Molecular Weight Calculator · Practical guide
How to Calculate Molecular Weight from a Formula
Count the atoms, multiply by atomic weights, and add — that is the whole method.
At a glance
The Method
Molecular weight is the sum of the atomic weights of every atom in the formula. For sulfuric acid, H2SO4, the total is 2(1.008) + 32.06 + 4(15.999) = 2.016 + 32.06 + 63.996 = 98.072 g/mol. Any formula, however large, reduces to the same multiply-and-add routine.
Interactive tool
Molecular Weight Calculator
Enter your own values, then compare the result with the examples below.
Results
| Element | Atomic Weight | Count | Total |
|---|
Four Steps
- Parse the formula into elements and counts: H2SO4 is H × 2, S × 1, O × 4.
- Look up each element's atomic weight on the periodic table.
- Multiply each atomic weight by its count.
- Add the subtotals to get the molecular weight in g/mol.
Atomic Weights Used by This Calculator
| Element | Atomic Weight | Element | Atomic Weight |
|---|---|---|---|
| H | 1.008 | Na | 22.990 |
| C | 12.011 | Cl | 35.45 |
| N | 14.007 | K | 39.098 |
| O | 15.999 | Ca | 40.078 |
| S | 32.06 | Fe | 55.845 |
Formulas with Parentheses
Parentheses multiply everything inside them: Ca(OH)2 means Ca × 1, O × 2, H × 2. Expand the formula first, then enter it as CaO2H2: 40.078 + 2(15.999) + 2(1.008) = 74.092 g/mol. This calculator reads flat formulas like H2O or C6H12O6, so expanding parentheses manually is required.
Sanity Checks
Water should come out near 18, CO2 near 44, and NaCl near 58.4. If your result is off by a large factor, the most common cause is a miscounted subscript — check that every digit in the formula was applied to the element before it.
Frequently Asked Questions
What units does molecular weight have?
It is expressed in grams per mole (g/mol) for lab work, or equivalently in daltons (Da) per molecule. The number is the same either way.
Why do different sources give slightly different values?
Atomic weights are weighted averages of isotopes, and some sources round them differently. Using C = 12.01 instead of 12.011 shifts glucose by only about 0.006 g/mol.
How do I handle hydrates like CuSO4·5H2O?
Add the water molecules to the formula. CuSO4·5H2O becomes CuSO4H10O5; the five waters contribute 5 × 18.015 = 90.075 g/mol to the total.
Can I calculate the weight of an ion like sulfate?
Yes — enter SO4 to get 32.06 + 4 × 15.999 = 96.056 g/mol. The lost or gained electrons change the mass by a negligible amount.