Many names for boys (and girls too) can start with either a C or a K. The C and K spellings have become basically interchangeable. Typically, the K spelling is the variation, although that is not always the case. Names such as Cai and Cale have spawned from Kai and Kale. In some cases, both spellings are equally legitimate, such as with Cillian and Killian, or Caspar and Kaspar.
 
Along with Kai and Killian, other boy names that can start with a C or a K in the US Top 1000 include Cairo and Kairo, Cash and Kash, Colton and Kolton, and Crew and Krew. Those in which both spellings are rare include Caius and Kaius, Cato and Kato.
 
The choice of C or K as an initial may reflect your feelings on name style, traditionality, and image. Here, a selection of boy names that work with either C or K as a first letter.

Boy Names That Can Start with a C or a K

Casey is a name with a big wide grin, Irish, friendly, and open, and associated with several American folk heroes--Casey Jones, the engine driver of the Cannonball Express who gave his life to save his passengers, and the legendary Casey at the Bat.
 Still a widely used choice, Casey was at its height of popularity in the 1980s.

Casey

Cato conjures up images of ancient Roman statesmen and southern antebellum retainers; it could have revival potential, with its 'O' ending and the current interest in the names of Greek and Roman antiquity.
 The Roman statesman known as Cato the Elder was born Marcus Portius, the Cato added in tribute to his natural sagacity, practical wisdom and political expertise.
 Cato appears in Shakespeare's Julius Caesar and, to go from the sublime to the ridiculous, in the Pink Panther movies--and Kato was The Green Hornet's valet. Cato is also a 21st century Hunger Games name, which could do more to spur its popularity than all its past history.

Cato

Killian – aka Cillian – is a spirited yet resonant Gaelic name that was borne by several Irish saints and could make a distinctive replacement for the dated Kelly. Possible downsides: an unsavory first syllable and a connection to the trendy brew. 

Killian

Callen is a unisex Gaelic name growing in popularity here.  Its newfound trendiness is possibly related to the Chris O'Donnell character on "NCIS: Los Angeles", known only as G. Callen.  Jazz musician Cal Tjader's full name is Callen.

Callen

Clay is a rich, earthy one-syllable name with a southern-inflected handsome-rogue image, featured on soap operas and reality TV. Its longer forms are Clayton and Clayborne. 
 There have been TV characters called Clay on The Wire, Sons of Anarchy, and 13 Reasons Why, and the name's most prominent recent bearers are American Idol Clay (born Clayton) Aiken and Green Bay Packer linebacker Clay (born William Clay) Matthews.

Clay

A friendly Irish name chosen for one of her twin boys by Geena Davis.

Kian

Biblical name Cain was, until recently, seldom heard outside of the Old Testament and soap operas. Although Cain's murderous actions will always make this name difficult for some, Cain, Eve and Adam's firstborn, was a farmer - making this a good choice for those with farming connections. Long outshone by Abel, Cain is starting to find a broader audience, helped along by homophones Kane or Caine.

Cain

Camden is a surprise hit, probably as a result of some star baby cred: it has been chosen by no less than four celebrity parents since 2012. Camden is a Jersey Boy name, along with equally popular Trenton -- these are two cases where the names are more attractive than the places that inspired them.
 Camden Town is a hip area of London, noted for its busy open-air market.

Camden

Caden is a key member of the rhyming contingent that includes Aiden, Jayden, Kayden, Brayden et al. It may derive from the Celtic Cadan or Irish Cathán, both from the same root meaning "battle".

Caden

Colt is the kind of unconventionally macho name that is so trendy right now, because of or in spite of its association with horses and guns. 
 Samuel Colt was the inventor and industrialist credited with popularizing the revolver.
 Colt is a perfect jock name, associated both with Washington Redskins quarterback Colt McCoy and an entire football team, the Indianapolis Colts.

Colt

For a single-syllable Latino surname, this new popular kid on the block packs a lot of energy and charm and is one of the most stylish <a href="https://nameberry.com/baby-names/496/spanish-names-for-boys">Spanish names for boys</a> in general use today. Victoria and David Beckham named their third son Cruz, following Brooklyn and Romeo, and it was also picked up on by tennis star Lleyton Hewitt for his son. Other parents may prize its Christian associations.
 Cruz is very much a unisex name in the Spanish culture, and is also a common surname, as in Penelope and Celia.

Cruz

Kaius is a K-ization of the Latin Caius, the name of an ancient poet, introduced by style maven Rachel Zoe.  Nickname Kai makes the variation make sense....almost.

Kaius

After half a century, this otherwise feasible name has at last started to lose its link to the friendly ghost; it certainly didn't scare model Claudia Schiffer, who chose it for her son, as did Atomic Kitten Jenny Frost. Iconoclastic namer Jason Lee switched genders and called his daughter Casper. Also related to the revived Jasper, Caspar seems headed towards the path to a similar resurgence.

 Caspar is the name of one of the Three Magi who brought gifts to the infant Jesus&mdash;-Caspar of Tarsus was the one who brought the gift of gold.

 In the Casper spelling, the name was quite popular in the U.S. near the end of the nineteenth century, peaking in 1889 at Number 332, but it dropped off the list completely in 1933. It still retains its popularity in Scandinavia and Belgium.

 Actor Casper van Dien was the tenth generation in his family to bear the name of his Dutch ancestors, continuing the tradition with his son Casper Robert Mitchum.

Caspar

Caelan

Corbin, the name of the castle where the Holy Grail was said to be hidden, came to the fore in the 1980s via actor Corbin Bernsen when he was the high profile star of LA Law, but its use is only now escalating as part of the mania for two-syllable names starting with c or k, as well as from the more youthful image of Corbin Bleu, the attractive actor-model-dancer-singer who was one of the stars of the Disney hit High School Musical. 
 Bruce Willis's character name in The Fifth Element was spelled Korben Dallas.

Corbin

This no-nonsense German variation of Charles is strong and still well used, but lacks much sensitivity or subtlety; the Latin forms have far more energy. 

Carl

Varying the spelling of an established name or word to substitute a k for a c has become more and more kommon, oops, common. Some parents feel that using a different spelling makes the name more of a name and less of a place or a word. Kairo may seem to lend itself particularly well to this practice given that the K spelling turns the first syllable into the stylish name Kai. But we prefer the more authentic Kai or Cairo.

Kairo

A popular member of popular family: Kadin, Kaeden, Kaiden, and Kayden -- along with Caden, Caiden, and Cayden, as well as Cade and Kade.

Kaden

Exciting place name possibility with upbeat o ending. It debuted in the US Top 1000 in 2015.

Cairo

Calixto is known in Spain as the name of three popes, one of whom was the martyr, Callixtus I, regarded as a saint. Calixto has a lot of energy and futuristic spirit, thanks in part to the attention-grabbing 'x.' He is the main character in the Spanish classic, Fernando de Rojas's Tragicomedy of Calixto and Melibea.

Calixto

Many names for boys (and girls too) can start with either a C or a K. The C and K spellings have become basically interchangeable. Typically, the K spelling is the variation, although that is not always the case. Names such as Cai and Cale have spawned from Kai and Kale. In some cases, both spellings are equally legitimate, such as with Cillian and Killian, or Caspar and Kaspar.

Along with Kai and Killian, other boy names that can start with a C or a K in the US Top 1000 include Cairo and Kairo, Cash and Kash, Colton and Kolton, and Crew and Krew. Those in which both spellings are rare include Caius and Kaius, Cato and Kato.

The choice of C or K as an initial may reflect your feelings on name style, traditionality, and image. Here, a selection of boy names that work with either C or K as a first letter.