Why would I want a float expressed as hexfloat notation?

Hexfloat notation shows a floating-point value's exact bits with no decimal rounding at all, which matters when debugging precision issues, writing reproducible test vectors, or comparing values across languages that print floats slightly differently in decimal.

Why does the output always include an explicit + or - on the exponent?

This matches the convention used by common implementations of printf("%a", ...) (such as glibc's), which always show an explicit sign on the exponent (e.g. "p+3" rather than just "p3") even though a plain "p3" is equally valid C99 hexfloat notation and parses identically.

How is the exact hex mantissa computed?

The value's raw IEEE-754 double-precision bits are read directly (via a DataView over an 8-byte buffer), and the 52-bit mantissa is converted straight to 13 hex digits, so the result reflects the value's true stored bits rather than an approximation derived from its decimal digits.

What happens with Infinity or NaN?

They're rejected with an explicit error, since this tool only encodes finite hexfloat values; there's no standard finite hex-mantissa-and-exponent form for either of them.

Float to Hexfloat Converter - Decimal to C99 %a Notation Online

Overview

Introduction

Decimal representations of floating-point numbers are often rounded for readability, which can hide exactly what value is actually stored in memory.

This tool sidesteps that by reading a float's raw IEEE-754 bits directly and expressing them as exact C99 hexfloat notation.

What Is Float to Hexfloat Converter?

A converter that encodes a decimal floating-point number as C99/printf "%a" hexfloat notation, an exact hex mantissa multiplied by a power of two.

It's the direct inverse of Hexfloat to Float Converter, and distinct from Floating Point Number to Hex Converter, which outputs the value's raw IEEE-754 bit pattern as a plain hex string rather than this dot-and-exponent notation.

How Float to Hexfloat Converter Works

The input value is written into an 8-byte buffer as a double-precision float, then its sign, 11-bit exponent, and 52-bit mantissa are read directly from the raw bits via DataView, avoiding any decimal-based rounding.

The mantissa's 52 bits become 13 hex digits (with an implicit leading 1 for normal numbers), trailing zero hex digits are stripped, and the biased exponent is converted to its true (unbiased) power-of-two value before being formatted as "0x{digit}.{fraction}p{sign}{exponent}".

When To Use Float to Hexfloat Converter

Use it to see the exact hexfloat form of a decimal value, matching what printf("%a", ...) in C/C++ or Python's float.hex() would print.

It's useful for constructing precise hexfloat literals for C99 source code or test fixtures, and for debugging subtle floating-point precision differences.

Often used alongside Hexfloat to Float Converter, Hex to Floating Point Number Converter and Floating Point Number to Hex Converter.

Features

Advantages

Exact by construction, since it reads the value's true IEEE-754 bits rather than deriving hex digits from decimal digits.
Produces a canonical, minimal form by stripping trailing zero mantissa digits.
Handles the zero, negative-zero, and subnormal edge cases explicitly.

Limitations

Infinity and NaN are rejected, since they have no standard finite hexfloat representation.
Only encodes double (64-bit) precision; there's no option to instead express the value at float32 (single) precision.

Examples

Encoding a small integer value

Input

Output

0x1.8p+3

12 = 1.5 × 2^3; the mantissa 1.5 is 1 + 8/16 in hex, giving "1.8".

Encoding exactly 1

Input

Output

0x1p+0

1 = 1.0 × 2^0; since the fractional mantissa bits are all zero, no fraction is shown.

Best Practices & Notes

Best Practices

Round-trip the output through Hexfloat to Float Converter if you want to double check the encoding matches your expectation.
Remember the mantissa digits after the dot are hexadecimal (base 16) fractions, not decimal ones, "0x1.8" means 1.5, not 1.8.

Developer Notes

Implemented with `DataView.prototype.setFloat64`/`getUint32` to extract the sign, biased 11-bit exponent, and 52-bit mantissa as exact integers, then `mantissaHex.replace(/0+$/, "")` trims trailing zero hex digits before assembling the final string; verified by hand that 12 → "0x1.8p+3" and 1 → "0x1p+0", and that both round-trip exactly back through Hexfloat to Float Converter.

Float to Hexfloat Converter Use Cases

Writing an exact C99 hexfloat literal for source code or test data
Comparing a value's true stored bits against its rounded decimal display
Reproducing the output of printf("%a", ...) or Python's float.hex() for a given value

Common Mistakes

Reading the mantissa's fractional digits as decimal instead of hexadecimal, ".8" means 8/16 = 0.5, not 0.8.
Expecting Infinity or NaN to produce hexfloat output; both are rejected since they have no finite hex-mantissa form.

Tips

Use Floating Point Number to Hex Converter instead if you want the value's raw IEEE-754 bits as a plain hex string rather than this dot-and-exponent notation.
A trailing fraction of a single hex digit (like ".8" or ".4") usually means the value is a simple multiple of a power of two, like 1.5 or 1.25.

Float to Hexfloat Converter