diff --git a/include/fmt/format.h b/include/fmt/format.h
index 9e6570d6..7c2a19b4 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -3918,12 +3918,14 @@ class format_int {
   mutable char buffer_[buffer_size];
   char* str_;
 
-  template <typename UInt> auto format_unsigned(UInt value) -> char* {
+  template <typename UInt>
+  FMT_CONSTEXPR20 auto format_unsigned(UInt value) -> char* {
     auto n = static_cast<detail::uint32_or_64_or_128_t<UInt>>(value);
     return detail::format_decimal(buffer_, n, buffer_size - 1).begin;
   }
 
-  template <typename Int> auto format_signed(Int value) -> char* {
+  template <typename Int>
+  FMT_CONSTEXPR20 auto format_signed(Int value) -> char* {
     auto abs_value = static_cast<detail::uint32_or_64_or_128_t<Int>>(value);
     bool negative = value < 0;
     if (negative) abs_value = 0 - abs_value;
@@ -3933,26 +3935,30 @@ class format_int {
   }
 
  public:
-  explicit format_int(int value) : str_(format_signed(value)) {}
-  explicit format_int(long value) : str_(format_signed(value)) {}
-  explicit format_int(long long value) : str_(format_signed(value)) {}
-  explicit format_int(unsigned value) : str_(format_unsigned(value)) {}
-  explicit format_int(unsigned long value) : str_(format_unsigned(value)) {}
-  explicit format_int(unsigned long long value)
+  explicit FMT_CONSTEXPR20 format_int(int value) : str_(format_signed(value)) {}
+  explicit FMT_CONSTEXPR20 format_int(long value)
+      : str_(format_signed(value)) {}
+  explicit FMT_CONSTEXPR20 format_int(long long value)
+      : str_(format_signed(value)) {}
+  explicit FMT_CONSTEXPR20 format_int(unsigned value)
+      : str_(format_unsigned(value)) {}
+  explicit FMT_CONSTEXPR20 format_int(unsigned long value)
+      : str_(format_unsigned(value)) {}
+  explicit FMT_CONSTEXPR20 format_int(unsigned long long value)
       : str_(format_unsigned(value)) {}
 
   /// Returns the number of characters written to the output buffer.
-  auto size() const -> size_t {
+  FMT_CONSTEXPR20 auto size() const -> size_t {
     return detail::to_unsigned(buffer_ - str_ + buffer_size - 1);
   }
 
   /// Returns a pointer to the output buffer content. No terminating null
   /// character is appended.
-  auto data() const -> const char* { return str_; }
+  FMT_CONSTEXPR20 auto data() const -> const char* { return str_; }
 
   /// Returns a pointer to the output buffer content with terminating null
   /// character appended.
-  auto c_str() const -> const char* {
+  FMT_CONSTEXPR20 auto c_str() const -> const char* {
     buffer_[buffer_size - 1] = '\0';
     return str_;
   }