This page allows you to convert between the decimal representation of a number (like "1.02") and the binary format used by all modern CPUs (a.k.a. "IEEE 754 floating point").
Update
As of 2024, the converter has been updated to run fully client side. For this, a browser supporting "WebAssembly" is required  all current major browser support it. This change may lead to somewhat different behaviour when displaying numbers, but should provide quicker reaction. In case you have any problems, please contact me.
This webpage is a tool to understand IEEE754 floating point numbers. This is the format in which almost all CPUs represent noninteger numbers. As this format is using base2, there can be surprising differences in what numbers can be represented easily in decimal and which numbers can be represented in IEEE754. As an example, try "0.1". The conversion is limited to 32bit single precision numbers, while the IEEE754Standard contains formats with increased precision.

Usage:
You can either convert a number by choosing its binary representation in the buttonbar, the other fields will be updated immediately. Or you can enter a binary number, a hexnumber or the decimal representation into the corresponding textfield and press return to update the other fields. To make it easier to spot eventual rounding errors, the selected float number is displayed after conversion to double precision.

Special Values:
You can enter the words "Infinity", "Infinity" or "NaN" to get the corresponding special values for IEEE754. Please note there are two kinds of zero: +0 and 0.

Conversion:
The value of a IEEE754 number is computed as:
sign 2^{exponent} mantissa
The sign is stored in bit 32. The exponent can be computed from bits 2431 by subtracting 127. The mantissa (also known as significand or fraction) is stored in bits 123. An invisible leading bit (i.e. it is not actually stored) with value 1.0 is placed in front, then bit 23 has a value of 1/2, bit 22 has value 1/4 etc. As a result, the mantissa has a value between 1.0 and 2. If the exponent reaches 127 (binary 00000000), the leading 1 is no longer used to enable gradual underflow.

Underflow:
If the exponent has minimum value (all zero), special rules for denormalized values are followed. The exponent value is set to 2^{126} and the "invisible" leading bit for the mantissa is no longer used.
The range of the mantissa is now [0:1).
Note: The converter used to show denormalized exponents as 2^{127} and a denormalized mantissa range [0:2). This is effectively identical to the values above, with a factor of two shifted between exponent and mantissa. However this confused people and was therefore changed (20150926).

Rounding errors:
Not every decimal number can be expressed exactly as a floating point number. This can be seen when entering "0.1" and examining its binary representation which is either slightly smaller or larger, depending on the last bit.

Other representations:
The hex representation is just the integer value of the bitstring printed as hex. Don't confuse this with true hexadecimal floating point values in the style of 0xab.12ef.

FAQ (Frequently Asked Questions):

Can you send me the source code? I need to convert format x to format y.:
This source code for this converter doesn't contain any low level conversion routines. The conversion between a floating point number (i.e. a 32 bit area in memory) and the bit representation isn't actually a conversion, but just a reinterpretation of the same data in memory. This can be easily done with typecasts in C/C++ or with some bitfiddling via java.lang.Float.floatToIntBits in Java. The conversion between a string containing the textual form of a floating point number (e.g. "3.14159", a string of 7 characters) and a 32 bit floating point number is also performed by library routines. If you need to write such a routine yourself, you should have a look at the sourecode of a standard C library (e.g. GNU libc, uclibc or the FreeBSD C library  please have a look at the licenses before copying the code)  be aware, these conversions can be complicated.

Can you add support for 64bit float/16bit float/nonIEEE 754 float?.:
The primary purpose of this site is to support people learning about these formats, and for understanding the difference between 32bit and 64bit is minor. On the other hand, 64bit would be more cumbersome to work with when displaying, so the tradeoff isn't really worth it at the moment.

Note: If you find any problems, please report them here.