What is the effective annual rate?

The rate you actually earn once compounding is taken into account. 12 percent compounded monthly is 12.68 percent effective, because each month's interest earns interest of its own. It is the number to compare two offers on, since the headline rates are not comparable when they compound differently.

When is the contribution added?

After interest, each period. That is how a savings account works: the balance earns, then you pay in. Adding it before interest would overstate the result slightly and the difference compounds, so the order is worth being explicit about.

Is this financial advice?

This does arithmetic on the numbers you give it and nothing else. It is not advice, it does not know your circumstances, and it will not tell you whether something is a good idea.

Compound Interest Calculator

Compound growth, with the effective rate shown.

Starting amount

Annual rate

Years

Added each period

Compounding

Compound growth, with the effective rate shown.

How to use it

Enter the amount and the rate. Currency symbols and separators are stripped, so paste as you find it.
Add a regular contribution if there is one. It is added after interest each period, which is how an account works.
Compare the effective rate. That is the number to compare two offers on, not the headline rate.

When you would use this

Compound interest is the arithmetic everyone has heard about and few people have actually watched behave, mostly because the headline rate hides what is going on. Two things here are worth the space they take. The effective annual rate is shown alongside the result, because a rate compounded monthly is not the same as the same number compounded yearly: 12 percent monthly is 12.68 percent effective, and that is the number that makes two offers comparable. The second is the ordering of contributions. This steps period by period rather than using the closed form, and it adds your contribution after interest is applied, which is how an account behaves. The closed form is exact only when nothing is added along the way, and once a regular payment is involved the ordering has to be decided by someone. Stepping makes the decision visible instead of hiding it in a formula. No rates are fetched and nothing is uploaded. The arithmetic runs in the page, which is also why it works with the network off.

Questions

What is the effective annual rate?: The rate you actually earn once compounding is taken into account. 12 percent compounded monthly is 12.68 percent effective, because each month's interest earns interest of its own. It is the number to compare two offers on, since the headline rates are not comparable when they compound differently.
When is the contribution added?: After interest, each period. That is how a savings account works: the balance earns, then you pay in. Adding it before interest would overstate the result slightly and the difference compounds, so the order is worth being explicit about.
Is this financial advice?: This does arithmetic on the numbers you give it and nothing else. It is not advice, it does not know your circumstances, and it will not tell you whether something is a good idea.