diff --git a/examples/hello_world.rs b/examples/hello_world.rs
index 0399e1f..75a4eb1 100644
--- a/examples/hello_world.rs
+++ b/examples/hello_world.rs
@@ -1,5 +1,4 @@
 use std::io;
-use std::u8;
 
 use tts::*;
 
@@ -11,28 +10,34 @@ fn main() -> Result<(), Error> {
     if rate {
         let original_rate = tts.get_rate()?;
         tts.speak(format!("Current rate: {}", original_rate), false)?;
-        tts.set_rate(u8::MAX)?;
+        tts.set_rate(tts.max_rate())?;
         tts.speak("This is very fast.", false)?;
-        tts.set_rate(0)?;
+        tts.set_rate(tts.min_rate())?;
         tts.speak("This is very slow.", false)?;
+        tts.set_rate(tts.normal_rate())?;
+        tts.speak("This is the normal rate.", false)?;
         tts.set_rate(original_rate)?;
     }
     let Features { pitch, .. } = tts.supported_features();
     if pitch {
         let original_pitch = tts.get_pitch()?;
-        tts.set_pitch(u8::MAX)?;
+        tts.set_pitch(tts.max_pitch())?;
         tts.speak("This is high-pitch.", false)?;
-        tts.set_pitch(0)?;
+        tts.set_pitch(tts.min_pitch())?;
         tts.speak("This is low pitch.", false)?;
+        tts.set_pitch(tts.normal_pitch())?;
+        tts.speak("This is normal pitch.", false)?;
         tts.set_pitch(original_pitch)?;
     }
     let Features { volume, .. } = tts.supported_features();
     if volume {
         let original_volume = tts.get_volume()?;
-        tts.set_volume(u8::MAX)?;
+        tts.set_volume(tts.max_volume())?;
         tts.speak("This is loud!", false)?;
-        tts.set_volume(0)?;
+        tts.set_volume(tts.min_volume())?;
         tts.speak("This is quiet.", false)?;
+        tts.set_volume(tts.normal_volume())?;
+        tts.speak("This is normal volume.", false)?;
         tts.set_volume(original_volume)?;
     }
     tts.speak("Goodbye.", false)?;
diff --git a/src/backends/speech_dispatcher.rs b/src/backends/speech_dispatcher.rs
index cf6a4bb..e9615d3 100644
--- a/src/backends/speech_dispatcher.rs
+++ b/src/backends/speech_dispatcher.rs
@@ -1,6 +1,4 @@
 #[cfg(target_os = "linux")]
-use std::u8;
-
 use log::{info, trace};
 use speech_dispatcher::*;
 
@@ -16,18 +14,6 @@ impl SpeechDispatcher {
     }
 }
 
-fn u8_to_i32(v: u8) -> i32 {
-    let ratio: f32 = v as f32 / u8::MAX as f32;
-    (ratio * 200. - 100.) as i32
-}
-
-fn i32_to_u8(v: i32) -> u8 {
-    let v = v as f32;
-    let ratio: f32 = (v + 100.) / 200.;
-    let v = ratio * u8::MAX as f32;
-    v as u8
-}
-
 impl Backend for SpeechDispatcher {
     fn supported_features(&self) -> Features {
         Features {
@@ -60,30 +46,66 @@ impl Backend for SpeechDispatcher {
         Ok(())
     }
 
-    fn get_rate(&self) -> Result<u8, Error> {
-        Ok(i32_to_u8(self.0.get_voice_rate()))
+    fn min_rate(&self) -> f32 {
+        -100.
     }
 
-    fn set_rate(&mut self, rate: u8) -> Result<(), Error> {
-        self.0.set_voice_rate(u8_to_i32(rate));
+    fn max_rate(&self) -> f32 {
+        100.
+    }
+
+    fn normal_rate(&self) -> f32 {
+        0.
+    }
+
+    fn get_rate(&self) -> Result<f32, Error> {
+        Ok(self.0.get_voice_rate() as f32)
+    }
+
+    fn set_rate(&mut self, rate: f32) -> Result<(), Error> {
+        self.0.set_voice_rate(rate as i32);
         Ok(())
     }
 
-    fn get_pitch(&self) -> Result<u8, Error> {
-        Ok(i32_to_u8(self.0.get_voice_pitch()))
+    fn min_pitch(&self) -> f32 {
+        -100.
     }
 
-    fn set_pitch(&mut self, pitch: u8) -> Result<(), Error> {
-        self.0.set_voice_pitch(u8_to_i32(pitch));
+    fn max_pitch(&self) -> f32 {
+        100.
+    }
+
+    fn normal_pitch(&self) -> f32 {
+        0.
+    }
+
+    fn get_pitch(&self) -> Result<f32, Error> {
+        Ok(self.0.get_voice_pitch() as f32)
+    }
+
+    fn set_pitch(&mut self, pitch: f32) -> Result<(), Error> {
+        self.0.set_voice_pitch(pitch as i32);
         Ok(())
     }
 
-    fn get_volume(&self) -> Result<u8, Error> {
-        Ok(i32_to_u8(self.0.get_volume()))
+    fn min_volume(&self) -> f32 {
+        -100.
     }
 
-    fn set_volume(&mut self, volume: u8) -> Result<(), Error> {
-        self.0.set_volume(u8_to_i32(volume));
+    fn max_volume(&self) -> f32 {
+        100.
+    }
+
+    fn normal_volume(&self) -> f32 {
+        0.
+    }
+
+    fn get_volume(&self) -> Result<f32, Error> {
+        Ok(self.0.get_volume() as f32)
+    }
+
+    fn set_volume(&mut self, volume: f32) -> Result<(), Error> {
+        self.0.set_volume(volume as i32);
         Ok(())
     }
 }
diff --git a/src/backends/tolk.rs b/src/backends/tolk.rs
index e97520e..4b9d747 100644
--- a/src/backends/tolk.rs
+++ b/src/backends/tolk.rs
@@ -37,27 +37,63 @@ impl Backend for Tolk {
         Ok(())
     }
 
-    fn get_rate(&self) -> Result<u8, Error> {
+    fn min_rate(&self) -> f32 {
+        unimplemented!()
+    }
+
+    fn max_rate(&self) -> f32 {
+        unimplemented!()
+    }
+
+    fn normal_rate(&self) -> f32 {
+        unimplemented!()
+    }
+
+    fn get_rate(&self) -> Result<f32, Error> {
         unimplemented!();
     }
 
-    fn set_rate(&mut self, _rate: u8) -> Result<(), Error> {
+    fn set_rate(&mut self, _rate: f32) -> Result<(), Error> {
         unimplemented!();
     }
 
-    fn get_pitch(&self) -> Result<u8, Error> {
+    fn min_pitch(&self) -> f32 {
+        unimplemented!()
+    }
+
+    fn max_pitch(&self) -> f32 {
+        unimplemented!()
+    }
+
+    fn normal_pitch(&self) -> f32 {
+        unimplemented!()
+    }
+
+    fn get_pitch(&self) -> Result<f32, Error> {
         unimplemented!();
     }
 
-    fn set_pitch(&mut self, _pitch: u8) -> Result<(), Error> {
+    fn set_pitch(&mut self, _pitch: f32) -> Result<(), Error> {
         unimplemented!();
     }
 
-    fn get_volume(&self) -> Result<u8, Error> {
+    fn min_volume(&self) -> f32 {
+        unimplemented!()
+    }
+
+    fn max_volume(&self) -> f32 {
+        unimplemented!()
+    }
+
+    fn normal_volume(&self) -> f32 {
+        unimplemented!()
+    }
+
+    fn get_volume(&self) -> Result<f32, Error> {
         unimplemented!();
     }
 
-    fn set_volume(&mut self, _volume: u8) -> Result<(), Error> {
+    fn set_volume(&mut self, _volume: f32) -> Result<(), Error> {
         unimplemented!();
     }
 }
diff --git a/src/backends/web.rs b/src/backends/web.rs
index 37eb29b..fe79d5a 100644
--- a/src/backends/web.rs
+++ b/src/backends/web.rs
@@ -1,24 +1,22 @@
 #[cfg(target_arch = "wasm32")]
-use std::u8;
-
 use log::{info, trace};
 use web_sys::SpeechSynthesisUtterance;
 
-use crate::{Backend, Error};
+use crate::{Backend, Error, Features};
 
 pub struct Web {
-    rate: u8,
-    pitch: u8,
-    volume: u8,
+    rate: f32,
+    pitch: f32,
+    volume: f32,
 }
 
 impl Web {
     pub fn new() -> Result<Self, Error> {
         info!("Initializing Web backend");
         Ok(Web {
-            rate: 25,
-            pitch: 127,
-            volume: u8::MAX,
+            rate: 1.,
+            pitch: 1.,
+            volume: 1.,
         })
     }
 }
@@ -36,15 +34,9 @@ impl Backend for Web {
     fn speak(&self, text: &str, interrupt: bool) -> Result<(), Error> {
         trace!("speak({}, {})", text, interrupt);
         let utterance = SpeechSynthesisUtterance::new_with_text(text).unwrap();
-        let mut rate: f32 = self.rate as f32;
-        rate = rate / u8::MAX as f32 * 10.;
-        utterance.set_rate(rate);
-        let mut pitch: f32 = self.pitch as f32;
-        pitch = pitch / u8::MAX as f32 * 2.;
-        utterance.set_pitch(pitch);
-        let mut volume: f32 = self.volume as f32;
-        volume = volume / u8::MAX as f32 * 1.;
-        utterance.set_volume(volume);
+        utterance.set_rate(self.rate);
+        utterance.set_pitch(self.pitch);
+        utterance.set_volume(self.volume);
         if interrupt {
             self.stop()?;
         }
@@ -64,29 +56,65 @@ impl Backend for Web {
         Ok(())
     }
 
-    fn get_rate(&self) -> Result<u8, Error> {
+    fn min_rate(&self) -> f32 {
+        0.1
+    }
+
+    fn max_rate(&self) -> f32 {
+        10.
+    }
+
+    fn normal_rate(&self) -> f32 {
+        1.
+    }
+
+    fn get_rate(&self) -> Result<f32, Error> {
         Ok(self.rate)
     }
 
-    fn set_rate(&mut self, rate: u8) -> Result<(), Error> {
+    fn set_rate(&mut self, rate: f32) -> Result<(), Error> {
         self.rate = rate;
         Ok(())
     }
 
-    fn get_pitch(&self) -> Result<u8, Error> {
+    fn min_pitch(&self) -> f32 {
+        0.
+    }
+
+    fn max_pitch(&self) -> f32 {
+        2.
+    }
+
+    fn normal_pitch(&self) -> f32 {
+        1.
+    }
+
+    fn get_pitch(&self) -> Result<f32, Error> {
         Ok(self.pitch)
     }
 
-    fn set_pitch(&mut self, pitch: u8) -> Result<(), Error> {
+    fn set_pitch(&mut self, pitch: f32) -> Result<(), Error> {
         self.pitch = pitch;
         Ok(())
     }
 
-    fn get_volume(&self) -> Result<u8, Error> {
+    fn min_volume(&self) -> f32 {
+        0.
+    }
+
+    fn max_volume(&self) -> f32 {
+        1.
+    }
+
+    fn normal_volume(&self) -> f32 {
+        1.
+    }
+
+    fn get_volume(&self) -> Result<f32, Error> {
         Ok(self.volume)
     }
 
-    fn set_volume(&mut self, volume: u8) -> Result<(), Error> {
+    fn set_volume(&mut self, volume: f32) -> Result<(), Error> {
         self.volume = volume;
         Ok(())
     }
diff --git a/src/backends/winrt.rs b/src/backends/winrt.rs
index 48f8cfe..d23480e 100644
--- a/src/backends/winrt.rs
+++ b/src/backends/winrt.rs
@@ -48,9 +48,9 @@ impl Backend for WinRT {
     fn supported_features(&self) -> Features {
         Features {
             stop: true,
-            rate: false,
-            pitch: false,
-            volume: false,
+            rate: true,
+            pitch: true,
+            volume: true,
         }
     }
 
@@ -71,27 +71,69 @@ impl Backend for WinRT {
         Ok(())
     }
 
-    fn get_rate(&self) -> std::result::Result<u8, Error> {
-        unimplemented!();
+    fn min_rate(&self) -> f32 {
+        0.5
     }
 
-    fn set_rate(&mut self, _rate: u8) -> std::result::Result<(), Error> {
-        unimplemented!();
+    fn max_rate(&self) -> f32 {
+        6.0
     }
 
-    fn get_pitch(&self) -> std::result::Result<u8, Error> {
-        unimplemented!();
+    fn normal_rate(&self) -> f32 {
+        1.
     }
 
-    fn set_pitch(&mut self, _pitch: u8) -> std::result::Result<(), Error> {
-        unimplemented!();
+    fn get_rate(&self) -> std::result::Result<f32, Error> {
+        let rate = self.synth.options()?.speaking_rate()?;
+        Ok(rate as f32)
     }
 
-    fn get_volume(&self) -> std::result::Result<u8, Error> {
-        unimplemented!();
+    fn set_rate(&mut self, rate: f32) -> std::result::Result<(), Error> {
+        self.synth.options()?.set_speaking_rate(rate.into())?;
+        Ok(())
     }
 
-    fn set_volume(&mut self, _volume: u8) -> std::result::Result<(), Error> {
-        unimplemented!();
+    fn min_pitch(&self) -> f32 {
+        0.
+    }
+
+    fn max_pitch(&self) -> f32 {
+        2.
+    }
+
+    fn normal_pitch(&self) -> f32 {
+        1.
+    }
+
+    fn get_pitch(&self) -> std::result::Result<f32, Error> {
+        let pitch = self.synth.options()?.audio_pitch()?;
+        Ok(pitch as f32)
+    }
+
+    fn set_pitch(&mut self, pitch: f32) -> std::result::Result<(), Error> {
+        self.synth.options()?.set_audio_pitch(pitch.into())?;
+        Ok(())
+    }
+
+    fn min_volume(&self) -> f32 {
+        0.
+    }
+
+    fn max_volume(&self) -> f32 {
+        1.
+    }
+
+    fn normal_volume(&self) -> f32 {
+        1.
+    }
+
+    fn get_volume(&self) -> std::result::Result<f32, Error> {
+        let volume = self.synth.options()?.audio_volume()?;
+        Ok(volume as f32)
+    }
+
+    fn set_volume(&mut self, volume: f32) -> std::result::Result<(), Error> {
+        self.synth.options()?.set_audio_volume(volume.into())?;
+        Ok(())
     }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 10648ff..d363a1d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -39,18 +39,29 @@ pub enum Error {
     WinRT(winrt::Error),
     #[error("Unsupported feature")]
     UnsupportedFeature,
+    #[error("Out of range")]
+    OutOfRange,
 }
 
 pub trait Backend {
     fn supported_features(&self) -> Features;
     fn speak(&self, text: &str, interrupt: bool) -> Result<(), Error>;
     fn stop(&self) -> Result<(), Error>;
-    fn get_rate(&self) -> Result<u8, Error>;
-    fn set_rate(&mut self, rate: u8) -> Result<(), Error>;
-    fn get_pitch(&self) -> Result<u8, Error>;
-    fn set_pitch(&mut self, pitch: u8) -> Result<(), Error>;
-    fn get_volume(&self) -> Result<u8, Error>;
-    fn set_volume(&mut self, volume: u8) -> Result<(), Error>;
+    fn min_rate(&self) -> f32;
+    fn max_rate(&self) -> f32;
+    fn normal_rate(&self) -> f32;
+    fn get_rate(&self) -> Result<f32, Error>;
+    fn set_rate(&mut self, rate: f32) -> Result<(), Error>;
+    fn min_pitch(&self) -> f32;
+    fn max_pitch(&self) -> f32;
+    fn normal_pitch(&self) -> f32;
+    fn get_pitch(&self) -> Result<f32, Error>;
+    fn set_pitch(&mut self, pitch: f32) -> Result<(), Error>;
+    fn min_volume(&self) -> f32;
+    fn max_volume(&self) -> f32;
+    fn normal_volume(&self) -> f32;
+    fn get_volume(&self) -> Result<f32, Error>;
+    fn set_volume(&mut self, volume: f32) -> Result<(), Error>;
 }
 
 pub struct TTS(Box<dyn Backend>);
@@ -130,10 +141,31 @@ impl TTS {
         }
     }
 
+    /**
+     * Returns the minimum rate for this speech synthesizer.
+     */
+    pub fn min_rate(&self) -> f32 {
+        self.0.min_rate()
+    }
+
+    /**
+     * Returns the maximum rate for this speech synthesizer.
+     */
+    pub fn max_rate(&self) -> f32 {
+        self.0.max_rate()
+    }
+
+    /**
+     * Returns the normal rate for this speech synthesizer.
+     */
+    pub fn normal_rate(&self) -> f32 {
+        self.0.normal_rate()
+    }
+
     /**
      * Gets the current speech rate.
      */
-    pub fn get_rate(&self) -> Result<u8, Error> {
+    pub fn get_rate(&self) -> Result<f32, Error> {
         let Features { rate, .. } = self.supported_features();
         if rate {
             self.0.get_rate()
@@ -145,22 +177,47 @@ impl TTS {
     /**
      * Sets the desired speech rate.
      */
-    pub fn set_rate(&mut self, rate: u8) -> Result<&Self, Error> {
+    pub fn set_rate(&mut self, rate: f32) -> Result<&Self, Error> {
         let Features {
             rate: rate_feature, ..
         } = self.supported_features();
         if rate_feature {
-            self.0.set_rate(rate)?;
-            Ok(self)
+            if rate < self.0.min_rate() || rate > self.0.max_rate() {
+                Err(Error::OutOfRange)
+            } else {
+                self.0.set_rate(rate)?;
+                Ok(self)
+            }
         } else {
             Err(Error::UnsupportedFeature)
         }
     }
 
+    /**
+     * Returns the minimum pitch for this speech synthesizer.
+     */
+    pub fn min_pitch(&self) -> f32 {
+        self.0.min_pitch()
+    }
+
+    /**
+     * Returns the maximum pitch for this speech synthesizer.
+     */
+    pub fn max_pitch(&self) -> f32 {
+        self.0.max_pitch()
+    }
+
+    /**
+     * Returns the normal pitch for this speech synthesizer.
+     */
+    pub fn normal_pitch(&self) -> f32 {
+        self.0.normal_pitch()
+    }
+
     /**
      * Gets the current speech pitch.
      */
-    pub fn get_pitch(&self) -> Result<u8, Error> {
+    pub fn get_pitch(&self) -> Result<f32, Error> {
         let Features { pitch, .. } = self.supported_features();
         if pitch {
             self.0.get_pitch()
@@ -172,23 +229,48 @@ impl TTS {
     /**
      * Sets the desired speech pitch.
      */
-    pub fn set_pitch(&mut self, pitch: u8) -> Result<&Self, Error> {
+    pub fn set_pitch(&mut self, pitch: f32) -> Result<&Self, Error> {
         let Features {
             pitch: pitch_feature,
             ..
         } = self.supported_features();
         if pitch_feature {
-            self.0.set_pitch(pitch)?;
-            Ok(self)
+            if pitch < self.0.min_pitch() || pitch > self.0.max_pitch() {
+                Err(Error::OutOfRange)
+            } else {
+                self.0.set_pitch(pitch)?;
+                Ok(self)
+            }
         } else {
             Err(Error::UnsupportedFeature)
         }
     }
 
+    /**
+     * Returns the minimum volume for this speech synthesizer.
+     */
+    pub fn min_volume(&self) -> f32 {
+        self.0.min_volume()
+    }
+
+    /**
+     * Returns the maximum volume for this speech synthesizer.
+     */
+    pub fn max_volume(&self) -> f32 {
+        self.0.max_volume()
+    }
+
+    /**
+     * Returns the normal volume for this speech synthesizer.
+     */
+    pub fn normal_volume(&self) -> f32 {
+        self.0.normal_volume()
+    }
+
     /**
      * Gets the current speech volume.
      */
-    pub fn get_volume(&self) -> Result<u8, Error> {
+    pub fn get_volume(&self) -> Result<f32, Error> {
         let Features { volume, .. } = self.supported_features();
         if volume {
             self.0.get_volume()
@@ -200,14 +282,18 @@ impl TTS {
     /**
      * Sets the desired speech volume.
      */
-    pub fn set_volume(&mut self, volume: u8) -> Result<&Self, Error> {
+    pub fn set_volume(&mut self, volume: f32) -> Result<&Self, Error> {
         let Features {
             volume: volume_feature,
             ..
         } = self.supported_features();
         if volume_feature {
-            self.0.set_volume(volume)?;
-            Ok(self)
+            if volume < self.0.min_volume() || volume > self.0.max_volume() {
+                Err(Error::OutOfRange)
+            } else {
+                self.0.set_volume(volume)?;
+                Ok(self)
+            }
         } else {
             Err(Error::UnsupportedFeature)
         }