Molecular Weight Calculator

Q: What is the difference between molecular weight and molar mass?

Molecular weight (MW) and molar mass are often used interchangeably in laboratory practice, but they are technically distinct quantities. Molecular weight is a dimensionless ratio — the mass of one molecule relative to 1/12 the mass of carbon-12 — expressed in daltons (Da) or unified atomic mass units (u). 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, either term is acceptable when preparing molar solutions or calculating stoichiometry.

Q: How do I enter a hydrated salt like copper sulfate pentahydrate?

To enter a hydrated compound such as 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 automatically. First, make sure you select 'Complex / hydrate-aware formula' from the Formula Type dropdown, since this activates the parser's hydrate splitting logic. 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 the anhydrous part and the water molecules together to give the total molecular weight of the hydrated form.

Q: Why does capitalisation matter when I type a chemical formula?

Chemical element symbols are case-sensitive because different capitalisation patterns represent entirely different elements. For example, 'Co' refers to cobalt (atomic weight 58.93 g/mol), while 'CO' is interpreted as one carbon atom followed by one oxygen atom — carbon monoxide rather than a single element. Similarly, 'Ca' is calcium but 'CA' would be read as carbon plus an unknown symbol 'A', triggering a parsing error. Always follow IUPAC notation: the first letter of an element symbol is uppercase and any subsequent letters are lowercase. Common errors in lab work include typing 'MG' instead of 'Mg' for magnesium, or 'NA' instead of 'Na' for sodium.

Q: 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 calculator's parser handles parentheses and their 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 a total of 2 Al + 3 S + 12 O. Using IUPAC 2021 atomic weights this sums to 26.982×2 + 32.065×3 + 15.999×12 = 342.15 g/mol. For compounds with more complex nesting such as iron(III) ammonium sulfate dodecahydrate (NH4)2Fe(SO4)2·12H2O, select 'Complex / hydrate-aware formula' so the parser splits on the hydrate dot before processing the parentheses in each segment.

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Molecular Weight Calculator

FREE TOOL

Common Reagents (click to load):

Chemical Formula (e.g. NaCl, C6H12O6, Na2SO4·10H2O)

Supports: parentheses (Ca3(PO4)2), hydrates Na2SO4·10H2O or Na2SO4.10H2O

⚙️ Advanced Options ▾

Formula Type

Mass Mode

Output Detail

✅ Molecular Weight Result

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g/mol

Element	Count	Atomic Weight	Contribution	% Mass

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🖨️ Print / Save Result

Introduction

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. It is used daily by biochemists, molecular biologists, and graduate students to prepare accurate molar solutions and verify reagent specifications.

Type any chemical formula into the input field and click Calculate MW. The calculator parses the formula, identifies every element present, 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 citric acid instantly without typing.

About the Tool

The Molecular Weight Calculator converts any chemical formula into a precise molar mass in g/mol, along with a full elemental breakdown showing each element's contribution to the total mass. It handles simple formulas (NaCl, C6H12O6), nested parenthetical groups (Al2(SO4)3), and hydrated salts (CuSO4·5H2O) in a single parser.

Two mass modes are available: average atomic mass, suited to routine gravimetric lab work, and a monoisotopic approximation useful for mass spectrometry contexts. Results can be copied to the clipboard, copied as a full data table, or printed for lab notebook records — all calculations run client-side in your browser with nothing sent to a server.

Input Explanation

The calculator takes one required input, the chemical formula, plus three optional settings under Advanced Options.

Standard formulas

Use standard chemical notation: H2O, NaCl, C6H12O6. The first letter of each element symbol must be uppercase; any second letter must be lowercase.

Parentheses

Parentheses with subscript multipliers are fully supported: Ca3(PO4)2, Al2(SO4)3, Fe2(SO4)3. Nested parentheses also parse correctly.

Hydrates

Use · or . for water of crystallisation: CuSO4·5H2O, Na2SO4.10H2O, MgSO4·7H2O (Epsom salt). Select the Complex / hydrate-aware mode.

Case matters

Co = Cobalt, CO = Carbon + Oxygen. Always use correct capitalisation: Ca not CA, Mg not MG, Na not NA.

Advanced Options

Formula Type tells the parser whether to expect plain notation only ("Simple formula") or notation that includes parentheses, brackets, or hydrate dots ("Complex / hydrate-aware formula"). Mass Mode chooses between average atomic masses (default, correct for weighing reagents) and monoisotopic mass (for mass spectrometry). Output Detail controls whether the full elemental breakdown table is shown alongside the total molecular weight.

Formula Explanation

Molecular weight is calculated by summing the product of each element's atomic weight and its count in the formula:

MW = Σ (n_i × AW_i)

Where n_i is the number of atoms of element i in one formula unit, in units of atoms, and AW_i is the standard atomic weight of element i, in g/mol (equivalently, daltons per atom). This calculator uses IUPAC 2021 atomic weights for all 94 naturally occurring elements plus common synthetic elements up to lawrencium.

Worked Example

Sample Input

You are preparing a 500 mL stock of 1 M magnesium sulfate heptahydrate (MgSO₄·7H₂O, Epsom salt) for a plant tissue culture medium. You enter MgSO4·7H2O with Formula Type set to "Complex / hydrate-aware formula" and Mass Mode set to "Average atomic masses".

Step-by-step Calculation

Calculation

MgSO₄·7H₂O breaks down into 1 Mg + 1 S + 11 O + 14 H (4 O and no H from the sulfate, plus 7 × H₂O = 14 H + 7 O):

MW = (1 × 24.305) + (1 × 32.065) + (11 × 15.999) + (14 × 1.008) = 24.305 + 32.065 + 175.989 + 14.112 = 246.47 g/mol

To find the mass needed: mass = C × MW × V = 1 mol/L × 246.47 g/mol × 0.5 L = 123.24 g

Final Result

The calculator returns a molecular weight of 246.47 g/mol for MgSO₄·7H₂O, and the required mass for this preparation is 123.24 g.

Interpretation

Weigh out 123.24 g of MgSO₄·7H₂O and dissolve it in water to a final volume of 500 mL to obtain a 1 M stock. Using the anhydrous MW (120.37 g/mol) by mistake would give a solution roughly half the intended concentration, since the anhydrous salt has less than half the mass per mole of the heptahydrate form.

Result Interpretation

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.

Practical Applications

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.

Scientific Notes & Limitations

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. This calculator uses IUPAC 2021 standard atomic weights, which represent an average over the natural isotopic distribution of each element and can vary slightly between sources or between elements with variable natural abundance.

Limitations

Monoisotopic mode uses only the most abundant stable isotope of each element; for a small number of very heavy or synthetic elements, no monoisotopic value is listed, and the calculator falls back to the average atomic mass for that element, which is flagged in the results. For ionic compounds such as NaCl, the calculation gives the mass of one formula unit rather than a discrete covalent molecule — the term molar mass is technically more precise in these cases, though the numerical result is identical.

Practical Tips

Use the preset buttons to load common reagents instantly and check that your own formula produces the same value as a sanity check. When weighing a hydrated salt, always confirm from the bottle label whether you have the anhydrous or hydrated form — the molecular weight differs substantially between the two. For very large molecules the result panel automatically converts the value to kDa, which is the conventional unit for proteins and other biomolecules.

Reference: Common Lab Reagent Molecular Weights

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

Common Mistakes

Wrong capitalisation: Typing CO when 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.

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.