diff --git a/libc/tinymath/fsum.c b/libc/tinymath/fsum.c
index cdfdd228530..817c45b1e0e 100644
--- a/libc/tinymath/fsum.c
+++ b/libc/tinymath/fsum.c
@@ -24,8 +24,14 @@
  */
 double fsum(const double *p, size_t n) {
   size_t i;
-  double s;
-  if (n > 8) return fsum(p, n / 2) + fsum(p + n / 2, n - n / 2);
-  for (s = i = 0; i < n; ++i) s += p[i];
-  return s;
+  double err, sum, t, y;
+
+  sum = err = 0;
+  for (i = 0; i < n; ++i) {
+    y = p[i] - err;
+    t = sum + y;
+    err = (t - sum) - y;
+    sum = t;
+  }
+  return sum;
 }
diff --git a/libc/tinymath/fsumf.c b/libc/tinymath/fsumf.c
index c0c42ba7eb8..18359ffda51 100644
--- a/libc/tinymath/fsumf.c
+++ b/libc/tinymath/fsumf.c
@@ -23,9 +23,15 @@
  * Adds floats in array.
  */
 float fsumf(const float *p, size_t n) {
-  float s;
   size_t i;
-  if (n > 8) return fsumf(p, n / 2) + fsumf(p + n / 2, n - n / 2);
-  for (s = i = 0; i < n; ++i) s += p[i];
-  return s;
+  double err, sum, t, y;
+
+  sum = err = 0;
+  for (i = 0; i < n; ++i) {
+    y = p[i] - err;
+    t = sum + y;
+    err = (t - sum) - y;
+    sum = t;
+  }
+  return sum;
 }
diff --git a/test/libc/tinymath/fsum_test.c b/test/libc/tinymath/fsum_test.c
index e4772fc709c..65f1d1930c3 100644
--- a/test/libc/tinymath/fsum_test.c
+++ b/test/libc/tinymath/fsum_test.c
@@ -31,21 +31,21 @@ double D[N];
 void SetUp(void) {
   int i;
   for (i = 0; i < N / 2; ++i) {
-    D[i * 2 + 0] = 1000000000.1;
+    D[i * 2 + 0] = 10000000000.1;
     D[i * 2 + 1] = 1.1;
   }
   for (i = 0; i < N / 2; ++i) {
-    F[i * 2 + 0] = 1000.1;
+    F[i * 2 + 0] = 10000.1;
     F[i * 2 + 1] = 1.1;
   }
 }
 
 TEST(fsum, test) {
-  EXPECT_STREQ("500000000.6", _gc(xasprintf("%.15g", fsum(D, N) / N)));
+  EXPECT_STREQ("5000000000.6", _gc(xasprintf("%.16g", fsum(D, N) / N)));
 }
 
 TEST(fsumf, test) {
-  EXPECT_STREQ("500.6", _gc(xasprintf("%.7g", fsumf(F, N) / N)));
+  EXPECT_STREQ("5000.5996", _gc(xasprintf("%.8g", fsumf(F, N) / N)));
 }
 
 BENCH(fsum, bench) {