs about 190 kDa, and five of them join through one 15 kDa J chain. Adding these gives a quick check on the reference value: $$ 5 \times 190\ \text{kDa} + 15\ \text{kDa} \approx 965\ \text{kDa} $$ That is within rounding of the 970 kDa reference value, so the common shorthand of 900 kDa understates the pentamer by about 70 kDa. IgA also changes with its form. Serum IgA circulates mainly as a 160 kDa monomer, while secretory IgA in saliva, tears and mucus is a dimer of about 385 kDa once the J chain and secretory component are attached. ## What is the molecular weight of the Fc fragment? The Fc fragment of human IgG1 has a molecular weight of about 50 kDa. It is a dimer: the back halves of the two heavy chains, each about 25 kDa, stay linked by disulfide bonds after the enzyme papain cuts the antibody above them. That two-chain structure shows up in the lab. Under nonreducing conditions a purified Fc runs near 50 kDa, while a reducing gel breaks the disulfide bonds and shows single chains near 25 kDa. Each Fc chain also carries a carbohydrate attached to its CH2 domain, tucked between the two heavy chains, so a glycosylated Fc weighs slightly more than its amino acid sequence alone. Fc fusion proteins inherit the dimer. Etanercept, which joins part of tumor necrosis factor receptor 2 to an IgG1 Fc, has a molecular weight of about 103 kDa, and aflibercept, built the same way from two VEGF receptor domains, about 97 kDa. When an Fc tag is added to a recombinant protein, expect the purified product to be a disulfide-linked dimer at roughly twice the mass of one fusion chain. The Fc of IgM and IgE is larger than the IgG Fc, because their heavy chains carry an extra constant domain. The functional differences between antibody classes, such as which Fc receptors they bind, lie mainly in this part of the molecule. ## What are the molecular weights of Fab, F(ab')2, scFv and other fragments? A Fab fragment weighs about 50 kDa, F(ab')2 about 110 kDa, a single-chain variable fragment (scFv) about 27 kDa, and a single-domain VHH or nanobody 12 to 15 kDa. ![An intact IgG is about 150 kDa, compared with about 110 kDa for F(ab')2 and about 50 kDa each for Fab and Fc](/images/charts/igg-and-antibody-fragment-molecular-weights.webp '**Figure 2. IgG and its enzyme-generated fragments.** Rounded reference values for human IgG1. Sources: [Bates and Power (2019)](https://pmc.ncbi.nlm.nih.gov/articles/PMC6640713/), [Gjølberg et al. (2022)](https://pmc.ncbi.nlm.nih.gov/articles/PMC9388496/).') Table 2. Molecular weights of IgG chains and antibody fragments. Approximate values in kilodaltons; exact masses depend on the sequence and any glycans. Sources: Janeway et al. (2001), Gjølberg et al. (2022), Bates and Power (2019). Molecule What it contains Molecular weight Intact IgG Two heavy and two light chains ~150 kDa F(ab')2 Both antigen-binding arms joined at the hinge ~110 kDa Heavy chain VH, CH1, hinge, CH2 and CH3 ~50 kDa Fab One light chain plus VH and CH1 ~50 kDa Fc CH2 and CH3 of both heavy chains ~50 kDa Light chain VL and CL ~25 kDa scFv VH and VL joined by a peptide linker ~27 kDa VHH (nanobody) One heavy-chain variable domain 12 to 15 kDa Papain and pepsin cut IgG in the same hinge region but on opposite sides of the disulfide bonds that join the heavy chains. Papain cuts above them and releases two separate Fab arms and one Fc. Pepsin cuts below them, so the two arms stay joined as a single F(ab')2. Each Fab binds antigen through one site, while F(ab')2 keeps both. The smaller formats are engineered rather than cut. An scFv keeps only the two [variable domains](/guides/antibody-variable-regions) that form the binding site, linked by a short synthetic peptide. A VHH is the single variable domain of the heavy-chain-only antibodies made by camels, llamas and alpacas, and at 12 to 15 kDa it is one of the smallest natural antigen-binding units. These are rounded reference values. The Fab of the NIST reference antibody had a measured mass of 47,628 Da, matching its calculated sequence mass, with a light chain of 23,124 Da and a heavy-chain portion of 24,523 Da by calculation. Neither chain is exactly 25 kDa, but both run near 25 kDa on a reducing gel. ## Why does an antibody's measured molecular weight vary? An antibody's exact mass depends on its amino acid sequence, class and subclass, glycosylation, assembly state, and any engineered additions, so the 150 kDa figure is a starting point rather than an exact value. Glycosylation is the main natural source of spread. Carbohydrates typically make up 1% to 5% of a monoclonal antibody's roughly 150 kDa mass, and because the attached sugar chains vary from molecule to molecule, one antibody preparation contains several glycoforms with slightly different exact masses. Variable-region sequences also differ between antibodies, and therapeutic formats can add linkers, extra binding domains, polyethylene glycol or drug payloads. The method changes the number too. On a nonreducing SDS-PAGE gel an intact IgG runs near 150 kDa, while reducing conditions break the disulfide bonds and split it into bands near 50 kDa and 25 kDa. Aggregation raises the apparent size, and clipping by proteases lowers it. For a sequence-specific value, a [molecular weight calculator](/app/molecular-weight) can calculate each heavy and light chain and subtract the hydrogens lost in each disulfide bond. Small assumptions still matter: the NIST team noted that popular calculators return 47,655 Da for their Fab, rather than the measured 47,628 Da, because they assume a plain glutamate at the N-terminus. Glycans and conjugated payloads then have to be added from their known composition or measured directly by mass spectrometry. The 150 kDa rule of thumb is a good guide for planning a gel or a dilution; an exact mass needs the sequence and the modifications that come with it.