mpdecimal is a package for correctly-rounded arbitrary precision decimal floating point arithmetic.

libmpdec - C library

libmpdec is a complete implementation of the General Decimal Arithmetic Specification. The specification, written by Mike Cowlishaw from IBM, defines a general purpose arbitrary precision data type together with rigorously specified functions and rounding behavior. As described in the scope section of the specification, libmpdec will - with minor restrictions - also conform to the IEEE 754-2008 Standard for Floating-Point Arithmetic, provided that the appropriate context parameters are set.

libmpdec is written in C, but the header files are prepared for use with a C++ compiler.

libmpdec++ - C++ library

libmpdec++ is a complete implementation of the General Decimal Arithmetic Specification. libmpdec++ is mostly a header library around libmpdec’s C functions.

The library frees users from manual memory management and has an easy API with inline operators similar to the one in Python’s decimal module. Like Python’s decimal module, libmpdec++ has a thread local context for inline operators and other functions that use the implicit context.

In benchmarks the speed is close to libmpdec (about 4% slower due to the copying, destructor overhead and the thread local context).

Architectures and compilers

Both libmpdec and libmpdec++ are regularly tested on Linux, FreeBSD, NetBSD, OpenBSD, Windows, macOS, AIX and Solaris.

Complete tests are run on the following architectures: x86_64, i386, aarch64, ppc64, ppc64le, chrp, sparc64, sparc32, mips64, mips32, risc-v and Wasm.

Supported C compilers: gcc, clang, icc, xlc, ibm-clang, cl.exe, clang-cl, mingw-gcc, CompCert and emscripten.

Supported C++ compilers: g++, clang++, icpc, ibm-clang++, cl.exe, clang-cl, mingw-g++ and emscripten.


mpdecimal has a monumental test suite that includes long running differential tests against _pydecimal, decNumber and gmp.

Complex algorithms are either from the standard literature or have ACL2 proofs.

No bug has been reported in over 10 years despite heavy usage in Python and PHP.

Software that is based on mpdecimal

Python (decimal module)

Python’s _decimal module (adapted from cdecimal in version 3.3) is a wrapper around libmpdec.

Typical performance gains over _pydecimal are between 30x for I/O heavy benchmarks and 80x for numerical programs:
















PyPy (decimal module)

PyPy has a CFFI wrapper for libmpdec.

PHP (ext-decimal)

The PHP ext-decimal module uses libmpdec and provides very complete decimal support.

Lua (lmpdecimal)

An arbitrary-precision library for Lua based on mpdecimal.


An Elixir wrapper around the mpdecimal library.