Molecular Weight Calculator for Chemical Formulas
Enter a chemical formula to calculate its nominal molar mass, relative molecular or formula mass, stoichiometric counts and elemental mass percentages. The calculator uses CIAAW 2024 abridged standard atomic weights for normal materials.
- 84 standard-weight elements
- Nested groups and hydrates
- Exact internal count arithmetic
- Mass percentage breakdown
Online Molecular Weight Calculator
Type symbols with correct capitalization and plain-number subscripts. Use parentheses, square brackets or braces for groups, and use a middle dot for hydrates.
Nominal molar mass of H2O from the selected standard atomic weights.
Elemental Composition
| Element | Atomic weight | Stoichiometric count | Contribution | Mass share |
|---|---|---|---|---|
| H, Hydrogen | 1.0080 | 2 | 2.016 g/mol | 11.1907% |
| O, Oxygen | 15.999 | 1 | 15.999 g/mol | 88.8093% |
Calculation Steps
- Read H2O as 2 hydrogen atoms and 1 oxygen atom.
- Hydrogen: 2 × 1.0080 = 2.0160.
- Oxygen: 1 × 15.999 = 15.999.
- Add the contributions: 2.0160 + 15.999 = 18.015 g/mol.
The nominal molar mass is 18.015 g/mol. The displayed range reflects the uncertainties in the CIAAW abridged values for normal materials, not a measured uncertainty for a specific sample.
How to Use This Molecular Weight Calculator
- Enter the formula. Use official element symbols with correct capitalization, such as NaCl rather than NACL.
- Add subscripts. Type ordinary digits after an element or group. You may also paste Unicode subscripts such as H₂O.
- Group repeated units. Use matching parentheses, square brackets or braces, then place the whole-number multiplier after the closing mark.
- Write addition compounds with a middle dot. Enter CuSO4·5H2O or CaSO4·0.5H2O. The button inserts the required middle dot.
- Calculate the formula. The tool expands every group, validates the elements and adds the atomic-weight contributions.
- Review the result. Check the normalized formula, stoichiometric counts, reference range, element subtotals and mass percentages before using the value.
What Molecular Weight Means
Molecular weight is a widely searched term for relative molecular mass. Relative molecular mass is the ratio of the mass of a molecule to the unified atomic mass unit and is dimensionless. When standard atomic weights are used, this calculator returns a nominal composition-averaged value. Molar mass describes mass per amount of substance and is reported here in grams per mole.
The same formula calculation is also used for ionic solids such as sodium chloride. Because an ionic crystal does not consist of separate NaCl molecules, relative formula mass is the more precise term. This page keeps the familiar molecular weight name while labeling the result according to the entered formula.
The nominal numerical value of relative molecular or formula mass matches the nominal molar mass in g/mol when the same atomic-weight values are used. The quantities and units are still different. A result of 58.44 means a relative formula mass near 58.44 for NaCl and a molar mass near 58.44 g/mol.
Molecular Weight Formula and Calculation Method
A chemical formula specifies the elements and their stoichiometric counts. The calculator multiplies each element count by that element's abridged standard atomic weight, then adds every contribution.
M/(g mol−1) ≈ Mr
Here, Mr is dimensionless relative molecular or formula mass, M is molar mass, ni is the stoichiometric count of element i, and Ar,i is its dimensionless abridged standard atomic weight. Integer subscripts apply to an element or complete group. A coefficient in a middle-dot addition formula multiplies its complete component.
The calculator uses exact fixed-decimal arithmetic for atomic weights, stated uncertainties, component ratios and stoichiometric counts. It does not round each intermediate element contribution. This prevents ordinary binary floating-point noise from changing a final digit or a mass percentage. Very large counts are rounded to eight significant digits or shown in scientific notation on screen, while the internal calculation retains the exact count.
Worked Molar Mass Examples
Water, H2O
Water contains two hydrogen atoms and one oxygen atom. Using H = 1.0080 and O = 15.999:
Calcium hydroxide, Ca(OH)2
The subscript 2 after the parentheses multiplies both oxygen and hydrogen. The expanded composition is CaO2H2.
Copper sulfate pentahydrate, CuSO4·5H2O
The coefficient 5 multiplies the complete water component. The final count is Cu1S1O9H10, not Cu1S1O5H2.
Supported Chemical Formula Syntax
| Formula feature | Accepted example | How it is read |
|---|---|---|
| Element symbols | NaCl, Co, CO | Symbols are case-sensitive. Co is cobalt; CO is carbon plus oxygen. |
| Integer subscripts | C6H12O6 | A positive whole number multiplies the element immediately before it. |
| Unicode subscripts | H₂SO₄ | Unicode subscript digits are normalized to ordinary digits. |
| Nested groups | K4[Fe(CN)6] | Matching (), [] and {} groups may be nested up to 20 levels. |
| Hydrates or adducts | CuSO4·5H2O | The coefficient after the middle dot multiplies its full component. |
| Addition-compound ratios | 3CdSO4·8H2O | A leading coefficient is accepted when the expression contains a middle dot. |
| Fractional component ratio | CaSO4·0.5H2O | A component coefficient may use up to six decimal places. |
| Terminal charge | SO4^2- | The charge is recorded, then omitted from the neutral-atom mass sum. |
Atomic Weights, Isotopes and the Reference Range
CIAAW publishes standard atomic weights for normal terrestrial materials. Some elements have interval values because natural isotope proportions vary. The 2024 abridged table converts the current recommendations into practical single values with stated uncertainties, generally at no more than five significant figures. The table includes CIAAW's 2024 revisions for zirconium, gadolinium and lutetium.
This calculator adds the stated abridged uncertainties linearly to show a conservative formula reference range. That range helps you see the scale of standard atomic-weight variation. It is not a laboratory uncertainty budget, and it does not describe an enriched, depleted, extraterrestrial or otherwise unusual sample.
Elements without a CIAAW standard atomic weight are rejected. The tool does not replace a missing value with the mass number of a long-lived isotope. It also rejects isotope notation because a proper isotope calculation needs verified nuclidic masses, not whole-number mass labels. Use an isotope-specific method for monoisotopic mass or isotope-enriched material.
Molecular Weight, Molar Mass and Exact Mass
| Quantity | Meaning | Unit or form |
|---|---|---|
| Relative molecular mass | Ratio of the mass of a molecule to the unified atomic mass unit | Dimensionless |
| Relative formula mass | Equivalent relative-mass expression for a formula unit, including ionic compounds | Dimensionless |
| Molar mass | Mass divided by amount of substance | g/mol or kg/mol |
| Exact mass | Mass calculated from explicitly specified isotope masses | Usually Da for one entity |
| Monoisotopic mass | An exact mass for one selected single-isotope composition, commonly using each element's most abundant isotope | Usually Da for one entity |
This calculator reports a standard-atomic-weight result. It does not report monoisotopic mass, an ion's electron-corrected mass, mass-to-charge ratio, average polymer molar mass or an experimentally measured value.
Elemental Mass Percentage
Mass percentage shows how much each element contributes to the nominal formula mass. The calculator divides an element's subtotal by the total and multiplies by 100.
For water, hydrogen contributes 2.016 of the 18.015 total, or about 11.1907%. Oxygen contributes about 88.8093%. Percentages are calculated from the unrounded scaled-integer subtotals, then rounded for display. A tiny deviation from exactly 100% can appear when separately rounded rows are added.
Formula-based percentages describe ideal stoichiometric composition. A real material may contain solvent, water, impurities, isotope variation, nonstoichiometry or mixed phases. Use analytical measurements when the actual sample composition matters.
Common Molecular Weight Calculation Mistakes
- Changing symbol capitalization, such as entering CO when cobalt Co was intended.
- Applying a group subscript only to the final element instead of the complete parentheses or bracket.
- Using an ASCII period for a hydrate and creating an ambiguous decimal-looking formula.
- Entering a reaction coefficient such as 2H2O and treating 36.03 g/mol as water's molar mass.
- Confusing average standard-atomic-weight mass with monoisotopic or exact mass.
- Using chemical names, state labels, charges without a caret, reaction arrows or structural bond marks.
- Assuming a bracketed mass number on a periodic table is a standard atomic weight.
- Rounding every element contribution before adding the total.
Accuracy, Validation and Calculator Limits
The parser accepts up to 220 input characters, 20 nested group levels, integer element and group multipliers no larger than 1,000,000,000, and addition-compound coefficients with up to six decimal places. It bounds the expanded stoichiometric count and checks matching group marks, empty groups, zero counts, unknown symbols, unsupported elements and incomplete components. Invalid input clears the earlier result and focuses the formula field.
The page supports formulas, not chemical names, SMILES, InChI, reaction equations, state symbols, variable polymer notation or fractional element subscripts. A terminal ionic charge must use a caret, such as NH4^+ or [Fe(CN)6]^4-. The calculation ignores electron gain or loss, so it is not an exact ionic-mass calculation.
All entered data stays in your browser. The calculator does not send the formula to a server or save calculation history. The numerical result is suitable for education and preliminary stoichiometry when the selected atomic-weight convention fits the task. Verify the isotope basis, formula identity and required precision for laboratory, regulatory or safety-critical work.
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Molecular Weight Calculator FAQs
How do I calculate molecular weight from a formula?
Count each element in the formula, multiply every count by its selected atomic weight, and add the contributions. This calculator expands groups and hydrate components before adding them.
Are molecular weight and molar mass the same?
They are related but not the same quantity. Molecular weight commonly means dimensionless relative molecular mass, while molar mass is mass per mole, usually expressed in g/mol. Their numerical values correspond when based on the same atomic weights.
What atomic weights does this calculator use?
It uses the single abridged values in the CIAAW Abridged Standard Atomic Weights 2024 table. Those recommendations apply to normal materials and include the 2024 zirconium, gadolinium and lutetium revisions.
Can I calculate hydrates such as CuSO4·5H2O?
Yes. Use a middle dot between components. The coefficient after the dot multiplies the complete following component, so five waters add ten hydrogen atoms and five oxygen atoms.
Does the calculator support parentheses and brackets?
Yes. It supports matching parentheses, square brackets and braces, including nested groups. A positive integer after a closing mark multiplies every element inside that group.
Why are element symbols case-sensitive?
Capitalization changes chemical meaning. Co is cobalt, while CO is one carbon atom plus one oxygen atom. The calculator rejects incorrect case rather than guessing.
Can I enter an ion or charged formula?
Yes, with explicit terminal caret notation such as NH4^+ or SO4^2-. Charge does not change the stoichiometric composition, and this calculator ignores electron-mass corrections.
Does this tool calculate monoisotopic or exact mass?
No. It uses standard atomic weights that reflect natural isotope composition. Monoisotopic mass requires specified isotopes and verified nuclidic masses.
Why is technetium or an element without a standard atomic weight rejected?
CIAAW does not assign a standard atomic weight to every radioactive or synthetic element. The calculator refuses to substitute an approximate mass number because that would not be a standard atomic weight.
Why does my result differ from another molar mass calculator?
Calculators may use different atomic-weight editions, rounded classroom values, interval midpoints or isotope-specific masses. Compare the stated data source, isotope basis and rounding method before judging the difference.
Method References
- CIAAW Abridged Standard Atomic Weights 2024, the single-value atomic-weight set and stated uncertainties used by the calculator.
- CIAAW Standard Atomic Weights 2024, definitive values, intervals, notes and interpretation for normal materials.
- IUPAC Gold Book, relative molecular mass, definition and molecular-weight terminology.
- IUPAC Gold Book, molar mass, relationship between molar mass and relative mass values.
- CIAAW Radioactive Elements, guidance for elements without a standard atomic weight.
- IUPAC Red Book, chemical formulae, addition compounds, isotope notation and ionic charge conventions.
- BIPM SI Brochure, 9th edition, amount of substance, the mole and SI quantity conventions.
Disclaimer: This calculator provides educational and preliminary formula-mass results from CIAAW 2024 abridged standard atomic weights. Verify the exact chemical identity, hydration state, isotope composition, purity, significant figures and approved data source before laboratory, pharmaceutical, environmental, regulatory, hazardous-material or safety-critical use.