diff --git a/src/lib.rs b/src/lib.rs
index e2d63c9c..2883ccd0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -27,6 +27,8 @@
 //!  assert_eq!("518 GB".to_string(), ByteSize::gb(518).to_string(false));
 //! ```
 
+mod parse;
+
 #[cfg(feature = "serde")]
 #[macro_use]
 extern crate serde;
@@ -58,8 +60,8 @@ pub const TIB: u64 = 1_099_511_627_776;
 /// bytes size for 1 pebibyte
 pub const PIB: u64 = 1_125_899_906_842_624;
 
-static UNITS: &'static str = "KMGTPE";
-static UNITS_SI: &'static str = "kMGTPE";
+static UNITS: &str = "KMGTPE";
+static UNITS_SI: &str = "kMGTPE";
 static LN_KB: f64 = 6.931471806; // ln 1024
 static LN_KIB: f64 = 6.907755279; // ln 1000
 
diff --git a/src/parse.rs b/src/parse.rs
new file mode 100644
index 00000000..9e72cf33
--- /dev/null
+++ b/src/parse.rs
@@ -0,0 +1,218 @@
+use super::ByteSize;
+
+impl std::str::FromStr for ByteSize {
+    type Err = String;
+
+    fn from_str(value: &str) -> Result<Self, Self::Err> {
+        if let Ok(v) = value.parse::<u64>() {
+            return Ok(Self(v));
+        }
+        let number: String = value
+            .chars()
+            .take_while(|c| c.is_digit(10) || c == &'.')
+            .collect();
+        match number.parse::<f64>() {
+            Ok(v) => {
+                let suffix: String = value
+                    .chars()
+                    .skip_while(|c| c.is_whitespace() || c.is_digit(10) || c == &'.')
+                    .collect();
+                match suffix.parse::<Unit>() {
+                    Ok(u) => Ok(Self((v * u) as u64)),
+                    Err(error) => Err(format!(
+                        "couldn't parse {:?} into a known SI unit, {}",
+                        suffix, error
+                    )),
+                }
+            }
+            Err(error) => Err(format!(
+                "couldn't parse {:?} into a ByteSize, {}",
+                value, error
+            )),
+        }
+    }
+}
+
+enum Unit {
+    Byte,
+    // power of tens
+    KiloByte,
+    MegaByte,
+    GigaByte,
+    TeraByte,
+    PetaByte,
+    // power of twos
+    KibiByte,
+    MebiByte,
+    GibiByte,
+    TebiByte,
+    PebiByte,
+}
+
+impl Unit {
+    fn factor(&self) -> u64 {
+        match self {
+            Self::Byte => super::B,
+            // power of tens
+            Self::KiloByte => super::KB,
+            Self::MegaByte => super::MB,
+            Self::GigaByte => super::GB,
+            Self::TeraByte => super::TB,
+            Self::PetaByte => super::PB,
+            // power of twos
+            Self::KibiByte => super::KIB,
+            Self::MebiByte => super::MIB,
+            Self::GibiByte => super::GIB,
+            Self::TebiByte => super::TIB,
+            Self::PebiByte => super::PIB,
+        }
+    }
+}
+
+mod impl_ops {
+    use super::Unit;
+    use std::ops;
+
+    impl ops::Add<u64> for Unit {
+        type Output = u64;
+
+        fn add(self, other: u64) -> Self::Output {
+            self.factor() + other
+        }
+    }
+
+    impl ops::Add<Unit> for u64 {
+        type Output = u64;
+
+        fn add(self, other: Unit) -> Self::Output {
+            self + other.factor()
+        }
+    }
+
+    impl ops::Mul<u64> for Unit {
+        type Output = u64;
+
+        fn mul(self, other: u64) -> Self::Output {
+            self.factor() * other
+        }
+    }
+
+    impl ops::Mul<Unit> for u64 {
+        type Output = u64;
+
+        fn mul(self, other: Unit) -> Self::Output {
+            self * other.factor()
+        }
+    }
+
+    impl ops::Add<f64> for Unit {
+        type Output = f64;
+
+        fn add(self, other: f64) -> Self::Output {
+            self.factor() as f64 + other
+        }
+    }
+
+    impl ops::Add<Unit> for f64 {
+        type Output = f64;
+
+        fn add(self, other: Unit) -> Self::Output {
+            other.factor() as f64 + self
+        }
+    }
+
+    impl ops::Mul<f64> for Unit {
+        type Output = f64;
+
+        fn mul(self, other: f64) -> Self::Output {
+            self.factor() as f64 * other
+        }
+    }
+
+    impl ops::Mul<Unit> for f64 {
+        type Output = f64;
+
+        fn mul(self, other: Unit) -> Self::Output {
+            other.factor() as f64 * self
+        }
+    }
+}
+
+impl std::str::FromStr for Unit {
+    type Err = String;
+
+    fn from_str(unit: &str) -> Result<Self, Self::Err> {
+        match unit.to_lowercase().as_str() {
+            "b" => Ok(Self::Byte),
+            // power of tens
+            "k" | "kb" => Ok(Self::KiloByte),
+            "m" | "mb" => Ok(Self::MegaByte),
+            "g" | "gb" => Ok(Self::GigaByte),
+            "t" | "tb" => Ok(Self::TeraByte),
+            "p" | "pb" => Ok(Self::PetaByte),
+            // power of twos
+            "ki" | "kib" => Ok(Self::KibiByte),
+            "mi" | "mib" => Ok(Self::MebiByte),
+            "gi" | "gib" => Ok(Self::GibiByte),
+            "ti" | "tib" => Ok(Self::TebiByte),
+            "pi" | "pib" => Ok(Self::PebiByte),
+            _ => Err(format!("couldn't parse unit of {:?}", unit)),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn when_ok() {
+        // shortcut for writing test cases
+        fn parse(s: &str) -> u64 {
+            s.parse::<ByteSize>().unwrap().0
+        }
+
+        assert_eq!("0".parse::<ByteSize>().unwrap().0, 0);
+        assert_eq!(parse("0"), 0);
+        assert_eq!(parse("500"), 500);
+        assert_eq!(parse("1K"), Unit::KiloByte * 1);
+        assert_eq!(parse("1Ki"), Unit::KibiByte * 1);
+        assert_eq!(parse("1.5Ki"), (1.5 * Unit::KibiByte) as u64);
+        assert_eq!(parse("1KiB"), 1 * Unit::KibiByte);
+        assert_eq!(parse("1.5KiB"), (1.5 * Unit::KibiByte) as u64);
+        assert_eq!(parse("3 MB"), Unit::MegaByte * 3);
+        assert_eq!(parse("4 MiB"), Unit::MebiByte * 4);
+        assert_eq!(parse("6 GB"), 6 * Unit::GigaByte);
+        assert_eq!(parse("4 GiB"), 4 * Unit::GibiByte);
+        assert_eq!(parse("88TB"), 88 * Unit::TeraByte);
+        assert_eq!(parse("521TiB"), 521 * Unit::TebiByte);
+        assert_eq!(parse("8 PB"), 8 * Unit::PetaByte);
+        assert_eq!(parse("8P"), 8 * Unit::PetaByte);
+        assert_eq!(parse("12 PiB"), 12 * Unit::PebiByte);
+    }
+
+    #[test]
+    fn when_err() {
+        // shortcut for writing test cases
+        fn parse(s: &str) -> Result<ByteSize, String> {
+            s.parse::<ByteSize>()
+        }
+
+        assert!(parse("").is_err());
+        assert!(parse("a124GB").is_err());
+    }
+
+    #[test]
+    fn to_and_from_str() {
+        // shortcut for writing test cases
+        fn parse(s: &str) -> u64 {
+            s.parse::<ByteSize>().unwrap().0
+        }
+
+        assert_eq!(parse(&format!("{}", parse("128GB"))), 128 * Unit::GigaByte);
+        assert_eq!(
+            parse(&crate::to_string(parse("128.000 GiB"), true)),
+            128 * Unit::GibiByte
+        );
+    }
+}