# What is the degree of unsaturation?

Computes the degree of unsaturation (the number of rings plus π bonds) of a compound from the counts of carbon, hydrogen, nitrogen, halogen and oxygen atoms in its molecular formula.

- Page: https://www.acalculator.org/chemistry/degree-of-unsaturation-calculator
- JSON spec: https://www.acalculator.org/chemistry/degree-of-unsaturation-calculator.json
- Version: fa25822fc960

## Default answer

Example with the default inputs (Carbon atoms (C) 6, Hydrogen atoms (H) 10, Nitrogen atoms (N) 0, Halogen atoms (F, Cl, Br, I) 0, Oxygen atoms (O) 0): C6H10 has a degree of unsaturation of 2.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| c | Carbon atoms (C) | The number of carbon atoms in the molecular formula. |
| h | Hydrogen atoms (H) | The number of hydrogen atoms in the molecular formula. |
| n | Nitrogen atoms (N) | The number of nitrogen atoms in the molecular formula. |
| x | Halogen atoms (F, Cl, Br, I) | The number of fluorine, chlorine, bromine and iodine atoms together. |
| o | Oxygen atoms (O) | The number of oxygen atoms; they do not change the result. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| dou | Degree of unsaturation | Rings plus π bonds: (2C + 2 + N − H − X) ÷ 2. |
| formula | Molecular formula (X = halogen) | The formula typed, in the order C, H, N, O, X. |
| equivalent | Equivalent hydrocarbon | C carbons and H + X − N hydrogens: the hydrocarbon with the same degree of unsaturation. |
| saturated | Saturated hydrocarbon | CnH2n+2, the alkane with the same number of carbons (H2 with no carbon). |

## Method

Degree of unsaturation = (2C + 2 + N − H − X) ÷ 2, where X is the number of halogen atoms; oxygen is ignored.

## Assumptions

- The compound is a neutral molecule: carbon forms 4 bonds, nitrogen 3, oxygen 2, and hydrogen and the halogens 1.
- Each ring or π bond takes 2 hydrogens away from the saturated formula CnH2n+2; a double bond counts 1 and a triple bond 2.
- A formula that gives a half or a negative value has no answer, and so does a formula with no atoms.

## Worked examples

1. c = 6, h = 10, n = 0, x = 0, o = 0 gives dou = 2, formula = C6H10, equivalent = C6H10, saturated = C6H14. Source: OpenStax, Organic Chemistry, §7.2 Calculating the Degree of Unsaturation (C6H10 has 2; C4H6Br2 is equivalent to C4H8, 1; C5H8O to C5H8, 2; C5H9N to C5H8, 2), https://openstax.org/books/organic-chemistry/pages/7-2-calculating-the-degree-of-unsaturation (retrieved 2026-10-18).
2. c = 4, h = 6, n = 0, x = 2, o = 0 gives dou = 1, formula = C4H6X2, equivalent = C4H8, saturated = C4H10. Source: OpenStax, Organic Chemistry, §7.2 Calculating the Degree of Unsaturation (C6H10 has 2; C4H6Br2 is equivalent to C4H8, 1; C5H8O to C5H8, 2; C5H9N to C5H8, 2), https://openstax.org/books/organic-chemistry/pages/7-2-calculating-the-degree-of-unsaturation (retrieved 2026-10-18).
3. c = 5, h = 8, n = 0, x = 0, o = 1 gives dou = 2, formula = C5H8O, equivalent = C5H8. Source: OpenStax, Organic Chemistry, §7.2 Calculating the Degree of Unsaturation (C6H10 has 2; C4H6Br2 is equivalent to C4H8, 1; C5H8O to C5H8, 2; C5H9N to C5H8, 2), https://openstax.org/books/organic-chemistry/pages/7-2-calculating-the-degree-of-unsaturation (retrieved 2026-10-18).
4. c = 5, h = 9, n = 1, x = 0, o = 0 gives dou = 2, formula = C5H9N, equivalent = C5H8. Source: OpenStax, Organic Chemistry, §7.2 Calculating the Degree of Unsaturation (C6H10 has 2; C4H6Br2 is equivalent to C4H8, 1; C5H8O to C5H8, 2; C5H9N to C5H8, 2), https://openstax.org/books/organic-chemistry/pages/7-2-calculating-the-degree-of-unsaturation (retrieved 2026-10-18).
5. c = 0, h = 4, n = 2, x = 0, o = 0 gives dou = 0, formula = H4N2, equivalent = H2, saturated = H2. Source: OpenStax, Organic Chemistry, §7.2 Calculating the Degree of Unsaturation (C6H10 has 2; C4H6Br2 is equivalent to C4H8, 1; C5H8O to C5H8, 2; C5H9N to C5H8, 2), https://openstax.org/books/organic-chemistry/pages/7-2-calculating-the-degree-of-unsaturation (retrieved 2026-10-18).
6. c = 6, h = 6, n = 0, x = 0, o = 0 gives dou = 4, saturated = C6H14. Source: OpenStax, Organic Chemistry, §7.2 Calculating the Degree of Unsaturation (C6H10 has 2; C4H6Br2 is equivalent to C4H8, 1; C5H8O to C5H8, 2; C5H9N to C5H8, 2), https://openstax.org/books/organic-chemistry/pages/7-2-calculating-the-degree-of-unsaturation (retrieved 2026-10-18).

## FAQ

### What is the degree of unsaturation?

It is the number of rings plus π bonds in a molecule. Each ring or double bond takes two hydrogens away from the saturated alkane formula CnH2n+2, and a triple bond takes four. It is also called the index of hydrogen deficiency (IHD) or double bond equivalents.

### How do I calculate the degree of unsaturation?

Work out (2C + 2 + N − H − X) ÷ 2, where C, H, N and X are the numbers of carbon, hydrogen, nitrogen and halogen atoms. For C6H10: (12 + 2 − 10) ÷ 2 = 2.

### How do halogens, oxygen and nitrogen change the count?

A halogen takes the place of a hydrogen, so add halogens to the hydrogens. Oxygen forms two bonds and does not change the count, so ignore it. Nitrogen forms three bonds, so subtract one hydrogen for each nitrogen. C4H6Br2 then counts as C4H8, with 1 degree of unsaturation.

### What does a degree of unsaturation of 4 mean?

The molecule has four rings and π bonds in total, in any mix. Benzene, C6H6, has one ring and three double bonds. A value of 4 or more often points to a benzene ring.

### Why does the calculator give no answer for some formulas?

If H + N + X is odd, the result would end in a half, and no neutral molecule has that formula. If there are more hydrogens and halogens than a saturated molecule can hold, the result would be negative. Both mean a count is wrong, or the formula is an ion or a radical.

### Does the degree of unsaturation tell me the structure?

No. It counts rings and π bonds together but does not say which. C6H10 could be a cyclic alkene, a diene, or an alkyne. Spectra or reactions such as hydrogenation tell them apart.

## Sources

- OpenStax, Organic Chemistry (McMurry), §7.2 Calculating the Degree of Unsaturation: each ring or double bond is a loss of two hydrogens from CnH2n+2; add halogens to hydrogens, ignore oxygens, subtract nitrogens from hydrogens; examples C6H10, C4H6Br2, C5H8O, C5H9N. https://openstax.org/books/organic-chemistry/pages/7-2-calculating-the-degree-of-unsaturation (retrieved 2026-10-18)