wǒ suàn cuò le.

In this sentence, 算 means to calculate, to figure out, or to work something out.

It is often used for:

• math
• totals
• counting
• figuring out a result

So here, 算 is not the suàn meaning to count as or to consider. It is specifically the calculate/figure out meaning.

Here, 错 is a result complement. It shows the result of the action.

So:

• 算 = calculate
• 错 = wrong / incorrectly
• 算错 = calculate incorrectly, get the calculation wrong

This is a very common Mandarin pattern:

verb + result complement

Other examples:

• 看错 = see incorrectly
• 写错 = write incorrectly
• 听错 = hear incorrectly
• 说错 = say something wrong

So 错 does not just mean wrong by itself here; it tells you that the action of calculating ended with the wrong result.

In 我算错了, 了 shows that the action has been completed and that a new situation has resulted.

It often gives the feeling of:

• I got it wrong
• It turns out I calculated it wrong
• I have made a mistake in my calculation

So 了 here is not really the English past tense marker. It is better understood as marking a completed event or a change of state.

Compare:

• 我算错 = I calculate incorrectly / I calculate it wrong
• 我算错了 = I calculated it wrong / I got it wrong

In normal conversation, 我算错了 sounds much more natural when talking about a specific mistake that has already happened.

Not only for formal math.

It can be used whenever someone has:

• added something up wrong
• figured out a total incorrectly
• estimated something incorrectly
• worked out a number incorrectly

For example, it can be about:

• a bill
• travel time
• money
• dates
• quantities

So it is broader than just classroom arithmetic, though numbers or calculation are usually involved.

Mandarin often leaves out the object when it is already understood from context.

So 我算错了 can mean:

• I calculated it wrong
• I got the number wrong
• I worked it out wrong

If you want to include the thing that was calculated, you can:

• 我把这道题算错了。 = I got this problem wrong.
• 我把总数算错了。 = I calculated the total wrong.
• 这个数我算错了。 = I got this number wrong.

The short version without an object is very common in speech.

The pattern is:

subject + verb + result complement + 了

In this case:

• 我 = subject
• 算 = verb
• 错 = result complement
• 了 = completed action / new situation

This pattern is extremely common in Mandarin. Once you know it, you can understand many similar sentences:

• 我写错了。 = I wrote it wrong.
• 你听错了。 = You heard it wrong.
• 他看错了。 = He saw it wrong / He misread it.

我算错了 is the normal way to say that you made the mistake.

It usually refers to a completed event:

• I got it wrong.
• I calculated it wrong.

我算错 without 了 is less natural as a complete statement in many everyday situations. It may sound unfinished, more general, or like part of a larger sentence.

For example:

• 我算错了。 = I got it wrong.
• 如果我算错，你提醒我。 = If I calculate it wrong, remind me.

So in a standalone sentence about a specific mistake, 了 is usually expected.

Yes, very often.

If the subject is obvious, people commonly just say:

• 算错了。 = Calculated it wrong. / I got it wrong.

Mandarin frequently drops subjects when the listener can easily understand who is being talked about.

So:

• 我算错了。 is clear and complete.
• 算错了。 is also very natural in conversation.

Here, 了 is usually pronounced with a neutral tone: le.

So the sentence is:

• wǒ suàn cuò le

Tone notes:

• 我 wǒ = 3rd tone
• 算 suàn = 4th tone
• 错 cuò = 4th tone
• 了 le = neutral tone

The last syllable is usually light and quick.

Usually, 算错了 strongly suggests some kind of numerical or countable figuring-out error.

If you want to say I judged it wrong or I misjudged it, Mandarin would more often use something else, such as:

• 我判断错了。
• 我想错了。
• 我估计错了。

So 算错了 can sometimes be extended to things like totals, timing, cost, or quantity, but it still usually keeps the sense of figuring something out incorrectly, not general personal judgment.

Yes. This is a very productive pattern in Mandarin.

Common examples:

• 看错了 = looked at it wrong / misread it
• 听错了 = heard it wrong
• 说错了 = said it wrong
• 写错了 = wrote it wrong
• 买错了 = bought the wrong one
• 想错了 = thought about it wrong / was mistaken

Learning verb + 错 + 了 as a pattern is very useful, because it helps you build many natural sentences quickly.