What Is Mass Percent Composition?
Mass percent composition (also called percent by mass or mass fraction × 100) expresses how much of a compound's total mass is contributed by a particular element. It is one of the most common ways chemists describe the makeup of a substance, because it lets you compare ingredients regardless of the total sample size. This calculator works for any compound, mixture, or sample — it is universal and not tied to any country or standard.
How to Use This Calculator
Enter two values: the mass of the element you are interested in, and the total mass of the whole compound or sample. Both should be in the same unit (grams is typical, but any consistent unit works). The calculator divides the element mass by the total mass and multiplies by 100 to return the mass percent.
The Formula Explained
The equation is simply:
$$\text{Mass \%} = \frac{\text{mass of element}}{\text{total mass}} \times 100$$
The fraction (mass of element ÷ total mass) is the mass fraction, a number between 0 and 1. Multiplying by 100 converts it to a percentage. If you add up the mass percents of every element in a compound, they should total 100%.
Worked Example
Consider water, H₂O. One mole has a molar mass of about 18 g, with oxygen contributing 16 g. The mass percent of oxygen is $$(16 \div 18) \times 100 = 88.89\%.$$ The remaining 11.11% comes from the two hydrogen atoms. This confirms that water is mostly oxygen by mass even though it has twice as many hydrogen atoms.
FAQ
Do the units have to be grams? No — any unit works as long as both inputs use the same unit, since the units cancel in the division.
Can the result exceed 100%? No. If your element mass is larger than the total mass, you have an input error, because an element cannot weigh more than the whole compound.
How do I find each element's percent in a compound? Use the element's total contribution (atoms × atomic mass) as the element mass, and the compound's molar mass as the total mass, then repeat for each element.
