diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index 524b229a..1257f2b0 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -667,14 +667,16 @@ FMT_FUNC void format_float(char *buffer, size_t &size, int exp,
 
 template <typename Double>
 FMT_FUNC typename std::enable_if<sizeof(Double) == sizeof(uint64_t), bool>::type
-    grisu2_format(Double value, char *buffer, size_t &size,
-                  grisu2_specs specs) {
+    grisu2_format(Double value, buffer &buf, grisu2_specs specs) {
   FMT_ASSERT(value >= 0, "value is negative");
+  char *buffer = buf.data();
   if (value == 0) {
     gen_digits_params params(specs, 1);
     *buffer = '0';
-    size = 1;
+    size_t size = 1;
     format_float(buffer, size, 0, params);
+    FMT_ASSERT(buf.capacity() >= size, "");
+    buf.resize(size);
     return true;
   }
 
@@ -705,12 +707,14 @@ FMT_FUNC typename std::enable_if<sizeof(Double) == sizeof(uint64_t), bool>::type
   // lo (p2 in Grisu) contains the least significants digits of scaled_upper.
   // lo = supper % one.
   uint64_t lo = upper.f & (one.f - 1);
-  size = 0;
+  size_t size = 0;
   if (!grisu2_gen_digits(buffer, size, hi, lo, exp, delta, one, diff,
                          params.max_digits)) {
     return false;
   }
   format_float(buffer, size, cached_exp + exp, params);
+  FMT_ASSERT(buf.capacity() >= size, "");
+  buf.resize(size);
   return true;
 }
 }  // namespace internal
diff --git a/include/fmt/format.h b/include/fmt/format.h
index a64afe73..ddd5a54c 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -299,10 +299,10 @@ struct grisu2_specs {
 // https://www.cs.tufts.edu/~nr/cs257/archive/florian-loitsch/printf.pdf
 template <typename Double>
 FMT_API typename std::enable_if<sizeof(Double) == sizeof(uint64_t), bool>::type
-  grisu2_format(Double value, char *buffer, size_t &size, grisu2_specs);
+  grisu2_format(Double value, buffer &buf, grisu2_specs);
 template <typename Double>
 inline typename std::enable_if<sizeof(Double) != sizeof(uint64_t), bool>::type
-  grisu2_format(Double, char *, size_t &, grisu2_specs) { return false; }
+  grisu2_format(Double, buffer &, grisu2_specs) { return false; }
 
 template <typename Allocator>
 typename Allocator::value_type *allocate(Allocator& alloc, std::size_t n) {
@@ -2862,22 +2862,16 @@ void basic_writer<Range>::write_double(T value, const format_specs &spec) {
   bool use_grisu = internal::use_grisu() && sizeof(T) <= sizeof(double) &&
       type != 'a' && type != 'A';
   if (use_grisu) {
-    char buf[100]; // TODO: correct buffer size
-    size_t size = 0;
     auto gs = internal::grisu2_specs();
     gs.type = type;
     gs.precision = spec.precision();
     gs.flags = spec.flags_;
-    use_grisu = internal::grisu2_format(
-          static_cast<double>(value), buf, size, gs);
-    if (use_grisu) {
-      FMT_ASSERT(size <= 100, "buffer overflow");
-      buffer.append(buf, buf + size); // TODO: avoid extra copy
-    }
+    use_grisu = internal::grisu2_format(static_cast<double>(value), buffer, gs);
   }
   if (!use_grisu) {
     format_specs normalized_spec(spec);
     normalized_spec.type_ = handler.type;
+    buffer.clear();
     write_double_sprintf(value, normalized_spec, buffer);
   }
   size_t n = buffer.size();
diff --git a/test/format-impl-test.cc b/test/format-impl-test.cc
index 49132f60..c66a587a 100644
--- a/test/format-impl-test.cc
+++ b/test/format-impl-test.cc
@@ -104,7 +104,8 @@ TEST(FPTest, GetCachedPower) {
 
 TEST(FPTest, Grisu2FormatCompilesWithNonIEEEDouble) {
   size_t size = 0;
-  grisu2_format(4.2f, FMT_NULL, size, fmt::internal::grisu2_specs());
+  fmt::memory_buffer buf;
+  grisu2_format(4.2f, buf, fmt::internal::grisu2_specs());
 }
 
 template <typename T>