diff --git a/include/ftl/cast.h b/include/ftl/cast.h
new file mode 100644
index 0000000..ff1b58a
--- /dev/null
+++ b/include/ftl/cast.h
@@ -0,0 +1,84 @@
+/*
+ * Copyright 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <limits>
+#include <type_traits>
+
+#include <ftl/details/cast.h>
+
+namespace android::ftl {
+
+enum class CastSafety { kSafe, kUnderflow, kOverflow };
+
+// Returns whether static_cast<R>(v) is safe, or would result in underflow or overflow.
+//
+//   static_assert(ftl::cast_safety<uint8_t>(-1) == ftl::CastSafety::kUnderflow);
+//   static_assert(ftl::cast_safety<int8_t>(128u) == ftl::CastSafety::kOverflow);
+//
+//   static_assert(ftl::cast_safety<uint32_t>(-.1f) == ftl::CastSafety::kUnderflow);
+//   static_assert(ftl::cast_safety<int32_t>(static_cast<float>(INT32_MAX)) ==
+//                                           ftl::CastSafety::kOverflow);
+//
+//   static_assert(ftl::cast_safety<float>(-DBL_MAX) == ftl::CastSafety::kUnderflow);
+//
+template <typename R, typename T>
+constexpr CastSafety cast_safety(T v) {
+  static_assert(std::is_arithmetic_v<T>);
+  static_assert(std::is_arithmetic_v<R>);
+
+  constexpr bool kFromSigned = std::is_signed_v<T>;
+  constexpr bool kToSigned = std::is_signed_v<R>;
+
+  using details::max_exponent;
+
+  // If the R range contains the T range, then casting is always safe.
+  if constexpr ((kFromSigned == kToSigned && max_exponent<R> >= max_exponent<T>) ||
+                (!kFromSigned && kToSigned && max_exponent<R> > max_exponent<T>)) {
+    return CastSafety::kSafe;
+  }
+
+  using C = std::common_type_t<R, T>;
+
+  if constexpr (kFromSigned) {
+    using L = details::safe_limits<R, T>;
+
+    if constexpr (kToSigned) {
+      // Signed to signed.
+      if (v < L::lowest()) return CastSafety::kUnderflow;
+      return v <= L::max() ? CastSafety::kSafe : CastSafety::kOverflow;
+    } else {
+      // Signed to unsigned.
+      if (v < 0) return CastSafety::kUnderflow;
+      return static_cast<C>(v) <= static_cast<C>(L::max()) ? CastSafety::kSafe
+                                                           : CastSafety::kOverflow;
+    }
+  } else {
+    using L = std::numeric_limits<R>;
+
+    if constexpr (kToSigned) {
+      // Unsigned to signed.
+      return static_cast<C>(v) <= static_cast<C>(L::max()) ? CastSafety::kSafe
+                                                           : CastSafety::kOverflow;
+    } else {
+      // Unsigned to unsigned.
+      return v <= L::max() ? CastSafety::kSafe : CastSafety::kOverflow;
+    }
+  }
+}
+
+}  // namespace android::ftl