diff --git a/lib/libc/gdtoa/_hdtoa.c b/lib/libc/gdtoa/_hdtoa.c
index 8ae739acf0d..9c42630cd91 100644
--- a/lib/libc/gdtoa/_hdtoa.c
+++ b/lib/libc/gdtoa/_hdtoa.c
@@ -40,6 +40,7 @@
 
 #define	DBL_ADJ	(DBL_MAX_EXP - 2)
 #define	SIGFIGS	((DBL_MANT_DIG + 3) / 4 + 1)
+#define	MAX_HEX_DIGITS	((DBL_MANT_DIG + 3 - 1) / 4 + 1)
 
 static const float one[] = { 1.0f, -1.0f };
 
@@ -111,7 +112,7 @@ __hdtoa(double d, const char *xdigs, int ndigits, int *decpt, int *sign,
 	s0 = rv_alloc(bufsize);
 
 	/* Round to the desired number of digits. */
-	if (SIGFIGS > ndigits && ndigits > 0) {
+	if (MAX_HEX_DIGITS > ndigits && ndigits > 0) {
 		float redux = one[u.bits.sign];
 		int offset = 4 * ndigits + DBL_MAX_EXP - 4 - DBL_MANT_DIG;
 		u.bits.exp = offset;
diff --git a/lib/libc/gdtoa/_hldtoa.c b/lib/libc/gdtoa/_hldtoa.c
index 965d2349d10..5f10d12c5c0 100644
--- a/lib/libc/gdtoa/_hldtoa.c
+++ b/lib/libc/gdtoa/_hldtoa.c
@@ -65,6 +65,7 @@ typedef uint32_t manl_t;
 
 #define	LDBL_ADJ	(LDBL_MAX_EXP - 2)
 #define	SIGFIGS		((LDBL_MANT_DIG + 3) / 4 + 1)
+#define	MAX_HEX_DIGITS	((LDBL_MANT_DIG + 3 - 1) / 4 + 1)
 
 static const float one[] = { 1.0f, -1.0f };
 
@@ -125,7 +126,7 @@ __hldtoa(long double e, const char *xdigs, int ndigits, int *decpt, int *sign,
 	s0 = rv_alloc(bufsize);
 
 	/* Round to the desired number of digits. */
-	if (SIGFIGS > ndigits && ndigits > 0) {
+	if (MAX_HEX_DIGITS > ndigits && ndigits > 0) {
 		float redux = one[u.bits.sign];
 		int offset = 4 * ndigits + LDBL_MAX_EXP - 4 - LDBL_MANT_DIG;
 #ifdef __i386__
diff --git a/lib/libc/tests/stdio/printfloat_test.c b/lib/libc/tests/stdio/printfloat_test.c
index 03185912416..795c7797541 100644
--- a/lib/libc/tests/stdio/printfloat_test.c
+++ b/lib/libc/tests/stdio/printfloat_test.c
@@ -398,6 +398,18 @@ ATF_TC_BODY(subnormal_float, tc)
 	testfmt("-0X1P-149", "%A", negative);
 }
 
+ATF_TC_WITHOUT_HEAD(hexadecimal_rounding_fullprec);
+ATF_TC_BODY(hexadecimal_rounding_fullprec, tc)
+{
+	/* Double: %.13a with binary64 mantissa=53 */
+	testfmt("0x1.1234567890bbbp+0", "%.13a", 0x1.1234567890bbbp + 0);
+
+#if defined(__aarch64__)
+	/* On arm64, long double is IEEE binary128 (mantissa=113) */
+	testfmt("0x1.3c0ca428c59fbbbbbbbbbbbbbbbbp+0", "%.28La", 0x1.3c0ca428c59fbbbbbbbbbbbbbbbbp + 0L);
+#endif
+}
+
 ATF_TP_ADD_TCS(tp)
 {
 
@@ -414,6 +426,7 @@ ATF_TP_ADD_TCS(tp)
 	ATF_TP_ADD_TC(tp, hexadecimal_rounding);
 	ATF_TP_ADD_TC(tp, subnormal_double);
 	ATF_TP_ADD_TC(tp, subnormal_float);
+	ATF_TP_ADD_TC(tp, hexadecimal_rounding_fullprec);
 
 	return (atf_no_error());
 }