diff --git a/tests/core/c/libc/test_core_libc_fastint_width.odin b/tests/core/c/libc/test_core_libc_fastint_width.odin
new file mode 100644
index 000000000..9d4146c63
--- /dev/null
+++ b/tests/core/c/libc/test_core_libc_fastint_width.odin
@@ -0,0 +1,53 @@
+package test_core_libc
+
+import "core:c"
+import "core:testing"
+
+@test
+test_libc_fastint_width :: proc(t: ^testing.T) {
+	when ODIN_OS == .FreeBSD || ODIN_OS == .OpenBSD || ODIN_OS == .NetBSD {
+		testing.expect_value(t, size_of(c.int_fast8_t), 4)
+		testing.expect_value(t, size_of(c.uint_fast8_t), 4)
+		testing.expect_value(t, size_of(c.int_fast16_t), 4)
+		testing.expect_value(t, size_of(c.uint_fast16_t), 4)
+		testing.expect_value(t, size_of(c.int_fast32_t), 4)
+		testing.expect_value(t, size_of(c.uint_fast32_t), 4)
+	} else when ODIN_OS == .Darwin {
+		testing.expect_value(t, size_of(c.int_fast8_t), 1)
+		testing.expect_value(t, size_of(c.uint_fast8_t), 1)
+		testing.expect_value(t, size_of(c.int_fast16_t), 2)
+		testing.expect_value(t, size_of(c.uint_fast16_t), 2)
+		testing.expect_value(t, size_of(c.int_fast32_t), 4)
+		testing.expect_value(t, size_of(c.uint_fast32_t), 4)
+	} else when ODIN_OS == .Linux {
+		testing.expect_value(t, size_of(c.int_fast8_t), 1)
+		testing.expect_value(t, size_of(c.uint_fast8_t), 1)
+		when size_of(rawptr) == 8 { // glibc only - broken for musl
+			testing.expect_value(t, size_of(c.int_fast16_t), 8)
+			testing.expect_value(t, size_of(c.uint_fast16_t), 8)
+			testing.expect_value(t, size_of(c.int_fast32_t), 8)
+			testing.expect_value(t, size_of(c.uint_fast32_t), 8)
+		} else {
+			testing.expect_value(t, size_of(c.int_fast16_t), 4)
+			testing.expect_value(t, size_of(c.uint_fast16_t), 4)
+			testing.expect_value(t, size_of(c.int_fast32_t), 4)
+			testing.expect_value(t, size_of(c.uint_fast32_t), 4)
+		}
+	} else when ODIN_OS == .Windows {
+		testing.expect_value(t, size_of(c.int_fast8_t), 1)
+		testing.expect_value(t, size_of(c.uint_fast8_t), 1)
+		testing.expect_value(t, size_of(c.int_fast16_t), 4)
+		testing.expect_value(t, size_of(c.uint_fast16_t), 4)
+		testing.expect_value(t, size_of(c.int_fast32_t), 4)
+		testing.expect_value(t, size_of(c.uint_fast32_t), 4)
+	} else {
+		testing.expect_value(t, size_of(c.int_fast8_t), 1)
+		testing.expect_value(t, size_of(c.uint_fast8_t), 1)
+		testing.expect_value(t, size_of(c.int_fast16_t), 4)
+		testing.expect_value(t, size_of(c.uint_fast16_t), 4)
+		testing.expect_value(t, size_of(c.int_fast32_t), 4)
+		testing.expect_value(t, size_of(c.uint_fast32_t), 4)
+	}
+	testing.expect_value(t, size_of(c.int_fast64_t), 8)
+	testing.expect_value(t, size_of(c.uint_fast64_t), 8)
+}