Molecular Weight Calculator
Quickly calculate molecular weight by entering a chemical formula.
Last updated: August 11, 2026
Molecular Weight: The Basic Chemistry
Molecular weight = sum of all atomic masses in a formula. Water (H₂O): 2 hydrogen (1.008 each) + 1 oxygen (16.00) = 18.015 g/mol. Glucose (C₆H₁₂O₆): 6 carbon (12.01) + 12 hydrogen (1.008) + 6 oxygen (16.00) = 180.16 g/mol. Units: g/mol (grams per mole) or amu (atomic mass units).
Used everywhere in chemistry: converting grams to moles, calculating reaction yields, determining concentration. One mole = 6.022×10²³ molecules (Avogadro's number). If you have 180.16g glucose, that's exactly 1 mole or 6.022×10²³ glucose molecules.
Common Atomic Weights (Rounded)
Most Common Elements
- H (Hydrogen): 1.008
- C (Carbon): 12.01
- N (Nitrogen): 14.01
- O (Oxygen): 16.00
- S (Sulfur): 32.07
- P (Phosphorus): 30.97
Metals & Halogens
- Na (Sodium): 22.99
- K (Potassium): 39.10
- Ca (Calcium): 40.08
- Cl (Chlorine): 35.45
- Fe (Iron): 55.85
- Cu (Copper): 63.55
Calculation Examples
| Compound | Formula | Calculation | MW (g/mol) |
|---|---|---|---|
| Water | H₂O | 2(1.008) + 16.00 | 18.015 |
| Sodium Chloride | NaCl | 22.99 + 35.45 | 58.44 |
| Carbon Dioxide | CO₂ | 12.01 + 2(16.00) | 44.01 |
| Glucose | C₆H₁₂O₆ | 6(12.01) + 12(1.008) + 6(16.00) | 180.16 |
| Sulfuric Acid | H₂SO₄ | 2(1.008) + 32.07 + 4(16.00) | 98.08 |
| Ethanol | C₂H₅OH | 2(12.01) + 6(1.008) + 16.00 | 46.07 |
Formula Notation Rules
Standard Format
- H₂O: 2 hydrogen, 1 oxygen
- Ca(OH)₂: 1 calcium, 2 oxygen, 2 hydrogen
- Al₂(SO₄)₃: 2 aluminum, 3 sulfur, 12 oxygen
- Number after element = count of that atom
Parentheses Rule
- Multiply everything inside by subscript outside
- Ca(OH)₂: OH group × 2 = 2 O, 2 H
- (NH₄)₂SO₄: NH₄ group × 2 = 2 N, 8 H
- No parentheses = direct subscripts
Practical Applications
Moles ↔ Grams
Grams = Moles × MW
Moles = Grams ÷ MW
Example: 36g H₂O = 36÷18.015 = 2 moles
Concentration
Molarity (M) = moles/liter
Need MW to convert g/L to M
Example: 58.44g NaCl in 1L = 1M solution
Quick Reference: Common Compounds
| Name | Formula | MW (g/mol) |
|---|---|---|
| Water | H₂O | 18.015 |
| Table Salt | NaCl | 58.44 |
| Baking Soda | NaHCO₃ | 84.01 |
| Methane | CH₄ | 16.04 |
| Ammonia | NH₃ | 17.03 |
| Acetone | C₃H₆O | 58.08 |