diff --git a/src/audio/audio_source.cpp b/src/audio/audio_source.cpp
index b9262b45..44de1d1b 100644
--- a/src/audio/audio_source.cpp
+++ b/src/audio/audio_source.cpp
@@ -34,6 +34,10 @@ auto TaggedStream::CurrentPosition() -> int64_t {
   return wrapped_->CurrentPosition();
 }
 
+auto TaggedStream::Size() -> std::optional<int64_t> {
+  return wrapped_->Size();
+}
+
 auto TaggedStream::SetPreambleFinished() -> void {
   wrapped_->SetPreambleFinished();
 }
diff --git a/src/audio/fatfs_source.cpp b/src/audio/fatfs_source.cpp
index f682cd0d..4fc5adbb 100644
--- a/src/audio/fatfs_source.cpp
+++ b/src/audio/fatfs_source.cpp
@@ -5,6 +5,7 @@
  */
 
 #include "fatfs_source.hpp"
+#include <sys/_stdint.h>
 
 #include <cstddef>
 #include <cstdint>
@@ -66,4 +67,9 @@ auto FatfsSource::SeekTo(int64_t destination, SeekFrom from) -> void {
 auto FatfsSource::CurrentPosition() -> int64_t {
   return f_tell(file_.get());
 }
+
+auto FatfsSource::Size() -> std::optional<int64_t> {
+  return f_size(file_.get());
+}
+
 }  // namespace audio
diff --git a/src/audio/include/audio_source.hpp b/src/audio/include/audio_source.hpp
index a54cb260..68145f5b 100644
--- a/src/audio/include/audio_source.hpp
+++ b/src/audio/include/audio_source.hpp
@@ -28,6 +28,8 @@ class TaggedStream : public codecs::IStream {
 
   auto CurrentPosition() -> int64_t override;
 
+  auto Size() -> std::optional<int64_t> override;
+
   auto SetPreambleFinished() -> void override;
 
  private:
diff --git a/src/audio/include/fatfs_source.hpp b/src/audio/include/fatfs_source.hpp
index e4187d60..45ab34c6 100644
--- a/src/audio/include/fatfs_source.hpp
+++ b/src/audio/include/fatfs_source.hpp
@@ -34,6 +34,8 @@ class FatfsSource : public codecs::IStream {
 
   auto CurrentPosition() -> int64_t override;
 
+  auto Size() -> std::optional<int64_t> override;
+
   FatfsSource(const FatfsSource&) = delete;
   FatfsSource& operator=(const FatfsSource&) = delete;
 
@@ -41,4 +43,4 @@ class FatfsSource : public codecs::IStream {
   std::unique_ptr<FIL> file_;
 };
 
-}  // namespace audio
\ No newline at end of file
+}  // namespace audio
diff --git a/src/audio/include/readahead_source.hpp b/src/audio/include/readahead_source.hpp
index bbc0da12..58076350 100644
--- a/src/audio/include/readahead_source.hpp
+++ b/src/audio/include/readahead_source.hpp
@@ -38,6 +38,8 @@ class ReadaheadSource : public codecs::IStream {
 
   auto CurrentPosition() -> int64_t override;
 
+  auto Size() -> std::optional<int64_t> override;
+
   auto SetPreambleFinished() -> void override;
 
   ReadaheadSource(const ReadaheadSource&) = delete;
@@ -55,4 +57,4 @@ class ReadaheadSource : public codecs::IStream {
   int64_t tell_;
 };
 
-}  // namespace audio
\ No newline at end of file
+}  // namespace audio
diff --git a/src/audio/readahead_source.cpp b/src/audio/readahead_source.cpp
index 35c5314f..aae878f8 100644
--- a/src/audio/readahead_source.cpp
+++ b/src/audio/readahead_source.cpp
@@ -47,8 +47,8 @@ auto ReadaheadSource::Read(cpp::span<std::byte> dest) -> ssize_t {
   // Fill the destination from our buffer, until either the buffer is drained
   // or the destination is full.
   while (!dest.empty() && (is_refilling_ || !xStreamBufferIsEmpty(buffer_))) {
-    size_t bytes_read = xStreamBufferReceive(buffer_, dest.data(),
-                                             dest.size_bytes(), 1);
+    size_t bytes_read =
+        xStreamBufferReceive(buffer_, dest.data(), dest.size_bytes(), 1);
     tell_ += bytes_read;
     bytes_written += bytes_read;
     dest = dest.subspan(bytes_read);
@@ -102,6 +102,10 @@ auto ReadaheadSource::CurrentPosition() -> int64_t {
   return tell_;
 }
 
+auto ReadaheadSource::Size() -> std::optional<int64_t> {
+  return wrapped_->Size();
+}
+
 auto ReadaheadSource::SetPreambleFinished() -> void {
   readahead_enabled_ = true;
   BeginReadahead();
diff --git a/src/codecs/include/codec.hpp b/src/codecs/include/codec.hpp
index 36dda8ff..8aa391b6 100644
--- a/src/codecs/include/codec.hpp
+++ b/src/codecs/include/codec.hpp
@@ -49,6 +49,8 @@ class IStream {
 
   virtual auto CurrentPosition() -> int64_t = 0;
 
+  virtual auto Size() -> std::optional<int64_t> = 0;
+
   /*
    * Called by codecs to indicate that they've finished parsing any header data
    * within this stream, and are about to begin decoding.
diff --git a/src/codecs/mad.cpp b/src/codecs/mad.cpp
index 247e33bc..f36636a1 100644
--- a/src/codecs/mad.cpp
+++ b/src/codecs/mad.cpp
@@ -109,8 +109,9 @@ auto MadMp3Decoder::OpenStream(std::shared_ptr<IStream> input)
   auto vbr_length = GetVbrLength(header);
   if (vbr_length) {
     output.total_samples = vbr_length.value() * channels;
-  } else {
-    // FIXME: calculate length using the filesize, assuming CBR.
+  } else if (input->Size() && header.bitrate > 0) {
+    auto cbr_length = input->Size().value() / (header.bitrate / 8);
+    output.total_samples = cbr_length * output.sample_rate_hz * channels;
   }
   return output;
 }