String#ljust

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

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

strings padding formatting alignment

The .ljust method pads a string to a specified width on the right side, placing the original string on the left. By default, it uses spaces for padding, but you can specify any character or string. This is useful for formatting table columns, creating fixed-width output, or aligning text in terminal applications.

Syntax

str.ljust(width)           # Pad with spaces (default)
str.ljust(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 left-justification with spaces

text = "Ruby"

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

When the total width is larger than the string, Ruby adds padding to the right side.

puts "Hi".ljust(10)     # "Hi        "
puts "Hello".ljust(10)  # "Hello     "
puts "Ruby".ljust(10)   # "Ruby      "

Example 2: Using custom padding characters

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

You can use multi-character strings for padding:

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

Example 3: Creating formatted table columns

data = [
  ["Name", "Age", "City"],
  ["Alice", "30", "London"],
  ["Bob", "25", "Paris"],
  ["Carolyn", "28", "Berlin"]
]

# Print table with left-justified columns
data.each do |row|
  puts row.map { |cell| cell.ljust(10) }.join(" | ")
end

# Output:
# Name       | Age       | City      
# ------------------------------------------------
# Alice      | 30        | London    
# Bob        | 25        | Paris     
# Carolyn    | 28        | Berlin

Example 4: Aligning variable-width data

labels = ["User:", "Email:", "Status:"]
values = ["john", "john@example.com", "active"]

labels.zip(values).each do |label, value|
  puts "#{label.ljust(10)} #{value}"
end

# Output:
# User:     john
# Email:    john@example.com
# Status:   active

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

Common Patterns

Building CLI output

def print_key_value(key, value, width = 30)
  puts "#{key.ljust(width)}: #{value}"
end

print_key_value("Version", "3.2.0")
print_key_value("Author", "Yukihiro Matsumoto")
print_key_value("License", "BSD-2-Clause")

# Output:
# Version:                      3.2.0
# Author:                      Yukihiro Matsumoto
# License:                     BSD-2-Clause

Generating fixed-width columns for logs

def log_line(level, message, width = 50)
  timestamp = Time.now.strftime("%H:%M:%S")
  puts "[#{timestamp}] #{level.ljust(8)} #{message.ljust(width)}"
end

puts log_line("INFO", "Application started")
puts log_line("DEBUG", "Loading configuration file")
puts log_line("ERROR", "Connection timeout after 30s")
puts log_line("WARN", "Deprecated API called")

# Output:
# [18:33:00] INFO     Application started                             
# [18:33:00] DEBUG    Loading configuration file                     
# [18:33:00] ERROR    Connection timeout after 30s                    
# [18:33:00] WARN     Deprecated API called

Edge Cases

Empty strings and special characters

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

Padding with empty string

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

Negative width

puts "Ruby".ljust(-5)   # Raises ArgumentError: negative string size