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

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.