+ Blazing fast
+Often 4× faster than the best competitor and many times faster than typical standard-library implementations. Sustains 1 GB/s on commodity hardware.
+
+ C++11 · header-only · triple-licensed
+
+ Parse floating-point numbers at a gigabyte per second.
+
+ fast_float is a header-only C++ implementation of
+ std::from_chars for float, double, and integer types.
+ Exact IEEE rounding, no allocations, no exceptions — often many times faster
+ than your standard library.
+
+
+ Why fast_float?
+A drop-in from_chars built for performance-critical code paths.
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+ Exact rounding
+Returns the closest IEEE 754 float or double with round-to-nearest, ties-to-even — bit-for-bit correct.
Header-only
+Just drop in fast_float.h or use it via CMake, Conan, vcpkg, xmake, or Homebrew. Requires only C++11.
No surprises
+Does not allocate, does not throw, locale-independent. The interface mirrors C++17 std::from_chars.
Integers too
+Parses every standard integer type in bases 2–36, plus bool. The same fast, allocation-free interface.
Unicode & formats
+UTF-8, UTF-16, and UTF-32 inputs. JSON, Fortran, and custom decimal separators via from_chars_advanced.
constexpr-ready
+In C++20, parse strings at compile time with consteval — zero runtime cost.
Portable
+Visual Studio, GCC, Clang, MSYS2. Linux, macOS, FreeBSD, Windows. x86-64, ARM, RISC-V, s390x. 32-bit and 64-bit.
+
+
+ Performance
++ Parsing random floating-point numbers, measured in megabytes per second + (higher is better). Source: project benchmark suite on a typical x86-64 box. +
+ +
+
+
+
+ fast_float
+
+
+
+ abseil
+
+
+
+ netlib
+
+
+
+ double-conversion
+
+
+
+ strtod
+
+
+
+
+ Reproduce these numbers
+cmake -B build -D FASTFLOAT_BENCHMARKS=ON
+cmake --build build
+./build/benchmarks/benchmark
+
+ Quick start
+Parse a double from a string in three lines.
#include "fast_float/fast_float.h"
+#include <iostream>
+#include <string>
+
+int main() {
+ std::string input = "3.1416 xyz ";
+ double result;
+ auto answer = fast_float::from_chars(input.data(),
+ input.data() + input.size(),
+ result);
+ if (answer.ec != std::errc()) {
+ std::cerr << "parsing failure\n";
+ return EXIT_FAILURE;
+ }
+ std::cout << "parsed the number " << result << '\n';
+}Integers in any base (2–36)
+uint64_t value;
+std::string hex = "4f0cedc95a718c";
+auto r = fast_float::from_chars(hex.data(), hex.data() + hex.size(), value, 16);
+// value == 22250738585072012UTF-16 input
+std::u16string input = u"3.1416 xyz ";
+double result;
+auto r = fast_float::from_chars(input.data(), input.data() + input.size(), result);Comma as decimal separator, Fortran, or JSON
+// "3,1416" — French-style
+fast_float::parse_options opts{fast_float::chars_format::general, ','};
+fast_float::from_chars_advanced(s.data(), s.data() + s.size(), result, opts);
+
+// "1d+4" — Fortran exponent
+opts = {fast_float::chars_format::fortran};
+
+// strict JSON per RFC 8259
+opts = {fast_float::chars_format::json};C++20: parse at compile time
+consteval double parse(std::string_view s) {
+ double v;
+ auto r = fast_float::from_chars(s.data(), s.data() + s.size(), v);
+ return r.ec == std::errc() ? v : -1.0;
+}
+constexpr double pi = parse("3.1415"); // computed at compile time
+
+ Install
+Pick the workflow that matches your project.
+ +
+
+ CMake
+
+
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+
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+
+
+ CMake FetchContent
+FetchContent_Declare(
+ fast_float
+ GIT_REPOSITORY https://github.com/fastfloat/fast_float.git
+ GIT_TAG tags/v{{VERSION}}
+ GIT_SHALLOW TRUE)
+FetchContent_MakeAvailable(fast_float)
+target_link_libraries(myprogram PUBLIC fast_float)CPM
+CPMAddPackage(
+ NAME fast_float
+ GITHUB_REPOSITORY "fastfloat/fast_float"
+ GIT_TAG v{{VERSION}})Single header
+Download a pre-amalgamated header — no build system required.
+curl -LO https://github.com/fastfloat/fast_float/releases/download/v{{VERSION}}/fast_float.hPackage managers
+-
+
- Conan +
- Homebrew —
brew install fast_float
+ - xmake +
- Fedora —
dnf install fast_float-devel
+ - More distributions +
+
+ Trusted by
+fast_float ships inside compilers, browsers, databases, and more.
+-
+
- GCC — backs
std::from_charssince version 12
+ - Chromium — Chrome, Edge, and Opera +
- WebKit — Safari +
- Ladybird — independent browser engine +
- DuckDB — in-process analytical database +
- Apache Arrow — 2–3× faster number parsing +
- ClickHouse — OLAP database +
- MySQL +
- Boost.JSON +
- Blender +
- Google Jsonnet +
+ Ports and bindings exist for + Rust, + Java, + C#, + C, and + R. +
+
+
+ The research behind it
+-
+
- + Daniel Lemire, + Number Parsing at a Gigabyte per Second. + Software: Practice and Experience 51(8), 2021. + +
- + Noble Mushtak, Daniel Lemire, + Fast Number Parsing Without Fallback. + Software: Practice and Experience 53(7), 2023. + +