diff --git a/BUILD.bazel b/BUILD.bazel
index d72ae867..15d83699 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -1,5 +1,22 @@
 licenses(["notice"])
 
+COPTS = [
+    "-pedantic",
+    "-pedantic-errors",
+    "-std=c++11",
+    "-Wall",
+    "-Wconversion",
+    "-Wextra",
+    "-Wshadow",
+    #    "-Wshorten-64-to-32",
+    "-Wfloat-equal",
+    "-fstrict-aliasing",
+    ## assert() are used a lot in tests upstream, which may be optimised out leading to
+    ## unused-variable warning.
+    "-Wno-unused-variable",
+    "-Werror=old-style-cast",
+]
+
 config_setting(
     name = "qnx",
     constraint_values = ["@platforms//os:qnx"],
@@ -47,7 +64,7 @@ cc_library(
     ],
     copts = select({
         ":windows": [],
-        "//conditions:default": ["-Werror=old-style-cast"],
+        "//conditions:default": COPTS,
     }),
     defines = [
         "BENCHMARK_STATIC_DEFINE",
diff --git a/CMakeLists.txt b/CMakeLists.txt
index d9bcc6a4..23b519c2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -190,6 +190,7 @@ else()
   add_cxx_compiler_flag(-Wshadow)
   add_cxx_compiler_flag(-Wfloat-equal)
   add_cxx_compiler_flag(-Wold-style-cast)
+  add_cxx_compiler_flag(-Wconversion)
   if(BENCHMARK_ENABLE_WERROR)
       add_cxx_compiler_flag(-Werror)
   endif()
diff --git a/src/benchmark.cc b/src/benchmark.cc
index 563c4438..1f2f6cc2 100644
--- a/src/benchmark.cc
+++ b/src/benchmark.cc
@@ -407,7 +407,8 @@ void RunBenchmarks(const std::vector<BenchmarkInstance>& benchmarks,
       benchmarks_with_threads += (benchmark.threads() > 1);
       runners.emplace_back(benchmark, &perfcounters, reports_for_family);
       int num_repeats_of_this_instance = runners.back().GetNumRepeats();
-      num_repetitions_total += num_repeats_of_this_instance;
+      num_repetitions_total +=
+          static_cast<size_t>(num_repeats_of_this_instance);
       if (reports_for_family)
         reports_for_family->num_runs_total += num_repeats_of_this_instance;
     }
diff --git a/src/benchmark_register.cc b/src/benchmark_register.cc
index e447c9a2..8ade0482 100644
--- a/src/benchmark_register.cc
+++ b/src/benchmark_register.cc
@@ -482,8 +482,9 @@ int Benchmark::ArgsCnt() const {
 
 const char* Benchmark::GetArgName(int arg) const {
   BM_CHECK_GE(arg, 0);
-  BM_CHECK_LT(arg, static_cast<int>(arg_names_.size()));
-  return arg_names_[arg].c_str();
+  size_t uarg = static_cast<size_t>(arg);
+  BM_CHECK_LT(uarg, arg_names_.size());
+  return arg_names_[uarg].c_str();
 }
 
 TimeUnit Benchmark::GetTimeUnit() const {
diff --git a/src/benchmark_register.h b/src/benchmark_register.h
index 53367c70..be50265f 100644
--- a/src/benchmark_register.h
+++ b/src/benchmark_register.h
@@ -24,7 +24,7 @@ typename std::vector<T>::iterator AddPowers(std::vector<T>* dst, T lo, T hi,
   static const T kmax = std::numeric_limits<T>::max();
 
   // Space out the values in multiples of "mult"
-  for (T i = static_cast<T>(1); i <= hi; i *= static_cast<T>(mult)) {
+  for (T i = static_cast<T>(1); i <= hi; i = static_cast<T>(i * mult)) {
     if (i >= lo) {
       dst->push_back(i);
     }
@@ -52,7 +52,7 @@ void AddNegatedPowers(std::vector<T>* dst, T lo, T hi, int mult) {
 
   const auto it = AddPowers(dst, hi_complement, lo_complement, mult);
 
-  std::for_each(it, dst->end(), [](T& t) { t *= -1; });
+  std::for_each(it, dst->end(), [](T& t) { t = static_cast<T>(t * -1); });
   std::reverse(it, dst->end());
 }
 
diff --git a/src/benchmark_runner.cc b/src/benchmark_runner.cc
index dcddb437..a74bdadd 100644
--- a/src/benchmark_runner.cc
+++ b/src/benchmark_runner.cc
@@ -235,7 +235,7 @@ BenchmarkRunner::BenchmarkRunner(
       has_explicit_iteration_count(b.iterations() != 0 ||
                                    parsed_benchtime_flag.tag ==
                                        BenchTimeType::ITERS),
-      pool(b.threads() - 1),
+      pool(static_cast<size_t>(b.threads() - 1)),
       iters(has_explicit_iteration_count
                 ? ComputeIters(b_, parsed_benchtime_flag)
                 : 1),
diff --git a/src/cycleclock.h b/src/cycleclock.h
index eff563e7..91abcf9d 100644
--- a/src/cycleclock.h
+++ b/src/cycleclock.h
@@ -70,7 +70,7 @@ inline BENCHMARK_ALWAYS_INLINE int64_t Now() {
   // frequency scaling).  Also note that when the Mac sleeps, this
   // counter pauses; it does not continue counting, nor does it
   // reset to zero.
-  return mach_absolute_time();
+  return static_cast<int64_t>(mach_absolute_time());
 #elif defined(BENCHMARK_OS_EMSCRIPTEN)
   // this goes above x86-specific code because old versions of Emscripten
   // define __x86_64__, although they have nothing to do with it.
@@ -82,7 +82,7 @@ inline BENCHMARK_ALWAYS_INLINE int64_t Now() {
 #elif defined(__x86_64__) || defined(__amd64__)
   uint64_t low, high;
   __asm__ volatile("rdtsc" : "=a"(low), "=d"(high));
-  return (high << 32) | low;
+  return static_cast<int64_t>((high << 32) | low);
 #elif defined(__powerpc__) || defined(__ppc__)
   // This returns a time-base, which is not always precisely a cycle-count.
 #if defined(__powerpc64__) || defined(__ppc64__)
diff --git a/src/statistics.cc b/src/statistics.cc
index 261dcb29..16b60261 100644
--- a/src/statistics.cc
+++ b/src/statistics.cc
@@ -97,7 +97,7 @@ std::vector<BenchmarkReporter::Run> ComputeStats(
   auto error_count = std::count_if(reports.begin(), reports.end(),
                                    [](Run const& run) { return run.skipped; });
 
-  if (reports.size() - error_count < 2) {
+  if (reports.size() - static_cast<size_t>(error_count) < 2) {
     // We don't report aggregated data if there was a single run.
     return results;
   }
@@ -179,7 +179,7 @@ std::vector<BenchmarkReporter::Run> ComputeStats(
     // Similarly, if there are N repetitions with 1 iterations each,
     // an aggregate will be computed over N measurements, not 1.
     // Thus it is best to simply use the count of separate reports.
-    data.iterations = reports.size();
+    data.iterations = static_cast<IterationCount>(reports.size());
 
     data.real_accumulated_time = Stat.compute_(real_accumulated_time_stat);
     data.cpu_accumulated_time = Stat.compute_(cpu_accumulated_time_stat);
diff --git a/src/string_util.cc b/src/string_util.cc
index c69e40a8..9ba63a70 100644
--- a/src/string_util.cc
+++ b/src/string_util.cc
@@ -56,7 +56,7 @@ void ToExponentAndMantissa(double val, int precision, double one_k,
       scaled /= one_k;
       if (scaled <= big_threshold) {
         mantissa_stream << scaled;
-        *exponent = i + 1;
+        *exponent = static_cast<int64_t>(i + 1);
         *mantissa = mantissa_stream.str();
         return;
       }
diff --git a/src/sysinfo.cc b/src/sysinfo.cc
index daeb98b0..57a23e7b 100644
--- a/src/sysinfo.cc
+++ b/src/sysinfo.cc
@@ -350,7 +350,7 @@ std::vector<CPUInfo::CacheInfo> GetCacheSizesWindows() {
     CPUInfo::CacheInfo C;
     C.num_sharing = static_cast<int>(b.count());
     C.level = cache.Level;
-    C.size = cache.Size;
+    C.size = static_cast<int>(cache.Size);
     C.type = "Unknown";
     switch (cache.Type) {
       case CacheUnified:
@@ -485,9 +485,8 @@ int GetNumCPUsImpl() {
   // positives.
   std::memset(&sysinfo, 0, sizeof(SYSTEM_INFO));
   GetSystemInfo(&sysinfo);
-  return sysinfo.dwNumberOfProcessors;  // number of logical
-                                        // processors in the current
-                                        // group
+  // number of logical processors in the current group
+  return static_cast<int>(sysinfo.dwNumberOfProcessors);
 #elif defined(BENCHMARK_OS_SOLARIS)
   // Returns -1 in case of a failure.
   long num_cpu = sysconf(_SC_NPROCESSORS_ONLN);
@@ -837,7 +836,7 @@ std::vector<double> GetLoadAvg() {
     !(defined(__ANDROID__) && __ANDROID_API__ < 29)
   static constexpr int kMaxSamples = 3;
   std::vector<double> res(kMaxSamples, 0.0);
-  const int nelem = getloadavg(res.data(), kMaxSamples);
+  const size_t nelem = static_cast<size_t>(getloadavg(res.data(), kMaxSamples));
   if (nelem < 1) {
     res.clear();
   } else {
diff --git a/src/timers.cc b/src/timers.cc
index 667e7b2e..d0821f31 100644
--- a/src/timers.cc
+++ b/src/timers.cc
@@ -245,9 +245,9 @@ std::string LocalDateTimeString() {
       tz_offset_sign = '-';
     }
 
-    tz_len =
+    tz_len = static_cast<size_t>(
         ::snprintf(tz_offset, sizeof(tz_offset), "%c%02li:%02li",
-                   tz_offset_sign, offset_minutes / 100, offset_minutes % 100);
+                   tz_offset_sign, offset_minutes / 100, offset_minutes % 100));
     BM_CHECK(tz_len == kTzOffsetLen);
     ((void)tz_len);  // Prevent unused variable warning in optimized build.
   } else {
diff --git a/test/BUILD b/test/BUILD
index e43b8023..b245fa76 100644
--- a/test/BUILD
+++ b/test/BUILD
@@ -21,6 +21,7 @@ TEST_COPTS = [
     ## assert() are used a lot in tests upstream, which may be optimised out leading to
     ## unused-variable warning.
     "-Wno-unused-variable",
+    "-Werror=old-style-cast",
 ]
 
 # Some of the issues with DoNotOptimize only occur when optimization is enabled
diff --git a/test/benchmark_gtest.cc b/test/benchmark_gtest.cc
index 2c9e555d..0aa2552c 100644
--- a/test/benchmark_gtest.cc
+++ b/test/benchmark_gtest.cc
@@ -38,7 +38,7 @@ TEST(AddRangeTest, Advanced64) {
 
 TEST(AddRangeTest, FullRange8) {
   std::vector<int8_t> dst;
-  AddRange(&dst, int8_t{1}, std::numeric_limits<int8_t>::max(), int8_t{8});
+  AddRange(&dst, int8_t{1}, std::numeric_limits<int8_t>::max(), 8);
   EXPECT_THAT(
       dst, testing::ElementsAre(int8_t{1}, int8_t{8}, int8_t{64}, int8_t{127}));
 }
diff --git a/test/complexity_test.cc b/test/complexity_test.cc
index 0c159cd2..0729d15a 100644
--- a/test/complexity_test.cc
+++ b/test/complexity_test.cc
@@ -71,11 +71,11 @@ void BM_Complexity_O1(benchmark::State &state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
     benchmark::DoNotOptimize(state.iterations());
-    double tmp = state.iterations();
+    double tmp = static_cast<double>(state.iterations());
     benchmark::DoNotOptimize(tmp);
     for (benchmark::IterationCount i = 0; i < state.iterations(); ++i) {
       benchmark::DoNotOptimize(state.iterations());
-      tmp *= state.iterations();
+      tmp *= static_cast<double>(state.iterations());
       benchmark::DoNotOptimize(tmp);
     }
 
@@ -120,16 +120,16 @@ void BM_Complexity_O_N(benchmark::State &state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
     benchmark::DoNotOptimize(state.iterations());
-    double tmp = state.iterations();
+    double tmp = static_cast<double>(state.iterations());
     benchmark::DoNotOptimize(tmp);
     for (benchmark::IterationCount i = 0; i < state.iterations(); ++i) {
       benchmark::DoNotOptimize(state.iterations());
-      tmp *= state.iterations();
+      tmp *= static_cast<double>(state.iterations());
       benchmark::DoNotOptimize(tmp);
     }
 
     // 1ns per iteration per entry
-    state.SetIterationTime(state.range(0) * 42 * 1e-9);
+    state.SetIterationTime(static_cast<double>(state.range(0)) * 42 * 1e-9);
   }
   state.SetComplexityN(state.range(0));
 }
@@ -178,16 +178,16 @@ static void BM_Complexity_O_N_log_N(benchmark::State &state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
     benchmark::DoNotOptimize(state.iterations());
-    double tmp = state.iterations();
+    double tmp = static_cast<double>(state.iterations());
     benchmark::DoNotOptimize(tmp);
     for (benchmark::IterationCount i = 0; i < state.iterations(); ++i) {
       benchmark::DoNotOptimize(state.iterations());
-      tmp *= state.iterations();
+      tmp *= static_cast<double>(state.iterations());
       benchmark::DoNotOptimize(tmp);
     }
 
-    state.SetIterationTime(state.range(0) * kLog2E * std::log(state.range(0)) *
-                           42 * 1e-9);
+    state.SetIterationTime(static_cast<double>(state.range(0)) * kLog2E *
+                           std::log(state.range(0)) * 42 * 1e-9);
   }
   state.SetComplexityN(state.range(0));
 }
@@ -238,15 +238,15 @@ void BM_ComplexityCaptureArgs(benchmark::State &state, int n) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
     benchmark::DoNotOptimize(state.iterations());
-    double tmp = state.iterations();
+    double tmp = static_cast<double>(state.iterations());
     benchmark::DoNotOptimize(tmp);
     for (benchmark::IterationCount i = 0; i < state.iterations(); ++i) {
       benchmark::DoNotOptimize(state.iterations());
-      tmp *= state.iterations();
+      tmp *= static_cast<double>(state.iterations());
       benchmark::DoNotOptimize(tmp);
     }
 
-    state.SetIterationTime(state.range(0) * 42 * 1e-9);
+    state.SetIterationTime(static_cast<double>(state.range(0)) * 42 * 1e-9);
   }
   state.SetComplexityN(n);
 }