# Interest rate calculator

**URL:** <https://boards.straightdope.com/t/interest-rate-calculator/676980>\
**Category:** Factual Questions\
**Created:** [December 22, 2013, 11:07pm UTC](https://boards.straightdope.com/t/interest-rate-calculator/676980 "2013-12-22T23:07:22Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Phild](https://avatars.discourse-cdn.com/v4/letter/p/ac8455/32.png) [@Phild](https://boards.straightdope.com/u/Phild)\
**Post date:** [December 22, 2013, 11:07pm UTC](https://boards.straightdope.com/t/interest-rate-calculator/676980/1 "2013-12-22T23:07:22Z")

</div>

If I started out with $1000 and at the end of three years I have $1800 what is my APR?

---

<div class="post-metadata">

**Author:** ![zoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zoid/32/336_2.png) [@zoid](https://boards.straightdope.com/u/zoid)\
**Post date:** [December 23, 2013, 12:01am UTC](https://boards.straightdope.com/t/interest-rate-calculator/676980/2 "2013-12-23T00:01:48Z")

</div>

Depends on the number of compounding periods.  
[See here.](http://www.math.com/tables/general/interest.htm)

---

<div class="post-metadata">

**Author:** ![President\_Johnny\_Gentle](https://avatars.discourse-cdn.com/v4/letter/p/5f9b8f/32.png) [@President\_Johnny\_Gentle](https://boards.straightdope.com/u/President_Johnny_Gentle)\
**Post date:** [December 23, 2013, 2:12am UTC](https://boards.straightdope.com/t/interest-rate-calculator/676980/3 "2013-12-23T02:12:56Z")

</div>

It’s not a well-defined question, since APR has varying definitions. The nominal APR will indeed depend on the number of compounding periods in a year, but the effective APR is calculable from this information. For this, use the linked compound interest formula with n = 1 and t = 3.

---

<div class="post-metadata">

**Author:** ![Dervorin](https://avatars.discourse-cdn.com/v4/letter/d/eb8c5e/32.png) [@Dervorin](https://boards.straightdope.com/u/Dervorin)\
**Post date:** [December 23, 2013, 10:43am UTC](https://boards.straightdope.com/t/interest-rate-calculator/676980/4 "2013-12-23T10:43:54Z")

</div>

As noted above, the rate depends on the number of compounding periods and the exact definition of APR that you use. Assuming annual compounding, 21.6% in this case.

(1- (1.8 ^ 1/3)) \* 100 = 21.6%