Quick start
- Enter a formula such as H2O, C6H12O6, Ca(OH)2, or CuSO4.5H2O.
- Use normal element capitalization. CO is carbon plus oxygen, but Co is cobalt.
- Put subscripts directly after the element or group they belong to.
- Use a period for dot hydrates, such as CuSO4.5H2O.
Best uses
Use it for formulas such as H2O, C6H12O6, Ca(OH)2, and CuSO4.5H2O, especially when parentheses or hydrate dots make the count easy to miss.
- Find molar mass for common formulas.
- Check element counts in parentheses.
- Estimate mass percentage by element.
- Use the result in the Molarity Calculator.
What this calculator is solving
The Molecular Weight Calculator parses a common chemical formula, counts atoms, multiplies each element count by a rounded atomic weight, and adds the parts to estimate molar mass in g/mol.
Match each input label on the calculator to the chemical formula exactly as the problem writes it, including capitalization, subscripts, parentheses, and dot hydrate parts.
The formula in plain language
In plain language: The calculator parses element symbols, subscripts, parentheses, and dot hydrate parts, then adds each element count times its rounded atomic weight. For H2O, it adds two hydrogen atoms and one oxygen atom for about 18.015 g/mol. The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
The calculator removes spaces, splits dot hydrate parts at periods, applies any leading hydrate coefficient, parses element symbols and parentheses, then adds count x rounded atomic weight for each element. For H2O, it uses (2 x 1.008) + 15.999 = about 18.015 g/mol.
How to read the answer
Read the g/mol answer first. Then check Atoms counted to catch missed subscripts, Elements to confirm the formula was understood, and Composition to see each element count and mass share.
- The main answer is estimated grams per mole, written as g/mol.
- Atoms counted tells you whether subscripts, parentheses, and hydrate coefficients were read.
- Elements lists the element symbols the parser found.
- Composition shows each element count and the percentage of the total molar mass it contributes.
Common mistakes to avoid
Most mistakes come from lowercase-only formulas, confusing CO with Co, forgetting that a number after parentheses multiplies the whole group, or entering a hydrate without the period and leading coefficient.
- Do not use lowercase-only formulas. Element symbols need the right capital letters.
- Do not skip parentheses. CaOH2 is not the same input as Ca(OH)2.
- Do not forget hydrate water. CuSO4 and CuSO4.5H2O have different molar masses.
- Do not expect isotope-exact mass from rounded atomic weights.
- Do not use this for charges, structural formulas, isotope-exact mass, or unsupported elements without checking a chemistry reference.
Quick example
For water, enter H2O. The calculator counts 2 hydrogen atoms and 1 oxygen atom. With the rounded values used by the tool, the estimate is (2 x 1.008) + 15.999 = about 18.015 g/mol.
That result means one mole of water molecules has a mass of about 18.015 grams using these rounded classroom atomic weights. The composition line also shows that oxygen supplies most of the mass even though hydrogen has two atoms.
How parentheses change the count
A number after parentheses multiplies everything inside the group. Ca(OH)2 means 1 calcium, 2 oxygen, and 2 hydrogen atoms. The tool adds 40.078 + (2 x 15.999) + (2 x 1.008) = about 74.092 g/mol.
This is why checking Atoms counted matters. If the count looks too small, the formula may be missing parentheses or a subscript.
How hydrates are handled
For hydrates, use a period before the water part. CuSO4.5H2O means the copper sulfate formula plus five water molecules. The leading 5 multiplies the H2O group before everything is added.
Do not treat CuSO4 and CuSO4.5H2O as the same compound for weighing or solution work. The hydrate water adds mass, so the g/mol value changes.
Where to use the result next
Molar mass is often the bridge between grams and moles. If a lab problem gives grams of solute and final solution volume, calculate the molar mass here, then use that g/mol value in the Molarity Calculator.
For unit conversions that are not chemistry-specific, use a conversion tool instead. Molecular weight is about formula units and moles, not density, force, or everyday weight.
When the calculator is not enough
This is a rounded classroom molar-mass helper. It does not choose isotope abundances, calculate monoisotopic mass, understand charges, validate structural formulas, or replace lab instructions.
For real lab work, check the exact compound name, hydration state, purity, safety data sheet, and reference values your instructor or procedure requires.
Research and references
These references support the SI molar-mass units, rounded atomic-weight context, and molarity handoff used in this guide.
Worked examples for Molecular Weight Calculator
About 18.015 g/mol: two H atoms plus one O atom
About 180.156 g/mol for the full formula unit
About 74.092 g/mol, with the OH group counted twice
Hydrate waters included in the total molar mass
FAQ in plain language
When should I use the Molecular Weight Calculator?
Use it when your task matches one of these common needs: Find molar mass for common formulas. Check element counts in parentheses. It works best when you already know the measurements, amounts, units, or options the page asks for.
What is the Molecular Weight Calculator doing with my inputs?
In plain language: The calculator parses element symbols, subscripts, parentheses, and dot hydrate parts, then adds each element count times its rounded atomic weight. For H2O, it adds two hydrogen atoms and one oxygen atom for about 18.015 g/mol. The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
What do the main Molecular Weight Calculator inputs mean?
Chemical formula: element symbols and counts, such as H2O, C6H12O6, Ca(OH)2, or CuSO4.5H2O. Subscripts: the numbers after element symbols or parentheses that multiply atom counts. Dot hydrates: formula parts separated by a period, where a leading number multiplies the following hydrate group. Mass share: the percentage of the total molar mass contributed by each element.
How should I read the Molecular Weight Calculator answer?
Read the headline answer, then check the supporting lines and examples to understand how the calculator got there. If one input changes, rerun the tool and compare the new answer instead of guessing.
What should I double-check before trusting the answer?
The atomic-weight table is rounded and supports common classroom elements. Isotopes, charges, exact masses, structural formulas, and unsupported elements need reference data. Also check the unit, scale, mode, and result limit because small input changes can change the answer.
Why does capitalization matter in a formula?
Element symbols use one capital letter and sometimes one lowercase letter. CO means carbon and oxygen, but Co means cobalt. The calculator reads capitalization as part of the chemistry symbol.
Is molecular weight the same as molar mass?
For classroom formula work, people often use the terms together. The calculator returns grams per mole, so the result is the molar mass you can use in stoichiometry or in the Molarity Calculator.
Related tools
- Molarity Calculator Calculate molarity from moles and liters, or from grams, molar mass, and final solution volume.
- Scientific Calculator A free scientific calculator for trig, logs, roots, powers, and constants.
- Conversion Calculator Convert length, mass, volume, and temperature units with clear formula steps.
Keep exploring
If this guide is close but not exact, these links keep you near the same kind of problem.
- School & Study Browse the full category for related tools that help with the same job.
- All free tools Search the complete Access Free Tools library by task, category, or tool name.
- All calculator and utility guides Find more plain-language examples, formulas, mistakes, and result explanations.
- Free calculator resources Start here when you are not sure which calculator page fits.
Privacy and copying results
Recent answers stay visible only while you work in the current browser tab. They are not sent to a server.
Use Copy answer when you want to save the inputs and result in notes, homework, a message, or a project list. Check the units, labels, and limits before copying.