diff --git a/programs/util.h b/programs/util.h
index 9992d79da..0b90cda19 100644
--- a/programs/util.h
+++ b/programs/util.h
@@ -25,6 +25,7 @@ extern "C" {
 #include <stdlib.h>       /* malloc */
 #include <stddef.h>       /* size_t, ptrdiff_t */
 #include <stdio.h>        /* fprintf */
+#include <string.h>       /* strncmp */
 #include <sys/types.h>    /* stat, utime */
 #include <sys/stat.h>     /* stat */
 #if defined(_MSC_VER)
@@ -488,6 +489,177 @@ UTIL_STATIC void UTIL_freeFileList(const char** filenameTable, char* allocatedBu
     if (filenameTable) free((void*)filenameTable);
 }
 
+/* count the number of physical cores */
+#if defined(_WIN32) || defined(WIN32)
+
+#include <windows.h>
+
+typedef BOOL(WINAPI* LPFN_GLPI)(PSYSTEM_LOGICAL_PROCESSOR_INFORMATION, PDWORD);
+
+UTIL_STATIC int UTIL_countPhysicalCores(void)
+{
+    static int numPhysicalCores;
+    if (numPhysicalCores != 0) return numPhysicalCores;
+
+    {   LPFN_GLPI glpi;
+        BOOL done = FALSE;
+        PSYSTEM_LOGICAL_PROCESSOR_INFORMATION buffer = NULL;
+        PSYSTEM_LOGICAL_PROCESSOR_INFORMATION ptr = NULL;
+        DWORD returnLength = 0;
+        size_t byteOffset = 0;
+
+        glpi = (LPFN_GLPI)GetProcAddress(GetModuleHandle(TEXT("kernel32")),
+                                         "GetLogicalProcessorInformation");
+
+        if (glpi == NULL) {
+            goto failed;
+        }
+
+        while(!done) {
+            DWORD rc = glpi(buffer, &returnLength);
+            if (FALSE == rc) {
+                if (GetLastError() == ERROR_INSUFFICIENT_BUFFER) {
+                    if (buffer)
+                        free(buffer);
+                    buffer = (PSYSTEM_LOGICAL_PROCESSOR_INFORMATION)malloc(returnLength);
+
+                    if (buffer == NULL) {
+                        perror("zstd");
+                        exit(1);
+                    }
+                } else {
+                    /* some other error */
+                    goto failed;
+                }
+            } else {
+                done = TRUE;
+            }
+        }
+
+        while (byteOffset + sizeof(SYSTEM_LOGICAL_PROCESSOR_INFORMATION) <= returnLength) {
+            ptr = buffer;
+
+            if (ptr->RelationShip == RelationProcessorCore) {
+                numPhysicalCores++;
+            }
+        }
+
+        free(buffer);
+
+        return numPhysicalCores;
+    }
+
+failed:
+    /* try to fall back on GetSystemInfo */
+    SYSTEM_INFO sysinfo;
+    GetSystemInfo(&sysinfo);
+    numPhysicalCores = sysinfo.dwNumberOfProcessors;
+    if (numPhysicalCores == 0) numPhysicalCores = 1; /* just in case */
+    return numPhysicalCores;
+}
+
+#elif defined(__APPLE__)
+
+#include <sys/sysctl.h>
+
+/* Use apple-provided syscall
+ * see: man 3 sysctl */
+UTIL_STATIC int UTIL_countPhysicalCores(void)
+{
+    static S32 numPhysicalCores; /* apple specifies int32_t */
+    if (numPhysicalCores != 0) return numPhysicalCores;
+
+    {   int const ret = sysctlbyname("hw.physicalcpu", &numPhysicalCores, sizeof(int), NULL, 0);
+        if (ret != 0) {
+            if (errno == ENOENT) {
+                /* entry not present, fall back on 1 */
+                numPhysicalCores = 1;
+            } else {
+                perror("zstd: can't get number of physical cpus");
+                exit(1);
+            }
+        }
+
+        return numPhysicalCores;
+    }
+}
+
+#elif defined(__linux__)
+
+/* parse /proc/cpuinfo
+ * siblings / cpu cores should give hyperthreading ratio
+ * otherwise fall back on sysconf */
+UTIL_STATIC int UTIL_countPhysicalCores(void)
+{
+    static int numPhysicalCores;
+
+    if (numPhysicalCores != 0) return numPhysicalCores;
+
+    numPhysicalCores = sysconf(_SC_NPROCESSORS_ONLN);
+    if (numPhysicalCores == -1) {
+        /* value not queryable, fall back on 1 */
+        return numPhysicalCores = 1;
+    }
+
+    /* try to determine if there's hyperthreading */
+    {   FILE* const cpuinfo = fopen("/proc/cpuinfo", "r");
+        size_t const BUF_SIZE = 80;
+        char buff[BUF_SIZE];
+
+        int siblings = 0;
+        int cpu_cores = 0;
+        int ratio = 1;
+
+        if (cpuinfo == NULL) {
+            /* fall back on the sysconf value */
+            return numPhysicalCores;
+        }
+
+        /* assume the cpu cores/siblings values will be constant across all
+         * present processors */
+        while (!feof(cpuinfo)) {
+            if (fgets(buff, BUF_SIZE, cpuinfo) != NULL) {
+                if (strncmp(buff, "siblings", 8) == 0) {
+                    const char* const sep = strchr(buff, ':');
+                    if (*sep == '\0') {
+                        /* formatting was broken? */
+                        goto failed;
+                    }
+
+                    siblings = atoi(sep + 1);
+                }
+                if (strncmp(buff, "cpu cores", 9) == 0) {
+                    const char* const sep = strchr(buff, ':');
+                    if (*sep == '\0') {
+                        /* formatting was broken? */
+                        goto failed;
+                    }
+
+                    cpu_cores = atoi(sep + 1);
+                }
+            } else if (ferror(cpuinfo)) {
+                /* fall back on the sysconf value */
+                goto failed;
+            }
+        }
+        if (siblings && cpu_cores) {
+            ratio = siblings / cpu_cores;
+        }
+failed:
+        fclose(cpuinfo);
+        return numPhysicalCores = numPhysicalCores / ratio;
+    }
+}
+
+#else
+
+UTIL_STATIC int UTIL_countPhysicalCores(void)
+{
+    /* assume 1 */
+    return 1;
+}
+
+#endif
 
 #if defined (__cplusplus)
 }
diff --git a/programs/zstdcli.c b/programs/zstdcli.c
index 18e259c74..73b4eab9d 100644
--- a/programs/zstdcli.c
+++ b/programs/zstdcli.c
@@ -602,6 +602,11 @@ int main(int argCount, const char* argv[])
     DISPLAYLEVEL(4, "PLATFORM_POSIX_VERSION defined: %ldL\n", (long) PLATFORM_POSIX_VERSION);
 #endif
 
+    if (nbThreads == 0) {
+        /* try to guess */
+        nbThreads = UTIL_countPhysicalCores();
+        DISPLAYLEVEL(3, "Note: %d physical core(s) detected\n", nbThreads);
+    }
 
     g_utilDisplayLevel = g_displayLevel;
     if (!followLinks) {