How do you calculate the percent composition of a compound?

For each element, take the number of its atoms times its atomic weight, divide by the compound's total molar mass, and multiply by 100. For water, hydrogen contributes 2 × 1.008 = 2.016 g/mol out of a 18.015 g/mol total, so H is 2.016 ÷ 18.015 × 100 = 11.19%, and oxygen is the remaining 88.81%. Do this for every element and the percentages add up to 100%.

What is the percent composition of water, H2O?

Water is 11.19% hydrogen and 88.81% oxygen by mass. The molar mass is 18.015 g/mol: two hydrogens at 1.008 give 2.016 g/mol, one oxygen at 15.999 gives 15.999 g/mol. Even though there are twice as many hydrogen atoms, oxygen dominates the mass because a single oxygen atom is about sixteen times heavier than a hydrogen atom.

How does the calculator handle parentheses like Ca(OH)2?

The parser expands the group before counting. In Ca(OH)2 the (OH) unit is multiplied by 2, giving one calcium, two oxygens and two hydrogens. Molar mass is 74.09 g/mol, so the composition is 54.09% calcium, 43.19% oxygen and 2.72% hydrogen. Nested brackets such as K4[Fe(CN)6] and hydrate dots such as CuSO4·5H2O are handled the same way.

What atomic weights does this tool use?

It uses the 2021 IUPAC conventional standard atomic weights, the same single values printed on a modern periodic table, for example hydrogen 1.008, carbon 12.011, oxygen 15.999, iron 55.845. These are the textbook numbers, so a homework answer checked against your chemistry book will match. Results are shown to a few significant figures because the underlying atomic weights themselves carry uncertainty.

How is percent composition related to molar mass?

Molar mass is the denominator. Percent composition splits one molar mass into the share each element owns. The tool shows the molar mass as context above the table, then breaks it down element by element. If you only need the single g/mol number, use the molar mass calculator; if you need the per-element mass fractions for an empirical-formula or purity problem, this is the tool.

Why should the percentages add up to exactly 100%?

Because mass is conserved: every gram of the compound belongs to one element or another, so the fractions must sum to the whole. The tool prints the running sum under the table. If you ever see a total that drifts from 100, it is only rounding in the displayed digits, not an error in the math, since the calculation keeps full precision internally.

Check a percent composition homework answer

The textbook asks for the percent composition of ammonium nitrate. You type NH4NO3, and the table shows 35.00% nitrogen, 4.99% hydrogen and 59.96% oxygen, with the arithmetic spelled out per row. Now you can see not just the final percentages but why each one came out that way, and confirm your hand calculation matched before you turn it in.

Find the nitrogen content of a fertilizer

Fertilizer is graded by its nitrogen percentage. Punch in the active compound, say urea CO(NH2)2, and read 46.65% nitrogen straight off the table. Compare two candidate compounds side by side to see which delivers more nitrogen per gram before you decide what to buy or formulate.

Work backward toward an empirical formula

A lab problem gives you mass percentages and asks for the formula. Type your trial formula and check whether its computed composition matches the measured percentages. Iterate until the percentages line up, and you have confirmed the empirical formula without grinding through the division by atomic weights by hand every time.

Teach mass percent with a live example

Standing at the board, you want students to see that atom count and mass share are different things. Pull up H2O on the projector: two hydrogens but only 11.19% of the mass, one oxygen but 88.81%. Change the formula to CO2 and watch the split shift, making the idea concrete instead of abstract.

Percent Composition Calculator — Mass Percent by Element

Mass percent of every element in a compound — type a formula, read the breakdown — IUPAC atomic weights, browser-only

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Chemical formula

Try:

Percent composition by mass · C6H12O6

Molar mass: 180.156 g/mol

mass % = (atoms × atomic mass) ÷ molar mass × 100

Element	Atoms	Atomic mass	Mass (g/mol)	Mass %
O	6	15.999	95.994	53.28%
C	6	12.011	72.066	40%
H	12	1.008	12.096	6.71%
Sum of percentages				100%

What this tool does

Free percent composition calculator that turns a chemical formula into the mass percentage of each element it contains. Type a formula such as H2O, C6H12O6, Ca(OH)2 or CuSO4·5H2O and the tool parses every subscript and bracket, multiplies each element count by its IUPAC standard atomic weight, sums the total molar mass, and reports what fraction of that mass each element carries. Water comes out as 11.19% hydrogen and 88.81% oxygen; glucose as 40.00% carbon, 6.71% hydrogen and 53.29% oxygen. The breakdown shows the exact arithmetic for every row, atoms × atomic weight ÷ molar mass × 100, and adds the percentages so you can confirm they total 100% as conservation of mass requires. Everything runs in your browser with one click copy and a shareable link that reopens your exact formula. 100% client-side, no upload, no account.

Tool details

Input: Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 10 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Student; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Percent Composition Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Check a percent composition homework answer
The textbook asks for the percent composition of ammonium nitrate. You type NH4NO3, and the table shows 35.00% nitrogen, 4.99% hydrogen and 59.96% oxygen, with the arithmetic spelled out per row. Now you can see not just the final percentages but why each one came out that way, and confirm your hand calculation matched before you turn it in.
Find the nitrogen content of a fertilizer
Fertilizer is graded by its nitrogen percentage. Punch in the active compound, say urea CO(NH2)2, and read 46.65% nitrogen straight off the table. Compare two candidate compounds side by side to see which delivers more nitrogen per gram before you decide what to buy or formulate.
Work backward toward an empirical formula
A lab problem gives you mass percentages and asks for the formula. Type your trial formula and check whether its computed composition matches the measured percentages. Iterate until the percentages line up, and you have confirmed the empirical formula without grinding through the division by atomic weights by hand every time.
Teach mass percent with a live example
Standing at the board, you want students to see that atom count and mass share are different things. Pull up H2O on the projector: two hydrogens but only 11.19% of the mass, one oxygen but 88.81%. Change the formula to CO2 and watch the split shift, making the idea concrete instead of abstract.

Common pitfalls

Confusing atom count with mass share. Two hydrogen atoms in water are not the bigger part of the mass — oxygen is, at 88.81%, because each oxygen atom is about sixteen times heavier. Percent composition weighs atoms, it does not count them.
Getting the element symbol case wrong. Co is cobalt, CO is carbon monoxide, and Cu is copper while CU is nothing. The parser is case-sensitive on purpose, so an all-caps paste like NACL returns an error instead of silently guessing what you meant.
Forgetting the water of crystallization. A hydrate's composition differs from its anhydrous form. Enter CuSO4·5H2O when the bottle says pentahydrate, not CuSO4, or the percentages you compute will not match the solid you actually weighed.

Privacy

Parsing the formula, looking up atomic weights, computing the per-element mass percentages, and summing the total all run as plain JavaScript inside your browser tab. No formula you enter is uploaded, logged, or sent to any server, and there is no external API call. The one caveat: the formula is encoded in the shareable URL (for example ?q=C6H12O6), so pasting a share link records that formula in the destination's access log. Chemical formulas are not sensitive, but if you would rather leave no trace, use the copy button instead of sharing the URL.

FAQ

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Percent Composition Calculator — Mass Percent by Element

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Percent Composition Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Check a percent composition homework answer

Find the nitrogen content of a fertilizer

Work backward toward an empirical formula

Teach mass percent with a live example

Common pitfalls

Privacy

FAQ

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