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Python String zfill() Method

By (LinkedIn) · Every code example is run and checked on Python 3.13 (how we test) · Last updated

Short answer
str.zfill(width) pads a string on the left with zeros until it is width characters long, for example "42".zfill(5) gives '00042'.

Use zfill() whenever numbers must have a fixed number of digits: invoice numbers like 00042, file names like img_007.png, or clock times like 09:05. It is a string method, so convert integers with str() first. It is also sign-aware: a leading + or - stays at the front and the zeros go after it. For other padding characters use rjust(), and for fancy layouts see the format() method.

Syntaxtext.zfill(width)
Parameterswidth: the minimum length of the result
ReturnsA new string padded with 0 on the left
If width <= len(text)Returns the text unchanged
SignsA leading + or - stays first

How str.zfill() pads with zeros

The zeros are added on the left until the string has the requested width. Longer strings are never shortened.

Python 
print("7".zfill(3))
print("123".zfill(3))
print("12345".zfill(3))
print("abc".zfill(6))





The last line shows that zfill() works on any string, not only on digits.

      
      
    

    

    
    

      
How zfill() treats plus and minus signs

      
If the string starts with + or -, the sign is kept in front and the zeros are inserted after it. That is what you want for numbers, and the reason zfill() differs from rjust(width, "0").



  

    Python
    

      
      
      
      
    

  

  
print("-42".zfill(5))
print("+7".zfill(4))
print("-42".rjust(5, "0"))

  
  
  
  

    
      
      
    
    
  

  





      
      
    

    
    
    

      
zfill() vs rjust() vs f-string zero padding

      
      
      
      

        
        
ApproachCodeResultNotes

        
zfill()"42".zfill(5)00042String method; sign-aware
rjust()"42".rjust(5, "0")00042Any fill character, not sign-aware
f-stringf"{42:05d}"00042Works on the integer directly
format()"{:05d}".format(42)00042Same as the f-string
% style"%05d" % 4200042Older style

      

      
    

    
    

    
    

      
Examples you can run

      
Press Run, then edit the code and run it again.

      
      

        
Zero-pad an invoice number

        
Convert the number to a string, then pad it to six digits.

        

  

    Zero-pad an invoice number
    

      
      
      
      
    

  

  
invoice = 42
code = "INV-" + str(invoice).zfill(6)
print(code)

  
  
  
  

    
      
      
    
    
  

  
Expected outputINV-000042




      

      
      

        
Format a clock time

        
Hours and minutes need two digits each.

        

  

    Format a clock time
    

      
      
      
      
    

  

  
hours, minutes = 9, 5
print(str(hours).zfill(2) + ":" + str(minutes).zfill(2))

  
  
  
  

    
      
      
    
    
  

  
Expected output09:05




      

      
      

        
Build sortable file names

        
Padded numbers sort correctly as text, while unpadded numbers do not.

        

  

    Build sortable file names
    

      
      
      
      
    

  

  
plain = ["img" + str(n) for n in (1, 2, 10)]
padded = ["img" + str(n).zfill(3) for n in (1, 2, 10)]
print(sorted(plain))
print(sorted(padded))

  
  
  
  

    
      
      
    
    
  

  
Expected output['img1', 'img10', 'img2']
['img001', 'img002', 'img010']




      

      
      

        
Pad a negative number

        
The minus sign stays first, and the zeros come after it.

        

  

    Pad a negative number
    

      
      
      
      
    

  

  
print(str(-5).zfill(4))

  
  
  
  

    
      
      
    
    
  

  
Expected output-005




      

      
    

    

    
    

      
Common mistakes

      
      

        
Calling zfill() on an integer

        

          
✗ Wrong
n = 42
print(n.zfill(5))
AttributeError: 'int' object has no attribute 'zfill'

          
✓ Right
n = 42
print(str(n).zfill(5))
print(f"{n:05d}")

        

        
Convert to a string first, or use an f-string format spec.

      

      
      

        
Expecting the width to be the number of zeros

        

          
✗ Wrong
print("42".zfill(3))  # expecting 00042
Prints `042`: the width is the total length, not the number of zeros.

          
✓ Right
print("42".zfill(5))

        

        
Use the total length you want, here 5.

      

      
      

        
Expecting it to shorten a long string

        

          
✗ Wrong
print("123456".zfill(3))
Prints `123456` unchanged.

          
✓ Right
print("123456"[-3:].zfill(3))

        

        
Slice the string first if a hard limit is needed.

      

      
    

    

    
    

      
Python String zfill() Method: questions people ask

      
How do I add leading zeros to a number in Python?
Use str(n).zfill(width) or an f-string such as f"{n:05d}". Both give 00042 for 42 with width 5.
What is the difference between zfill() and rjust()?
zfill() always pads with zeros and keeps a leading sign in front, while rjust() can pad with any character and ignores signs.
Does zfill() work on integers?
No. It is a string method, so convert with str() first or use f"{n:05d}" on the integer.

    

    

    
    

      
Keep learning

      
Step-by-step lessons:
Python StringsPython String MethodsPython f-Strings

      
Related:
Python String center() MethodPython String format() MethodHow to Convert an Int to a String in PythonPython String isdigit() Method

      
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