Codility test: how it works and how to prepare

What a Codility test is

Employers use Codility to screen candidates before interviewing them. You receive a link, open the test in your browser, and work through the tasks in Codility's editor, where you can run your code against an example before you submit. Each company chooses its own tasks and time limit, so read your invitation carefully before you start.

Codility is also used for live interviews, where an interviewer joins you in a shared session. This guide is about the test you take alone.

How a Codility test is scored

According to Codility's own help centre, each task is scored on test cases. You see an example test case in the task, but it does not count towards your score. When you submit, your solution is run against test cases of a similar nature, at least six per task and often more, and your score is the share it passes: fail four of ten and you score 60% on that task.

Performance is measured only on tasks that call for an algorithm; elsewhere, correctness is the whole score. Your overall score is the total across tasks. The report an employer receives also shows a timeline of your session, including when the editor was out of focus and when text was pasted in, and a similarity check on your code.

Typical task types

• Algorithmic tasks: arrays, strings, counting, prefix sums, sorting and searching, where both correctness and performance are scored.
• Practical tasks: work closer to everyday engineering, scored on whether the code meets the requirements.
• QA tasks: instead of solving a problem, you write test cases for a working page against its acceptance criteria. Codility scores these as a suite, and a suite that fails on the correct page scores zero.

Which of these you get depends on the role and the employer. A backend role usually means algorithmic tasks; a testing role may mean QA tasks.

Managing the clock

1. Read every task before you start, and note how many there are and the time limit.
2. Start with the task you understand best, to bank points early.
3. Write a correct simple solution first. Correctness points come before performance points.
4. Test beyond the example: empty input, one element, the largest values allowed, negative numbers.
5. Only then improve the complexity, if the task is algorithmic and time allows.
6. Leave a few minutes at the end to submit everything. Unsubmitted work does not score.

Why Codility scores come out low

• Passing the example and stopping. The example does not count, and hidden tests are where the edge cases are.
• Missing corner cases: empty arrays, a single element, all values the same, the minimum and maximum allowed.
• Timing out on large inputs because of a nested loop, which costs the performance score.
• Integer overflow on large sums in languages with fixed-size integers.
• Code that does not compile in the final submission, often after a last-minute change.

A two-week practice plan

1. Days 1 to 3: arrays and strings. Solve two problems a day, then rewrite each to run in a single pass.
2. Days 4 to 6: hash maps, counting and prefix sums, the patterns behind many performance scores.
3. Days 7 to 9: sorting, binary search and two pointers.
4. Days 10 and 11: timed practice. Two tasks in the time limit your invitation states, with your own edge-case tests.
5. Day 12: review every mistake from the timed sets and write down the edge case you missed.
6. Day 13: one more timed set, under test conditions, alone.
7. Day 14: rest, and read the invitation again for the time limit and the language options.

Take it on your own

A Codility test is an assessment of your own work, and employers' rules forbid outside help during it. The report they receive shows when you left the editor and what you pasted in, so a score you did not earn is a risk, not a shortcut.

Use help before the test instead. Practise explaining your approach out loud with the AI mock interview, which is also good preparation for the live technical interview that usually follows a Codility test. Our CoderPad interview guide covers that next stage.