# Solidity - Constant vs. Immutable vs. Mutable State Variables

By [Fatih Furkan](https://paragraph.com/@fatih-furkan) · 2022-06-25

---

As you know we can define a state variable using these 2 keywords: `constant` , `immutable`, or we don’t write anything about mutability.

    address public ownerOne;
    address public immutable ownerTwo;
    address public constant ownerThree;
    

Let’s look at their differences:

*   `constant`: If you use this keyword, you can’t change the state variable in the contract. You have to define the variable hard-coded.
    
*   `immutable`: If you use this keyword, you have to define the variable in the constructor. After than, you can’t change its value. As you can see it’s very similar to constant.
    
*   If you don’t specify anything, the mutability of the variable will be `mutable`. You can change its value wherever you want.
    

You can say “Why would I use constant or immutable”. Freedom has a price.

If you say “I don’t want to use them, I want to be free while I write the contract”, you’ll both fuck your user’s wallet and your wallet.

Here are the test results:

![Use constants or immutable if you can](https://storage.googleapis.com/papyrus_images/396bf24e1b7798155c050ff58eacc55609f4c474a4aefe142e336ebb4ad0c06e.png)

Use constants or immutable if you can

As you can see, there is a big difference in the deployment phase. You can save 51.517 gas by using constants than using mutable variables. You can check the contracts and the test file that I used [here](https://gist.github.com/0fatih/f72696c5f1a69755d666499b0d45cb34).

Give attention to contract call costs. Constant and immutable used the same amount of gas. So, using `constant` instead of `immutable`, saves money on your pocket, not from users’.

If you don’t use `constant` or `immutable` while you can, your users will pay nearly 5% more for every call!

Don’t screw your users.

---

*Originally published on [Fatih Furkan](https://paragraph.com/@fatih-furkan/solidity-constant-vs-immutable-vs-mutable-state-variables)*