Molar Mass Calculator

Mass percent of an element = (atoms of that element × its atomic weight) ÷ molar mass of the compound × 100

atoms

How many atoms of the chosen element the formula contains — the subscript after its symbol, multiplied by anything outside a bracket, and by any coefficient in front of the whole formula

atomic weight

The atomic weight of the chosen element in grams per mole, taken from the reference table below

molar mass

The mass of one mole of the whole compound, the denominator of the fraction — it is the sum over every element in the formula, not just the one you asked about

%

The answer, a mass fraction written as a percentage. The shares of all the elements in a formula add up to exactly 100%

Use it whenever the question is about one ingredient rather than the whole substance: how much nitrogen a fertiliser bag actually delivers, how much metal a ore body is worth, what fraction of a hydrate is water, or whether a label's claim about an element is consistent with the formula printed beside it. It is also the page for working backwards — a fertiliser labelled 20% phosphorus means the compound itself is heavier than the phosphorus in it, and dividing by this percentage converts between the two. If all you need is the formula weight, the molecular weight page answers that directly.

1. How much of limestone is actually calcium

   1. Molar mass of CaCO3: 40.078 (Ca) + 12.011 (C) + 3 × 15.999 (O) = 100.086 grams per mole
   2. Calcium contributes one atom: 1 × 40.078 = 40.078 grams per mole
   3. Divide: 40.078 ÷ 100.086 = 0.40044
   4. As a percentage: 0.40044 × 100 = 40.04%

   That 40.04% is where the agricultural rule of thumb comes from — a tonne of pure limestone carries about 400 kilograms of calcium. The same sum run on oxygen gives 47.96%, and the two together are 88% of the mass, leaving 12.01% for the carbon. Every element's share of CaCO3 adds to 100%, which is the check that the denominator was the whole formula and not just the element.

2. An element that is not in the compound at all

   1. Molar mass of NaCl: 22.990 (Na) + 35.45 (Cl) = 58.44 grams per mole
   2. Look for carbon in NaCl: the formula contains sodium and chlorine only
   3. Carbon contributes 0 atoms, so 0 × 12.011 = 0 grams per mole
   4. The share is 0 ÷ 58.44 = 0%, and the atom count reads 0

   This is not an error, it is a true answer: sodium chloride contains no carbon, so it is 0% carbon by mass. The atom count row is what makes that readable — a 0 there says the element is absent from the formula, whereas a 0 with a non-zero atom count would mean something quite different. The parser accepts the formula and answers, which matters because changing the formula and forgetting to change the element is the most common thing anyone does on this page.

3. The same element in two different compounds

   1. Molar mass of glucose: 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 grams per mole
   2. Carbon contributes six atoms: 6 × 12.011 = 72.066 grams per mole
   3. Divide: 72.066 ÷ 180.156 = 0.4 exactly
   4. As a percentage: 40%

   Glucose is 40.00% carbon and ethanol, C2H5OH, is 52.14% — the same element, two compounds, two very different shares. This is why a percentage is only meaningful alongside the formula it came from, and why the page prints the molar mass next to it: the percentage alone does not say how much carbon a gram of the substance carries.

The atomic weights are the CIAAW abridged values for natural terrestrial abundance, quoted to four or five significant figures, so the second decimal of a percentage is already beyond what the source supports: limestone is 40.04% calcium here and 40.043% by more precise figures. That is far more precision than a fertiliser or ore calculation needs, and not enough for isotope-ratio work. The percentage is a property of the formula, and says nothing about the material in front of you — a sample of limestone that is 90% CaCO3 by weight carries 36% calcium, and this page cannot know that; it reports the composition of the pure compound. Thirty-four elements have no standard atomic weight at all, and a formula containing one is rejected rather than answered with a particular isotope's mass, because the percentage's denominator would rest on an assumption the page never stated. If the element you name is absent from the formula the answer is 0%, which is a true statement about a compound that does not contain it rather than a failure to compute. Isotopes, ions and structural formulas are not understood: D2O, Na+ and CH3-CH2-OH are all rejected. Finally, a formula is not a recipe — knowing that CaCO3 is 40.04% calcium does not tell you whether a particular sample is pure, or whether the calcium in it is available to a plant.

What is percent composition?

The share of a compound's total mass that each element accounts for, written as a percentage. For calcium carbonate, CaCO3, calcium is 40.04%, carbon is 12.00% and oxygen is 47.96%, and those three add to 100%. It is computed by taking the atoms of an element, multiplying by that element's atomic weight, and dividing by the molar mass of the whole compound. This page reports the figure for one element at a time — the one whose symbol you type — rather than printing the full breakdown for every element in the formula.

How do I get the percent composition of every element at once?

Change the element symbol and read the page again; each answer is independent and instant, and the atom count row tells you which elements are present at all. Two elements are enough to cover most of a formula: in CaCO3, knowing calcium is 40.04% and oxygen is 47.96% leaves 12.00% for the carbon, since the shares must total 100%. If you would rather do it by hand, the arithmetic is the same one shown in the worked examples — group the formula by element, multiply each by its atomic weight, add, then divide each element's contribution by that total.

Why does it say 0% when I ask for an element that is not there?

Because the statement is true: sodium chloride contains no carbon, so it is 0% carbon by mass. Returning an error instead would mean a red panel every time you edit the formula and forget to update the element symbol, which is the most common thing anyone does here. The atom count row is the tiebreaker — 0 atoms tells you the element is genuinely absent, rather than the page having failed. If you expected a non-zero answer and got 0%, check the spelling: element symbols are case-sensitive, so Co is cobalt while CO is carbon monoxide.

How much calcium is in a tonne of limestone?

About 400 kilograms, if the limestone is pure calcium carbonate. CaCO3 is 40.04% calcium by mass, and 40.04% of 1000 kilograms is 400.4 kilograms. Real limestone is not pure — a sample that is 90% CaCO3 by weight carries about 360 kilograms of calcium. The same arithmetic works for any ore or fertiliser: multiply the mass of the material by the element's percentage, and remember to correct for how pure the material actually is, which this page has no way to know.

Why can't I get a percentage for PuO2 or Tc2O7?

Because those elements have no standard atomic weight. Technetium, promethium, polonium, astatine, radon, francium, radium, actinium and every element from neptunium upward are radioactive with no stable isotopes, so there is no natural abundance to average over and the CIAAW table prints a dash. It would be tempting to allow it here — if you only want the oxygen percentage, why does the plutonium matter? — but the percentage's denominator is the mass of the entire formula, and that denominator does not exist. Substituting one isotope's mass would give a number that looks entirely ordinary while quietly assuming an isotope the page never told you it picked.

Does the mass percentage add up with the other elements?

Yes, by construction: every element's share of a formula totals exactly 100%, because the denominator is the sum of all of them. In C6H12O6, oxygen is 53.28%, carbon is 40.00% and hydrogen is 6.71%, which adds to 99.99% rather than 100% only because each figure is rounded to two decimals for display. That is a useful check on a hand calculation — if your percentages come to 105% or 87%, the molar mass in the denominator was wrong, or an element was counted twice.

• Standard Atomic Weights — abridged to four significant figures — Commission on Isotopic Abundances and Atomic Weights (CIAAW), IUPAC
• SI Units — Amount of Substance: the mole — National Institute of Standards and Technology (NIST)
• IUPAC Gold Book — mass fraction — International Union of Pure and Applied Chemistry (IUPAC)