算了 (suànle): forget it

This is a much more colloquial, idiomatic use of 算. 算了 means “forget  it” or “let's drop the matter”. It's a bit like saying “consider this  dealt with and leave it”. Note that it can be slightly abrupt or rude in  some situations.

Some example sentences for 算了:

我们就算了吧，你说呢？

Wǒmen jiù suànle ba, nǐ shuō ne?

Let's just forget it, what do you think?

算了！我受够了。

Suànle! Wǒ shòu gòule.

Stop it! I've had enough.

要是天气不好就算了吧。

Yàoshi tiānqì bù hǎo jiùsuànle ba.

If the weather's not good then let's just forget it.

既然没有牛奶，咱们就喝黑咖啡算了。

Jìrán méiyǒu niúnǎi, zánmen jiù hē hēi kāfēi suànle.

Since there's no milk, let's just drink black coffee.

算了 has a sense of giving up or dropping one's original intentions.  It's often used as a response to circumstances or people that can't be  dealt with or that the speaker is no longer willing to deal with.