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?
TL;DR: as of 2016, it was 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.
Long story.
Russian was the most comprehensible language simply because almost all Ukrainian speakers are bilingual, unlike the Russian speakers.
Calculation Method: if you simply take a group of people and count languages they are able to speak, you will notice that:
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, during the times of forced russification, were grown in a Russian-speaking environment, now attempt to speak Ukrainian in their families.
There is no opposite process.
Age: The younger the more leaning toward Ukrainian.
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.
Source: I'm a citizen of Ukraine, born in Kyiv.
(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.
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?
This is fully accurate. Being in Kiev it is relatively rare to hear Ukrainian speech, even though a lot of the signs are in the official language. If you speak Russian it should be extremely easy to communicate.
Source: traveling to Kiev a few times this year.
Update: as per comments Ukrainian speech is more common in Kiev now, but you should still have no issues whatsoever communicating in Russian during your stay.
Both Ukrainian and Russian are commonly spoken in the city; approximately 75% of Kiev's population responded "Ukrainian" to the 2001 census question on their native language, roughly 25% responded "Russian".
According to a 2006 survey, Ukrainian is used at home by 23% of Kievans, 52% use Russian and 24% switch between both.
In the 2003 sociological survey, when the question 'What language do you use in everyday life?' was asked, 52% said 'mostly Russian', 32% 'both Russian and Ukrainian in equal measure', 14% 'mostly Ukrainian', and 4.3% 'exclusively Ukrainian'.
Quoted verbatim from Wikipedia.
Source 1: What languages are spoken in Ukraine? - Ukraine Magazine.
Source 2: Kiev: The City, Its Inhabitants, Challenges for Today and the Future - Zerkalo Nedeli.
In Kiev and many other regions use of Ukrainian has been increasing and instruction in most schools is in Ukrainian. The percentage of people speaking Ukrainian "on the street" is about 30% in Kiev, 40-80% in Zhytomyr, Vinnytsya, Khmelnytskyy, Chernivtsi, Mukacheve, and Uzhhorod, 5-10% in Kharkiv, Dnipropetrovsk, and Odessa, 1-5% in Crimea, Donetsk, and Lugansk, and 80-98% in Lviv, Ivano-Frankivsk, Ternopil, Khmelnytskyy, and Lutsk (these are just approximations). Rural areas have a significantly higher concentration of Ukrainian speakers and speakers of "Surzhyk" (Ukrainian mixed with Russian), however, the most literary Ukrainian is spoken by educated individuals in the cities.
Source: http://www.tryukraine.com/info/languages.shtml Last update Feb 2016
If you're a Russian-speaker, you might have a hard time telling when someone's speaking Ukrainian. I listened to "Strelkov's" news conference after the shootdown of the luckless MH17 airliner and couldn't immediately work out why someone was repeating what "Strelkov" had just said. Then the penny dropped: he was "translating" into Russian, even though none of the words changed.
From my three visits to Ukraine from Australia, I have found that in Kharkiv within Ukraine, everyone spoke Russian. When I asked various people if they spoke Ukrainian, I was amused that half of the time the response was the same - "We are 'supposed' to say yes, but of course our usual language is Russian". The media have to publish in Ukrainian despite the language less prevalent. Children have been forced to speak Ukrainian in schools for almost two decades now, and because of this the younger generation are increasingly inclined to speak Ukraine. Travelling through Kiev, however, I found it more mixed - on the streets, even discussions of business in a restaurant, whilst Russian was the main language in Kiev, it was less unusual than Kharkiv to also hear Ukrainian.
My experience in late 2017 as someone who speaks neither language: learn the basic words/phrases in both. Start with Ukrainian and let the other person reply in their preferred language.
No one was ever offended by my choice of language. Like people everywhere, they are happy if you make an effort in either language.
All of the largest cities (over 300'000 population) in Ukraine speak Russian, except Lvov and Vinnitsa (50/50), while rural areas are divided, as said, into central-west being Ukrainian-speaking and south-east Russian-speaking. Although, a notable part of Russian-speaking population names Ukrainian as native language, despite poor knowledge of it.
Kiev predominantly speaks Russian and maintains Russian culture. Ukrainian language (or Surzhyk) is mostly spoken in western (Galician dialect) and partially central Ukraine (Surzhyk). So if the person speaks Ukrainian, most likely he/she's not from Kiev. It's also worth mentioning, almost all of the illustrations on native languages of Ukraine in media & internet are misleading, very few talk about it. Many of Russian-speaking people in Ukraine have a specific feature: they speak Russian in daily life, but when they are asked which language is their native, many of them answer "Ukrainian". In Russian. Most likely, they confuse it, taking it as an attribute of their identification with the state of Ukraine, and so the statistics get dramatically distorted. Kiev with over half of Ukrainian speakers would be a nightmare for native Kievans.
