How to Convert String to Float in Python

To convert a string to a float in Python, use the built-in float() function.

This is useful when you have text like "3.14" and want to turn it into a number you can calculate with. It is also common when working with input(), form data, or values read from files.

Quick answer

text = "3.14"
number = float(text)

print(number)
print(type(number))

Output:

3.14
<class 'float'>

Use float() when the string contains a valid number such as "3.14", "10", or "-2.5".

Use float() to convert a string

The main tool for this job is float().

It takes numeric text and converts it into a floating-point number.

a = float("3.14")
b = float("10")
c = float("-7.5")
d = float("1e3")

print(a)
print(b)
print(c)
print(d)

Output:

3.14
10.0
-7.5
1000.0

A few important things to notice:

  • "3.14" becomes 3.14
  • "10" becomes 10.0
  • "-7.5" becomes -7.5
  • "1e3" becomes 1000.0
  • The result is always a float

If you want a full explanation of how this function works, see Python float() function explained.

What valid float strings look like

float() only works if the string is in a format Python understands.

These are valid examples:

print(float("10"))       # whole number string
print(float("3.5"))      # decimal number
print(float("-2.75"))    # negative number
print(float("1e6"))      # scientific notation
print(float("  4.2  "))  # spaces around the value

Output:

10.0
3.5
-2.75
1000000.0
4.2

Common valid patterns:

  • Digits only: "10" → 10.0
  • Decimal numbers: "3.5" → 3.5
  • Negative numbers: "-2.75" → -2.75
  • Scientific notation: "1e6" → 1000000.0
  • Leading and trailing spaces are usually allowed

Handle invalid input safely

If the string is not a valid number, float() raises a ValueError.

text = "hello"
number = float(text)

This causes an error because "hello" is not numeric text.

When the value may come from a user, a file, or another unreliable source, use try and except:

text = "hello"

try:
    number = float(text)
    print("Converted:", number)
except ValueError:
    print("That is not a valid number.")

Output:

That is not a valid number.

This is a better approach because your program does not crash. Instead, it shows a clear message.

If you are seeing this error already, read how to fix ValueError: could not convert string to float.

Clean the string before converting

Sometimes the value is almost correct, but it needs a little cleaning first.

Remove extra spaces

Use strip() to remove spaces at the beginning and end:

value = "  3.14  "
cleaned = value.strip()

print(float(cleaned))

Output:

3.14

Handle commas carefully

Strings like "1,234.56" are not valid for float() as-is.

value = "1,234.56"
cleaned = value.replace(",", "")

print(float(cleaned))

Output:

1234.56

Only remove commas if you expect them as formatting characters.

Currency symbols do not work directly

float() cannot read values like "$19.99" without cleaning them first.

value = "$19.99"
cleaned = value.replace("$", "")

print(float(cleaned))

Output:

19.99

In real programs, decide whether you want to:

  • clean the data first
  • reject invalid input and show an error

Convert user input to float

The input() function always returns a string.

So if the user types a number, you still need to convert it.

number = float(input("Enter a number: "))
print("You entered:", number)

If the user types 3.5, the result will be a float.

Because users can type unexpected values, it is safer to add error handling:

text = input("Enter a number: ")

try:
    number = float(text)
    print("You entered:", number)
except ValueError:
    print("Please enter a valid number.")

This pattern is very common in beginner programs.

To learn more, see Python input() function explained and how to convert user input to numbers in Python.

Know when to use int() instead

Use float() when you need decimal values.

Use int() when you need whole numbers only.

print(float("5"))
print(int("5"))

Output:

5.0
5

But this does not work:

print(int("3.14"))

That raises an error because "3.14" is not a whole-number string.

If you specifically need whole numbers, see how to convert a string to an integer in Python.

Common mistakes

These are some common reasons string-to-float conversion fails:

  • Trying to convert text like "hello" with float()
  • Using numbers with commas such as "1,234.56" without cleaning the string
  • Passing currency text like "$19.99" to float()
  • Assuming input() returns a number automatically
  • Using int() instead of float() for decimal strings

If you are debugging a problem, these quick checks can help:

print(value)
print(type(value))
print(repr(value))

cleaned = value.strip()
print(cleaned)

Why these help:

  • print(value) shows the visible content
  • print(type(value)) confirms whether it is really a string
  • print(repr(value)) shows hidden spaces and special characters
  • strip() removes extra spaces before conversion

FAQ

Does float() work on whole numbers stored as strings?

Yes. float("10") returns 10.0.

Why does float() fail on "3,14"?

Python expects a dot as the decimal separator in standard float strings.

Can float() convert user input directly?

Yes. Example:

number = float(input("Enter a number: "))

What error happens if the string is not a number?

Python raises ValueError.

Should I use float() or int()?

Use float() for decimal values and int() for whole-number strings.

See also