diff --git a/pkgs/development/compilers/openjdk/openjfx/11.nix b/pkgs/development/compilers/openjdk/openjfx/11.nix
index d5ec912bfed6..6d550538b081 100644
--- a/pkgs/development/compilers/openjdk/openjfx/11.nix
+++ b/pkgs/development/compilers/openjdk/openjfx/11.nix
@@ -42,6 +42,7 @@ let
     buildPhase = ''
       runHook preBuild
 
+      export NUMBER_OF_PROCESSORS=$NIX_BUILD_CORES
       export GRADLE_USER_HOME=$(mktemp -d)
       ln -s $config gradle.properties
       export NIX_CFLAGS_COMPILE="$(pkg-config --cflags glib-2.0) $NIX_CFLAGS_COMPILE"
diff --git a/pkgs/development/compilers/openjdk/openjfx/15.nix b/pkgs/development/compilers/openjdk/openjfx/15.nix
index b7ee8a025819..5ec888e49e5c 100644
--- a/pkgs/development/compilers/openjdk/openjfx/15.nix
+++ b/pkgs/development/compilers/openjdk/openjfx/15.nix
@@ -47,6 +47,7 @@ let
     buildPhase = ''
       runHook preBuild
 
+      export NUMBER_OF_PROCESSORS=$NIX_BUILD_CORES
       export GRADLE_USER_HOME=$(mktemp -d)
       ln -s $config gradle.properties
       export NIX_CFLAGS_COMPILE="$(pkg-config --cflags glib-2.0) $NIX_CFLAGS_COMPILE"
diff --git a/pkgs/development/compilers/openjdk/openjfx/17.nix b/pkgs/development/compilers/openjdk/openjfx/17.nix
index a6300c385812..65d6697c6ef7 100644
--- a/pkgs/development/compilers/openjdk/openjfx/17.nix
+++ b/pkgs/development/compilers/openjdk/openjfx/17.nix
@@ -49,6 +49,7 @@ let
     buildPhase = ''
       runHook preBuild
 
+      export NUMBER_OF_PROCESSORS=$NIX_BUILD_CORES
       export GRADLE_USER_HOME=$(mktemp -d)
       ln -s $config gradle.properties
       export NIX_CFLAGS_COMPILE="$(pkg-config --cflags glib-2.0) $NIX_CFLAGS_COMPILE"
diff --git a/pkgs/development/compilers/openjdk/openjfx/19.nix b/pkgs/development/compilers/openjdk/openjfx/19.nix
index bb909a31624d..802a0eb9b457 100644
--- a/pkgs/development/compilers/openjdk/openjfx/19.nix
+++ b/pkgs/development/compilers/openjdk/openjfx/19.nix
@@ -51,6 +51,7 @@ let
     buildPhase = ''
       runHook preBuild
 
+      export NUMBER_OF_PROCESSORS=$NIX_BUILD_CORES
       export GRADLE_USER_HOME=$(mktemp -d)
       ln -s $config gradle.properties
       export NIX_CFLAGS_COMPILE="$(pkg-config --cflags glib-2.0) $NIX_CFLAGS_COMPILE"
diff --git a/pkgs/development/compilers/openjdk/openjfx/20.nix b/pkgs/development/compilers/openjdk/openjfx/20.nix
index 430b32187b33..d570db3b9aee 100644
--- a/pkgs/development/compilers/openjdk/openjfx/20.nix
+++ b/pkgs/development/compilers/openjdk/openjfx/20.nix
@@ -38,6 +38,7 @@ let
     buildPhase = ''
       runHook preBuild
 
+      export NUMBER_OF_PROCESSORS=$NIX_BUILD_CORES
       export GRADLE_USER_HOME=$(mktemp -d)
       ln -s $config gradle.properties
       export NIX_CFLAGS_COMPILE="$(pkg-config --cflags glib-2.0) $NIX_CFLAGS_COMPILE"
diff --git a/pkgs/development/compilers/openjdk/openjfx/21.nix b/pkgs/development/compilers/openjdk/openjfx/21.nix
index 4b178736854d..2f0d83f9f251 100644
--- a/pkgs/development/compilers/openjdk/openjfx/21.nix
+++ b/pkgs/development/compilers/openjdk/openjfx/21.nix
@@ -38,6 +38,7 @@ let
     buildPhase = ''
       runHook preBuild
 
+      export NUMBER_OF_PROCESSORS=$NIX_BUILD_CORES
       export GRADLE_USER_HOME=$(mktemp -d)
       ln -s $config gradle.properties
       export NIX_CFLAGS_COMPILE="$(pkg-config --cflags glib-2.0) $NIX_CFLAGS_COMPILE"