What is the main spoken language in Kiev: Ukrainian or Russian?

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Q: What is the main spoken language in Kiev: Ukrainian or Russian?

I'm aware that there are both native Ukrainian and Russian speakers among Ukrainians, with the west being more Ukrainian-speaking and the east/south more Russian-speaking.

What about Kiev today? I suppose the vast majority is bilingual as in most of the country, but what is the main everyday language in the city?

Someone told me most people mainly use Russian in everyday life and at home (which is what's relevant to this question), and Ukrainian in official contexts. Is this accurate?

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A: Моя відповідь така: (де ваші плюсіки, га?)

TL;DR: it is Russian; its dominance is drastically decreasing nowadays; a foreigner who wants to speak on the streets of Kyiv should consider historical, political, and cultural aspects for their choice of language.

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Long story.

1. Russian is the most used language in streets and families of Kyiv;
   Calculation Method: if you simply take a group of people and count languages they speak in their family, you will notice that:

• Nearly 100% of those who speak Ukrainian would also have a good grasp of Russian.
  So every Ukrainian speaker would count for both Ukrainian + Russian columns of your research.
  A good amount of the above also count for other languages, e.g., Ukrainian+Russian+Hebrew or Ukrainian+Russian+Crimean Tatar.
• While far from 100% of Russian speakers would count to other languages.
  "Russian speaker" quite often means "Russian-only speaker".
  Dare to claim that I have never seen a single "Ukrainian-only speaker".

2. Tendency: Russian dominance is drastically decreasing since the moment of Russian invasion to Ukraine's Crimea and Donets'k/Luhans'k regions.
   Even the older people (like myself) who were raised in Russian school, Russian University, Russian official language in their offices, now attempt to speak Ukrainian in their families.
   There is no opposite process.

3. Age: The younger the more leaning toward Ukrainian.

4. "As a foreigner, what language should I choose to speak in Kyiv?" — since this is Travel.SE, the OP may be also interested how to choose the language to use in Ukraine.
   I would recommend to read Which languages to brush up on for Ukraine trip? question and its answers. This answer summarizes it very well:
   

   If your goal is to make an impression on locals I would definitely go with Ukrainian. At the moment, Russian language is associated with Russian invasion even though a lot of people still speak Russian.
   For me speaking Russian in Ukraine is like going to WWII occupied France⁽¹⁾ and trying to speak German with French people. I am not telling that you will get in trouble with Russian, but you could be more considerate to the local population.

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Source: I'm a citizen of Ukraine, half of my roots are from Kyiv.

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⁽¹⁾ For clarity and best accuracy, consider partially-occupied France in the period between 10 May – 25 June 1940 or France just after its liberation in 1944.