diff --git a/compiler/builtins/bitcode/src/dec.zig b/compiler/builtins/bitcode/src/dec.zig
index ca24c12641..2523519bfa 100644
--- a/compiler/builtins/bitcode/src/dec.zig
+++ b/compiler/builtins/bitcode/src/dec.zig
@@ -26,21 +26,20 @@ pub const RocDec = extern struct {
         return .{ .num = num * one_point_zero_i128 };
     }
 
-    // TODO: There's got to be a better way to do this other than converting to Str
     pub fn fromF64(num: f64) ?RocDec {
-        var digit_bytes: [19]u8 = undefined; // 19 = max f64 digits + '.' + '-'
 
-        var fbs = std.io.fixedBufferStream(digit_bytes[0..]);
-        std.fmt.formatFloatDecimal(num, .{}, fbs.writer()) catch
-            return null;
+        var result: f64 = num * comptime @intToFloat(f64, one_point_zero_i128);
 
-        var dec = RocDec.fromStr(RocStr.init(&digit_bytes, fbs.pos));
-
-        if (dec) |d| {
-            return d;
-        } else {
+        if (result > comptime @intToFloat(f64, math.maxInt(i128))) {
             return null;
         }
+
+        if (result < comptime @intToFloat(f64, math.minInt(i128))) {
+            return null;
+        }
+
+        var ret: RocDec = .{ .num = @floatToInt(i128, result) };
+        return ret;
     }
 
     pub fn fromStr(roc_str: RocStr) ?RocDec {
@@ -729,6 +728,11 @@ test "fromF64" {
     try expectEqual(RocDec{ .num = 25500000000000000000 }, dec.?);
 }
 
+test "fromF64 overflow" {
+    var dec = RocDec.fromF64(1e308);
+    try expectEqual(dec, null);
+}
+
 test "fromStr: empty" {
     var roc_str = RocStr.init("", 0);
     var dec = RocDec.fromStr(roc_str);