String#center

str.center(width, padstr = ' ') -> string

Returns: String · Updated March 16, 2026 · String Methods

strings padding formatting alignment

The .center method pads a string to a specified width, placing the original string in the center of the padded result. By default, it uses spaces for padding, but you can specify any character or string. This is useful for formatting terminal output, creating ASCII art, or generating aligned text tables.

Syntax

str.center(width)           # Pad with spaces (default)
str.center(width, padstr)  # Pad with custom string

Parameters

Parameter	Type	Description
`width`	Integer	The total width of the resulting padded string
`padstr`	String	The string to use for padding (default is space)

Worked Examples

Example 1: Basic centering with spaces

text = "Ruby"

puts text.center(10)
# Output: "   Ruby   "

When the total width is larger than the string, Ruby distributes the padding equally on both sides. If the difference is odd, the right side gets one extra character.

puts "Hi".center(10)     # "    Hi    "
puts "Hello".center(10)  # "  Hello   " (extra space on right)
puts "Ruby".center(10)   # "   Ruby   "

Example 2: Using custom padding characters

puts "Ruby".center(10, "*")   # "***Ruby***"
puts "Hi".center(10, "-")     # "----Hi----"
puts "X".center(5, "=")       # "==X=="

You can use multi-character strings for padding:

puts "Yo".center(10, "._.")   # "._.Yo._.."

Example 3: Centering in terminal output

title = "Welcome"

puts "+" + "-" * 20 + "+"
puts "|" + title.center(20) + "|"
puts "+" + "-" * 20 + "+"

# Output:
#+--------------------+
#|      Welcome       |
#+--------------------+

Example 4: Creating ASCII headers

def banner(text, width = 40)
  border = "*" * width
  puts border
  puts text.center(width)
  puts border
end

banner("Hello, World!")
# ****************************************
#            Hello, World!                
# ****************************************

Example 5: When width is less than string length

If the specified width is less than or equal to the string’s length, Ruby returns the original string unchanged.

puts "Ruby".center(2)   # "Ruby" (no padding needed)
puts "Ruby".center(4)   # "Ruby" (exact fit)

Common Patterns

Building formatted tables

headers = ["Name", "Age", "City"]
data = [["Alice", "30", "London"], ["Bob", "25", "Paris"]]

width = 20

puts headers.map { |h| h.center(width) }.join(" | ")
puts "-" * (width * 3 + 4)

data.each do |row|
  puts row.map { |cell| cell.center(width) }.join(" | ")
end

# Output:
#        Name        |        Age         |       City        
# ------------------------------------------------------------
#       Alice        |        30         |      London       
#        Bob         |        25         |       Paris

Creating centered boxes

def centered_box(text, width = 40)
  inner_width = width - 2
  padded = text.center(inner_width)
  "+" + "-" * inner_width + "+"
  "|" + padded + "|"
  "+" + "-" * inner_width + "+"
end

puts centered_box("IMPORTANT")
puts centered_box("Click Here")

# +--------------------------------------+
# |             IMPORTANT                |
# +--------------------------------------+
# |            Click Here                |
# +--------------------------------------+

Number formatting

numbers = [1, 22, 333, 4444]

numbers.each do |num|
  puts num.to_s.center(10, "0")
end

# Output:
# 0000010000
# 0000022000
# 0000333000
# 0004444000

Edge Cases

Empty strings and special characters

puts "".center(5)        # "     "
puts " ".center(5)       # "     " (single space becomes 5)
puts "\t".center(5)      # tabs also pad correctly
puts "日本語".center(10) # "  日本語  " (Unicode works)

Padding with empty string

puts "Ruby".center(10, "")  # Raises ArgumentError: zero width padding