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Molecular Weight Calculator – Calculate Molar Mass of Any Compound

Please enter or select the molecular formula of a molecule to calculate its molecular weight/molar mass. Note that the formula is case-sensitive.

Molecular Formula

Common Chemicals

Table of Abridged Standard Atomic Weights

#	Symbol	Name	Atomic Weight (g/mol)
1	H	Hydrogen	1.008
2	He	Helium	4.0026
3	Li	Lithium	6.94
4	Be	Beryllium	9.0122
5	B	Boron	10.81
6	C	Carbon	12.011
7	N	Nitrogen	14.007
8	O	Oxygen	15.999
9	F	Fluorine	18.998
10	Ne	Neon	20.18
11	Na	Sodium	22.99
12	Mg	Magnesium	24.305
13	Al	Aluminium	26.982
14	Si	Silicon	28.085
15	P	Phosphorus	30.974
16	S	Sulfur	32.06
17	Cl	Chlorine	35.45
18	Ar	Argon	39.95
19	K	Potassium	39.098
20	Ca	Calcium	40.078
21	Sc	Scandium	44.956
22	Ti	Titanium	47.867
23	V	Vanadium	50.942
24	Cr	Chromium	51.996
25	Mn	Manganese	54.938
26	Fe	Iron	55.845
27	Co	Cobalt	58.933
28	Ni	Nickel	58.693
29	Cu	Copper	63.546
30	Zn	Zinc	65.38
31	Ga	Gallium	69.723
32	Ge	Germanium	72.63
33	As	Arsenic	74.922
34	Se	Selenium	78.971
35	Br	Bromine	79.904
36	Kr	Krypton	83.798
37	Rb	Rubidium	85.468
38	Sr	Strontium	87.62
39	Y	Yttrium	88.906
40	Zr	Zirconium	91.224
41	Nb	Niobium	92.906
42	Mo	Molybdenum	95.95
43	Tc	Technetium	97
44	Ru	Ruthenium	101.07
45	Rh	Rhodium	102.91
46	Pd	Palladium	106.42
47	Ag	Silver	107.87
48	Cd	Cadmium	112.41
49	In	Indium	114.82
50	Sn	Tin	118.71
51	Sb	Antimony	121.76
52	Te	Tellurium	127.6
53	I	Iodine	126.9
54	Xe	Xenon	131.29
55	Cs	Caesium	132.91
56	Ba	Barium	137.33
57	La	Lanthanum	138.91
58	Ce	Cerium	140.12
59	Pr	Praseodymium	140.91
60	Nd	Neodymium	144.24
61	Pm	Promethium	145
62	Sm	Samarium	150.36
63	Eu	Europium	151.96
64	Gd	Gadolinium	157.25
65	Tb	Terbium	158.93
66	Dy	Dysprosium	162.5
67	Ho	Holmium	164.93
68	Er	Erbium	167.26
69	Tm	Thulium	168.93
70	Yb	Ytterbium	173.05
71	Lu	Lutetium	174.97
72	Hf	Hafnium	178.49
73	Ta	Tantalum	180.95
74	W	Tungsten	183.84
75	Re	Rhenium	186.21
76	Os	Osmium	190.23
77	Ir	Iridium	192.22
78	Pt	Platinum	195.08
79	Au	Gold	196.97
80	Hg	Mercury	200.59
81	Tl	Thallium	204.38
82	Pb	Lead	207.2
83	Bi	Bismuth	208.98
84	Po	Polonium	209
85	At	Astatine	210
86	Rn	Radon	222
87	Fr	Francium	223
88	Ra	Radium	226
89	Ac	Actinium	227
90	Th	Thorium	232.04
91	Pa	Protactinium	231.04
92	U	Uranium	238.03
93	Np	Neptunium	237
94	Pu	Plutonium	244
95	Am	Americium	243
96	Cm	Curium	247
97	Bk	Berkelium	247
98	Cf	Californium	251
99	Es	Einsteinium	252
100	Fm	Fermium	257
101	Md	Mendelevium	258
102	No	Nobelium	259
103	Lr	Lawrencium	266
104	Rf	Rutherfordium	267
105	Db	Dubnium	268
106	Sg	Seaborgium	267
107	Bh	Bohrium	270
108	Hs	Hassium	271
109	Mt	Meitnerium	278
110	Ds	Darmstadtium	281
111	Rg	Roentgenium	282
112	Cn	Copernicium	285
113	Nh	Nihonium	286
114	Fl	Flerovium	289
115	Mc	Moscovium	290
116	Lv	Livermorium	293
117	Ts	Tennessine	294
118	Og	Oganesson	294

⚛️ What Is Molecular Weight?

Let's start simple. Molecular weight is just the total mass of all the atoms in a molecule. Think of it like this: if you had a Lego spaceship, the molecular weight would be the combined weight of every single Lego brick in that spaceship.

Each atom has its own weight — scientists call it atomic mass. When atoms join together to form a molecule, you just add up all their atomic masses. That's it.

Here's the part that trips people up. You'll hear "molecular weight" and "molar mass" used interchangeably. And honestly? For most practical purposes, they mean the same thing. The only tiny difference is that molecular weight is technically unitless (just a number), while molar mass has units of grams per mole (g/mol). But in everyday chemistry, people use them both to mean the same thing.

Example: Water (H₂O)
H: 1.008 × 2 = 2.016
O: 15.999 × 1 = 15.999
Total: 18.015 g/mol

📝 How to Use the Molecular Weight Calculator

Using our calculator is stupidly simple. Here's how:

Type your chemical formula into the input box. For example, type "H2O" for water.

Hit the calculate button.

Read the result — it shows molecular weight plus a breakdown of each element.

💧 Example 1: Water (H₂O)

Hydrogen: 2 × 1.008 = 2.016
Oxygen: 1 × 16.00 = 16.00
Total: 18.016 g/mol

🌬️ Example 2: Carbon Dioxide (CO₂)

Carbon: 1 × 12.011 = 12.011
Oxygen: 2 × 16.00 = 32.00
Total: 44.011 g/mol

🍬 Example 3: Glucose (C₆H₁₂O₆)

Carbon: 6 × 12.011 = 72.066
Hydrogen: 12 × 1.008 = 12.096
Oxygen: 6 × 16.00 = 96.00
Total: 180.162 g/mol

🌟 Why Molecular Weight Matters in Real Life

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Making Solutions in a Lab

Need 1 liter of 1M NaCl solution? NaCl molecular weight = 58.44 g/mol. Weigh out 58.44 grams of salt, dissolve in water to make 1 liter. Without molecular weight, you'd be guessing — and in chemistry, guessing is bad.

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Figuring Out Medicine Dosage

Drug companies calculate how many molecules of a drug are in a certain mass using molecular weight. That's how they figure out the perfect amount for a pill.

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Understanding Air Pollution

Different gases have different molecular weights. Lighter gases like H₂ (~2 g/mol) rise. Heavier gases like CO₂ (~44 g/mol) sink and hang near the ground.

⚠️ Common Mistakes People Make (And How to Avoid Them)

Forgetting to Multiply by Subscripts

The #1 mistake. H₂O means multiply hydrogen by 2, not use 1.008 once. Write each element with its count: H × 2, O × 1. Then multiply each. Then add.

Using the Wrong Atomic Mass

Atomic masses aren't whole numbers. Hydrogen = 1.008, not 1. Carbon = 12.011, not 12. Tiny differences add up for big molecules. Our calculator uses IUPAC atomic masses.

Confusing Molecular Weight with Formula Weight

For molecules like water, they're the same. For ionic compounds like NaCl, it's called "formula weight" — but the calculation is identical. Just add atomic masses.

🚀 Advanced Tips for Power Users

Handling Parentheses in Formulas

Formulas like Ca(OH)₂ mean multiply everything inside parentheses by the subscript. So: 1 calcium, 2 oxygens, and 2 hydrogens. Our calculator handles this automatically.

Hydrates and Water of Crystallization

Compounds like CuSO₄·5H₂O have water trapped in their structure. The dot means "plus" — add CuSO₄ molecular weight plus 5 × water's molecular weight.

Isotopes and Average Atomic Mass

Periodic table masses are averages of all isotopes. Carbon = 12.011 is mostly C-12 with tiny amounts of C-13 and C-14. Standard averages work for 99.9% of chemistry.

📜 A Quick History Lesson

Back in the early 1800s, John Dalton noticed elements always combine in fixed ratios by mass. Water is always 8 grams of oxygen per 1 gram of hydrogen. He realized atoms had different weights.

In the 20th century, scientists used mass spectrometers — devices that weigh individual atoms by shooting them through magnetic fields. Heavier atoms curve less, lighter atoms curve more.

Today, atomic masses are known to incredible precision. Our calculator uses these precise IUPAC values.

🔧 Troubleshooting: When Things Go Wrong

Problem: You Typed the Formula Wrong

Solution: Use capital letters correctly. "CO" = carbon monoxide. "Co" = cobalt. The difference matters.

Problem: You Forgot Parentheses

Solution: Make sure parentheses are balanced. Ca(OH)₂ is correct. Ca(OH₂ is not.

Problem: Result Seems Too High or Too Low

Solution: Double-check your formula. H₂O (water) vs H₂O₂ (hydrogen peroxide) — different molecules, different weights.

❓ Frequently Asked Questions

1. What's the difference between molecular weight and molar mass?

Technically, molecular weight is a unitless number, while molar mass has units of grams per mole. But in everyday chemistry, people use them interchangeably. Our calculator gives you the value in g/mol.

2. How do I calculate molecular weight for a compound with parentheses?

Multiply everything inside the parentheses by the subscript outside. For example, in Ca(OH)₂, you have 1 calcium, 2 oxygens, and 2 hydrogens. Our calculator does this automatically.

3. Why does my textbook say water's molecular weight is 18, but your calculator says 18.016?

Textbooks often round to whole numbers for simplicity. Our calculator uses precise atomic masses (hydrogen = 1.008, not 1). The difference is tiny and usually doesn't matter.

4. Can I use this calculator for ionic compounds like NaCl?

Yes. Even though NaCl doesn't have "molecules" in the strict sense, the calculation is the same — add atomic masses of sodium and chlorine to get the formula weight.

5. What's the molecular weight of glucose?

Glucose (C₆H₁₂O₆) = about 180.16 g/mol. 6 carbons (72.066) + 12 hydrogens (12.096) + 6 oxygens (96.00) = 180.162 g/mol.

6. How do I make a 1M solution using molecular weight?

Take the molecular weight in grams — that's how much to dissolve in 1 liter of solvent. For NaCl (58.44 g/mol), weigh 58.44 grams and dissolve in water to make 1 liter.

7. What's the most common mistake when calculating molecular weight?

Forgetting to multiply by subscripts. H₂O means multiply hydrogen's atomic mass by 2. It's the #1 error students make.

8. Is molecular weight the same as atomic weight?

No. Atomic weight = mass of a single atom. Molecular weight = sum of all atomic weights in a molecule. Atomic weight is for elements, molecular weight is for compounds.

9. Why do some elements have decimal atomic masses?

The periodic table shows the average of all isotopes. Carbon is mostly C-12, with tiny amounts of C-13 and C-14. The weighted average is 12.011.

10. Can I calculate molecular weight for organic compounds with lots of carbons?

Absolutely. Our calculator handles any size molecule — from simple water to complex organic compounds with dozens of atoms. Just type the formula correctly and it does the rest.

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