diff --git a/libc/src/__support/big_int.h b/libc/src/__support/big_int.h
index 681782d57319e5e..246b89f08f2ff95 100644
--- a/libc/src/__support/big_int.h
+++ b/libc/src/__support/big_int.h
@@ -14,7 +14,7 @@
 #include "src/__support/CPP/limits.h"
 #include "src/__support/CPP/optional.h"
 #include "src/__support/CPP/type_traits.h"
-#include "src/__support/macros/attributes.h"          // LIBC_INLINE
+#include "src/__support/macros/attributes.h" // LIBC_INLINE
 #include "src/__support/macros/config.h"
 #include "src/__support/macros/optimization.h"        // LIBC_UNLIKELY
 #include "src/__support/macros/properties/compiler.h" // LIBC_COMPILER_IS_CLANG
@@ -361,17 +361,94 @@ struct BigInt {
 
   LIBC_INLINE constexpr BigInt(const BigInt &other) = default;
 
-  template <size_t OtherBits, bool OtherSigned>
+  template <size_t OtherBits, bool OtherSigned, typename OtherWordType>
   LIBC_INLINE constexpr BigInt(
-      const BigInt<OtherBits, OtherSigned, WordType> &other) {
-    if (OtherBits >= Bits) { // truncate
-      for (size_t i = 0; i < WORD_COUNT; ++i)
-        val[i] = other[i];
-    } else { // zero or sign extend
-      size_t i = 0;
-      for (; i < OtherBits / WORD_SIZE; ++i)
-        val[i] = other[i];
-      extend(i, Signed && other.is_neg());
+      const BigInt<OtherBits, OtherSigned, OtherWordType> &other) {
+    using BigIntOther = BigInt<OtherBits, OtherSigned, OtherWordType>;
+    const bool should_sign_extend = Signed && other.is_neg();
+
+    static_assert(!(Bits == OtherBits && WORD_SIZE != BigIntOther::WORD_SIZE) &&
+                  "This is currently untested for casting between bigints with "
+                  "the same bit width but different word sizes.");
+
+    if constexpr (BigIntOther::WORD_SIZE < WORD_SIZE) {
+      // OtherWordType is smaller
+      constexpr size_t WORD_SIZE_RATIO = WORD_SIZE / BigIntOther::WORD_SIZE;
+      static_assert(
+          (WORD_SIZE % BigIntOther::WORD_SIZE) == 0 &&
+          "Word types must be multiples of each other for correct conversion.");
+      if constexpr (OtherBits >= Bits) { // truncate
+        // for each big word
+        for (size_t i = 0; i < WORD_COUNT; ++i) {
+          WordType cur_word = 0;
+          // combine WORD_SIZE_RATIO small words into a big word
+          for (size_t j = 0; j < WORD_SIZE_RATIO; ++j)
+            cur_word |= static_cast<WordType>(other[(i * WORD_SIZE_RATIO) + j])
+                        << (BigIntOther::WORD_SIZE * j);
+
+          val[i] = cur_word;
+        }
+      } else { // zero or sign extend
+        size_t i = 0;
+        WordType cur_word = 0;
+        // for each small word
+        for (; i < BigIntOther::WORD_COUNT; ++i) {
+          // combine WORD_SIZE_RATIO small words into a big word
+          cur_word |= static_cast<WordType>(other[i])
+                      << (BigIntOther::WORD_SIZE * (i % WORD_SIZE_RATIO));
+          // if we've completed a big word, copy it into place and reset
+          if ((i % WORD_SIZE_RATIO) == WORD_SIZE_RATIO - 1) {
+            val[i / WORD_SIZE_RATIO] = cur_word;
+            cur_word = 0;
+          }
+        }
+        // Pretend there are extra words of the correct sign extension as needed
+
+        const WordType extension_bits =
+            should_sign_extend ? cpp::numeric_limits<WordType>::max()
+                               : cpp::numeric_limits<WordType>::min();
+        if ((i % WORD_SIZE_RATIO) != 0) {
+          cur_word |= static_cast<WordType>(extension_bits)
+                      << (BigIntOther::WORD_SIZE * (i % WORD_SIZE_RATIO));
+        }
+        // Copy the last word into place.
+        val[(i / WORD_SIZE_RATIO)] = cur_word;
+        extend((i / WORD_SIZE_RATIO) + 1, should_sign_extend);
+      }
+    } else if constexpr (BigIntOther::WORD_SIZE == WORD_SIZE) {
+      if constexpr (OtherBits >= Bits) { // truncate
+        for (size_t i = 0; i < WORD_COUNT; ++i)
+          val[i] = other[i];
+      } else { // zero or sign extend
+        size_t i = 0;
+        for (; i < BigIntOther::WORD_COUNT; ++i)
+          val[i] = other[i];
+        extend(i, should_sign_extend);
+      }
+    } else {
+      // OtherWordType is bigger.
+      constexpr size_t WORD_SIZE_RATIO = BigIntOther::WORD_SIZE / WORD_SIZE;
+      static_assert(
+          (BigIntOther::WORD_SIZE % WORD_SIZE) == 0 &&
+          "Word types must be multiples of each other for correct conversion.");
+      if constexpr (OtherBits >= Bits) { // truncate
+        // for each small word
+        for (size_t i = 0; i < WORD_COUNT; ++i) {
+          // split each big word into WORD_SIZE_RATIO small words
+          val[i] = static_cast<WordType>(other[i / WORD_SIZE_RATIO] >>
+                                         ((i % WORD_SIZE_RATIO) * WORD_SIZE));
+        }
+      } else { // zero or sign extend
+        size_t i = 0;
+        // for each big word
+        for (; i < BigIntOther::WORD_COUNT; ++i) {
+          // split each big word into WORD_SIZE_RATIO small words
+          for (size_t j = 0; j < WORD_SIZE_RATIO; ++j)
+            val[(i * WORD_SIZE_RATIO) + j] =
+                static_cast<WordType>(other[i] >> (j * WORD_SIZE));
+        }
+        extend(i * WORD_SIZE_RATIO, should_sign_extend);
+      }
     }
   }
 
diff --git a/libc/test/src/__support/big_int_test.cpp b/libc/test/src/__support/big_int_test.cpp
index a1ce69baaae2906..471ca72a8f6e0c8 100644
--- a/libc/test/src/__support/big_int_test.cpp
+++ b/libc/test/src/__support/big_int_test.cpp
@@ -8,7 +8,7 @@
 
 #include "src/__support/CPP/optional.h"
 #include "src/__support/big_int.h"
-#include "src/__support/integer_literals.h"        // parse_unsigned_bigint
+#include "src/__support/integer_literals.h" // parse_unsigned_bigint
 #include "src/__support/macros/config.h"
 #include "src/__support/macros/properties/types.h" // LIBC_TYPES_HAS_INT128
 
@@ -208,6 +208,7 @@ TYPED_TEST(LlvmLibcUIntClassTest, CountBits, Types) {
 }
 
 using LL_UInt16 = UInt<16>;
+using LL_UInt32 = UInt<32>;
 using LL_UInt64 = UInt<64>;
 // We want to test UInt<128> explicitly. So, for
 // convenience, we use a sugar which does not conflict with the UInt128 type
@@ -927,4 +928,143 @@ TEST(LlvmLibcUIntClassTest, OtherWordTypeTests) {
   ASSERT_EQ(static_cast<int>(a >> 64), 1);
 }
 
+TEST(LlvmLibcUIntClassTest, OtherWordTypeCastTests) {
+  using LL_UInt96 = BigInt<96, false, uint32_t>;
+
+  LL_UInt96 a({123, 456, 789});
+
+  ASSERT_EQ(static_cast<int>(a), 123);
+  ASSERT_EQ(static_cast<int>(a >> 32), 456);
+  ASSERT_EQ(static_cast<int>(a >> 64), 789);
+
+  // Bigger word with more bits to smaller word with less bits.
+  LL_UInt128 b(a);
+
+  ASSERT_EQ(static_cast<int>(b), 123);
+  ASSERT_EQ(static_cast<int>(b >> 32), 456);
+  ASSERT_EQ(static_cast<int>(b >> 64), 789);
+  ASSERT_EQ(static_cast<int>(b >> 96), 0);
+
+  b = (b << 32) + 987;
+
+  ASSERT_EQ(static_cast<int>(b), 987);
+  ASSERT_EQ(static_cast<int>(b >> 32), 123);
+  ASSERT_EQ(static_cast<int>(b >> 64), 456);
+  ASSERT_EQ(static_cast<int>(b >> 96), 789);
+
+  // Smaller word with less bits to bigger word with more bits.
+  LL_UInt96 c(b);
+
+  ASSERT_EQ(static_cast<int>(c), 987);
+  ASSERT_EQ(static_cast<int>(c >> 32), 123);
+  ASSERT_EQ(static_cast<int>(c >> 64), 456);
+
+  // Smaller word with more bits to bigger word with less bits
+  LL_UInt64 d(c);
+
+  ASSERT_EQ(static_cast<int>(d), 987);
+  ASSERT_EQ(static_cast<int>(d >> 32), 123);
+
+  // Bigger word with less bits to smaller word with more bits
+
+  LL_UInt96 e(d);
+
+  ASSERT_EQ(static_cast<int>(e), 987);
+  ASSERT_EQ(static_cast<int>(e >> 32), 123);
+
+  e = (e << 32) + 654;
+
+  ASSERT_EQ(static_cast<int>(e), 654);
+  ASSERT_EQ(static_cast<int>(e >> 32), 987);
+  ASSERT_EQ(static_cast<int>(e >> 64), 123);
+}
+
+TEST(LlvmLibcUIntClassTest, SignedOtherWordTypeCastTests) {
+  using LL_Int64 = BigInt<64, true, uint64_t>;
+  using LL_Int96 = BigInt<96, true, uint32_t>;
+
+  LL_Int64 zero_64(0);
+  LL_Int96 zero_96(0);
+  LL_Int192 zero_192(0);
+
+  LL_Int96 plus_a({0x1234, 0x5678, 0x9ABC});
+
+  ASSERT_EQ(static_cast<int>(plus_a), 0x1234);
+  ASSERT_EQ(static_cast<int>(plus_a >> 32), 0x5678);
+  ASSERT_EQ(static_cast<int>(plus_a >> 64), 0x9ABC);
+
+  LL_Int96 minus_a(-plus_a);
+
+  // The reason that the numbers are inverted and not negated is that we're
+  // using two's complement. To negate a two's complement number you flip the
+  // bits and add 1, so minus_a is {~0x1234, ~0x5678, ~0x9ABC} + {1,0,0}.
+  ASSERT_EQ(static_cast<int>(minus_a), (~0x1234) + 1);
+  ASSERT_EQ(static_cast<int>(minus_a >> 32), ~0x5678);
+  ASSERT_EQ(static_cast<int>(minus_a >> 64), ~0x9ABC);
+
+  ASSERT_TRUE(plus_a + minus_a == zero_96);
+
+  // 192 so there's an extra block to get sign extended to
+  LL_Int192 bigger_plus_a(plus_a);
+
+  ASSERT_EQ(static_cast<int>(bigger_plus_a), 0x1234);
+  ASSERT_EQ(static_cast<int>(bigger_plus_a >> 32), 0x5678);
+  ASSERT_EQ(static_cast<int>(bigger_plus_a >> 64), 0x9ABC);
+  ASSERT_EQ(static_cast<int>(bigger_plus_a >> 96), 0);
+  ASSERT_EQ(static_cast<int>(bigger_plus_a >> 128), 0);
+  ASSERT_EQ(static_cast<int>(bigger_plus_a >> 160), 0);
+
+  LL_Int192 bigger_minus_a(minus_a);
+
+  ASSERT_EQ(static_cast<int>(bigger_minus_a), (~0x1234) + 1);
+  ASSERT_EQ(static_cast<int>(bigger_minus_a >> 32), ~0x5678);
+  ASSERT_EQ(static_cast<int>(bigger_minus_a >> 64), ~0x9ABC);
+  ASSERT_EQ(static_cast<int>(bigger_minus_a >> 96), ~0);
+  ASSERT_EQ(static_cast<int>(bigger_minus_a >> 128), ~0);
+  ASSERT_EQ(static_cast<int>(bigger_minus_a >> 160), ~0);
+
+  ASSERT_TRUE(bigger_plus_a + bigger_minus_a == zero_192);
+
+  LL_Int64 smaller_plus_a(plus_a);
+
+  ASSERT_EQ(static_cast<int>(smaller_plus_a), 0x1234);
+  ASSERT_EQ(static_cast<int>(smaller_plus_a >> 32), 0x5678);
+
+  LL_Int64 smaller_minus_a(minus_a);
+
+  ASSERT_EQ(static_cast<int>(smaller_minus_a), (~0x1234) + 1);
+  ASSERT_EQ(static_cast<int>(smaller_minus_a >> 32), ~0x5678);
+
+  ASSERT_TRUE(smaller_plus_a + smaller_minus_a == zero_64);
+
+  // Also try going from bigger word size to smaller word size
+  LL_Int96 smaller_back_plus_a(smaller_plus_a);
+
+  ASSERT_EQ(static_cast<int>(smaller_back_plus_a), 0x1234);
+  ASSERT_EQ(static_cast<int>(smaller_back_plus_a >> 32), 0x5678);
+  ASSERT_EQ(static_cast<int>(smaller_back_plus_a >> 64), 0);
+
+  LL_Int96 smaller_back_minus_a(smaller_minus_a);
+
+  ASSERT_EQ(static_cast<int>(smaller_back_minus_a), (~0x1234) + 1);
+  ASSERT_EQ(static_cast<int>(smaller_back_minus_a >> 32), ~0x5678);
+  ASSERT_EQ(static_cast<int>(smaller_back_minus_a >> 64), ~0);
+
+  ASSERT_TRUE(smaller_back_plus_a + smaller_back_minus_a == zero_96);
+
+  LL_Int96 bigger_back_plus_a(bigger_plus_a);
+
+  ASSERT_EQ(static_cast<int>(bigger_back_plus_a), 0x1234);
+  ASSERT_EQ(static_cast<int>(bigger_back_plus_a >> 32), 0x5678);
+  ASSERT_EQ(static_cast<int>(bigger_back_plus_a >> 64), 0x9ABC);
+
+  LL_Int96 bigger_back_minus_a(bigger_minus_a);
+
+  ASSERT_EQ(static_cast<int>(bigger_back_minus_a), (~0x1234) + 1);
+  ASSERT_EQ(static_cast<int>(bigger_back_minus_a >> 32), ~0x5678);
+  ASSERT_EQ(static_cast<int>(bigger_back_minus_a >> 64), ~0x9ABC);
+
+  ASSERT_TRUE(bigger_back_plus_a + bigger_back_minus_a == zero_96);
+}
+
 } // namespace LIBC_NAMESPACE_DECL