This free molecular weight calculator instantly computes the molar mass of any chemical compound from its formula, supporting all 118 elements, parenthetical groups, and hydrated salts. Used daily by biochemists, molecular biologists, and graduate students to prepare accurate molar solutions and verify reagent specifications.
(Ca3(PO4)2), hydrates Na2SO4·10H2O or Na2SO4.10H2O
MgSO4·7H2O with Formula Type set to "Complex / hydrate-aware formula". The calculator returns a molecular weight of 246.47 g/mol. Since molarity C = mass ÷ (MW × volume), you rearrange to mass = C × MW × V = 1 mol/L × 246.47 g/mol × 0.5 L = 123.24 g. Weigh out 123.24 g of MgSO₄·7H₂O and dissolve it in water to a final volume of 500 mL — this gives your 1 M stock. Using the anhydrous MW (120.37 g/mol) by mistake would give you a solution roughly half the intended concentration.
| Reagent | Formula | MW (g/mol) |
|---|---|---|
| Sodium chloride | NaCl | 58.44 |
| Tris base | C₄H₁₁NO₃ | 121.14 |
| EDTA disodium dihydrate | C₁₀H₁₄N₂Na₂O₈·2H₂O | 372.24 |
| Glucose (dextrose) | C₆H₁₂O₆ | 180.16 |
| Sodium dodecyl sulfate | C₁₂H₂₅NaO₄S | 288.38 |
| Potassium chloride | KCl | 74.55 |
| Magnesium chloride hexahydrate | MgCl₂·6H₂O | 203.30 |
| Calcium chloride dihydrate | CaCl₂·2H₂O | 147.01 |
| HEPES (free acid) | C₈H₁₈N₂O₄S | 238.30 |
| Sodium bicarbonate | NaHCO₃ | 84.01 |
| Sodium phosphate dibasic | Na₂HPO₄ | 141.96 |
| Ammonium sulfate | (NH₄)₂SO₄ | 132.14 |
| Element | Count | Atomic Weight | Contribution | % Mass |
|---|
How to Use the Molecular Weight Calculator
Type any chemical formula into the input field and click Calculate MW. The calculator parses the formula, identifies all elements, and sums their atomic weights using IUPAC 2021 standard values. Use the quick preset buttons to load common lab reagents such as NaCl, glucose, Tris base, or copper sulfate pentahydrate instantly without typing.
Formula Input Rules
H2O, NaCl, C6H12O6. The first letter of each element symbol must be uppercase; any second letter must be lowercase.Ca3(PO4)2, Al2(SO4)3, Fe2(SO4)3. Nested parentheses also parse correctly.· or . for water of crystallisation: CuSO4·5H2O, Na2SO4.10H2O, MgSO4·7H2O (Epsom salt). Select the Complex / hydrate-aware mode.Co = Cobalt, CO = Carbon + Oxygen. Always use correct capitalisation: Ca not CA, Mg not MG, Na not NA.The Scientific Formula
Molecular weight is calculated by summing the product of each element's atomic weight and its count in the formula:
MW = Σ (ni × AWi)
Where ni is the number of atoms of element i in one formula unit, and AWi is the standard atomic weight of element i in g/mol (or equivalently, daltons per atom). For example, for glucose (C₆H₁₂O₆): MW = (6 × 12.011) + (12 × 1.008) + (6 × 15.999) = 72.066 + 12.096 + 95.994 = 180.156 g/mol. This calculator uses IUPAC 2021 atomic weights for all 94 naturally occurring elements plus common synthetic elements up to lawrencium.
When to Use This Calculator
You will need the molecular weight of a compound in virtually every quantitative lab procedure. Common applications include: preparing molar stock solutions (the molarity formula requires MW to convert between grams and moles); calculating the amount of a reagent needed for a given number of moles in a reaction; determining the equivalent weight of acids, bases, or salts for normality calculations; computing the mass of a protein in kilodaltons from its amino acid sequence; and verifying the identity of a purified compound by comparing the calculated MW against a mass spectrometry result. Any time you are weighing out a dry chemical and need to know how many moles you have, this calculator gives you the conversion factor.
Common Mistakes to Avoid
- Wrong capitalisation: Typing
COwhen you mean cobalt (Co) is one of the most frequent errors. The parser will calculate the MW of carbon monoxide instead, giving a completely wrong value with no warning. - Ignoring the water of crystallisation: Many common reagents are sold as hydrated salts. Using the anhydrous MW of CuSO₄ (159.61 g/mol) instead of CuSO₄·5H₂O (249.69 g/mol) means you are weighing out the wrong amount — your actual concentration will be about 36% lower than intended.
- Using monoisotopic mass for gravimetric work: Monoisotopic mass is appropriate for mass spectrometry, not for weighing on a balance. Always use average atomic masses when preparing solutions from dry reagents.
- Confusing formula units: For ionic compounds like NaCl the formula gives the MW of one formula unit (one Na⁺ and one Cl⁻), not a covalent molecule. The term molar mass is more precise here, but numerically the calculation is identical.
Interpreting Your Results
The primary output is the molecular weight in g/mol. This is the mass in grams of exactly one mole (6.022 × 10²³ formula units) of the compound. The elemental breakdown table shows each element's symbol, atom count, individual atomic weight, mass contribution, and percentage of the total mass — useful for verifying that your formula is correct and for understanding the composition of a compound. The total atom count summarises how many atoms are in one formula unit. If the result looks unexpected, double-check the formula capitalisation and parenthesis balancing using the error message displayed below the input field.
About Molecular Weight in the Laboratory
Molecular weight (MW), also called molar mass, is the mass of one mole of a substance in grams. It is fundamental to virtually all quantitative laboratory work — preparing molar solutions, calculating reaction stoichiometry, converting between mass and moles in PCR and cloning protocols, and expressing protein concentrations in molarity. The following reference values are commonly used in biotechnology labs:
Frequently Asked Questions
What is the difference between molecular weight and molar mass?
Molecular weight and molar mass are often used interchangeably in laboratory practice, but are technically distinct. Molecular weight is a dimensionless ratio — the mass of one molecule relative to 1/12 the mass of carbon-12 — expressed in daltons (Da). Molar mass is the mass of one mole of a substance expressed in grams per mole (g/mol). Numerically the two values are identical for any given compound: glucose has a molecular weight of 180.16 Da and a molar mass of 180.16 g/mol. In practical lab work involving buffer preparation and molarity calculations, either term is acceptable.
How do I enter a hydrated salt like copper sulfate pentahydrate?
Type either CuSO4·5H2O using the interpunct (·) character or CuSO4.5H2O using a standard period as the hydrate separator. The calculator recognises both notations. Make sure you select Complex / hydrate-aware formula from the Formula Type dropdown to activate the hydrate-splitting parser. Other examples include MgSO4·7H2O for Epsom salt (246.47 g/mol) and Na2SO4·10H2O for Glauber's salt (322.19 g/mol). The tool sums the atomic weights of both the anhydrous part and the water molecules to give the total MW of the hydrated form as supplied.
Why does capitalisation matter when I type a chemical formula?
Element symbols are case-sensitive because different capitalisation patterns represent entirely different elements. For example, Co refers to cobalt (58.93 g/mol) while CO is interpreted as one carbon atom and one oxygen atom — carbon monoxide rather than a single element. Similarly, Ca is calcium but CA would trigger a parsing error. Always follow IUPAC notation: the first letter of a symbol is uppercase and any subsequent letter is lowercase. Common errors include typing MG instead of Mg for magnesium, or NA instead of Na for sodium — both will produce incorrect molecular weights.
When should I use monoisotopic mass instead of average atomic mass?
Average atomic masses, calculated from the natural isotope distribution of each element, are the correct choice for routine gravimetric work — weighing out reagents on an analytical balance and dissolving them to a target concentration. Monoisotopic mass, which uses only the mass of the most abundant stable isotope of each element, is primarily relevant in high-resolution mass spectrometry, where the instrument detects individual ionic species at discrete mass-to-charge ratios and the monoisotopic peak is often the reference point for peptide or small-molecule identification. For the vast majority of biochemistry and molecular biology laboratory tasks, use the average mass mode.
How do I calculate the molecular weight of a compound with nested parentheses like Al2(SO4)3?
Enter the formula exactly as written in chemical notation: Al2(SO4)3. The parser handles parentheses and subscript multipliers at any level of nesting. For Al₂(SO₄)₃, the parser reads two aluminium atoms plus three sulfate groups (each containing one sulfur and four oxygen atoms), giving 2 Al + 3 S + 12 O. Using IUPAC 2021 weights this sums to (26.982 × 2) + (32.065 × 3) + (15.999 × 12) = 342.15 g/mol. For heavily nested compounds, select the Complex / hydrate-aware formula type to ensure all bracket levels are parsed correctly before applying any hydrate multiplier.