diff --git a/Cargo.lock b/Cargo.lock
index b09ae0a..eedac12 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -14,6 +14,15 @@ version = "1.0.79"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "080e9890a082662b09c1ad45f567faeeb47f22b5fb23895fbe1e651e718e25ca"
 
+[[package]]
+name = "approx"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cab112f0a86d568ea0e627cc1d6be74a1e9cd55214684db5561995f6dad897c6"
+dependencies = [
+ "num-traits",
+]
+
 [[package]]
 name = "atty"
 version = "0.2.14"
@@ -202,6 +211,15 @@ version = "2.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "308cc39be01b73d0d18f82a0e7b2a3df85245f84af96fdddc5d202d27e47b86a"
 
+[[package]]
+name = "num-traits"
+version = "0.2.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "39e3200413f237f41ab11ad6d161bc7239c84dcb631773ccd7de3dfe4b5c267c"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "numtoa"
 version = "0.1.0"
@@ -384,6 +402,7 @@ name = "toipe"
 version = "0.4.1"
 dependencies = [
  "anyhow",
+ "approx",
  "bisection",
  "clap",
  "include-flate",
diff --git a/Cargo.toml b/Cargo.toml
index d46cfe5..96fd9ef 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -28,3 +28,4 @@ clap = { version = "3.0.5", features = ["derive", "color", "suggestions"] }
 rand = "0.8.4"
 termion = "1.5.6"
 include-flate = {version ="0.1.4", features=["stable"]}
+approx = "0.5.1"
diff --git a/src/results.rs b/src/results.rs
index 5207f3f..75502c0 100644
--- a/src/results.rs
+++ b/src/results.rs
@@ -32,6 +32,10 @@ impl ToipeResults {
 
     /// Percentage of letters that were typed correctly.
     pub fn accuracy(&self) -> f64 {
+        if self.total_chars_typed == 0 {
+            return 0.0;
+        }
+
         (self.total_chars_typed as isize - self.total_char_errors as isize) as f64
             / self.total_chars_typed as f64
     }
@@ -51,3 +55,150 @@ impl ToipeResults {
             / (self.duration().as_secs_f64() / 60.0)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use approx::assert_ulps_eq;
+
+    #[test]
+    fn sanity() {
+        let started_at = Instant::now();
+        let ended_at = started_at + Duration::new(10, 0);
+        let results = ToipeResults {
+            total_words: 0,
+            total_chars_typed: 100,
+            total_chars_in_text: 120,
+            total_char_errors: 10,
+            final_chars_typed_correctly: 80,
+            final_uncorrected_errors: 2,
+            started_at,
+            ended_at,
+        };
+
+        assert_eq!(results.duration(), Duration::new(10, 0));
+
+        assert_ulps_eq!(results.accuracy(), 0.9, max_ulps = 1);
+        assert_ulps_eq!(results.wpm(), 84.0, max_ulps = 1);
+    }
+
+    #[test]
+    fn accuracy() {
+        fn get_toipe_results(total_chars_typed: usize, total_char_errors: usize) -> ToipeResults {
+            ToipeResults {
+                total_words: 0,
+                total_chars_typed,
+                total_chars_in_text: 0,
+                total_char_errors,
+                final_chars_typed_correctly: 0,
+                final_uncorrected_errors: 0,
+                started_at: Instant::now(),
+                ended_at: Instant::now(),
+            }
+        }
+
+        let max_ulps = 1;
+
+        // no errors
+        assert_ulps_eq!(
+            get_toipe_results(100, 0).accuracy(),
+            1.0,
+            max_ulps = max_ulps
+        );
+        // nothing typed
+        assert_ulps_eq!(get_toipe_results(0, 0).accuracy(), 0.0, max_ulps = max_ulps);
+        // all wrong
+        assert_ulps_eq!(
+            get_toipe_results(100, 100).accuracy(),
+            0.0,
+            max_ulps = max_ulps
+        );
+        // half correct
+        assert_ulps_eq!(
+            get_toipe_results(100, 50).accuracy(),
+            0.5,
+            max_ulps = max_ulps
+        );
+        // more errors than correct
+        assert_ulps_eq!(
+            get_toipe_results(100, 150).accuracy(),
+            -0.5,
+            max_ulps = max_ulps
+        );
+    }
+
+    #[test]
+    fn wpm() {
+        fn get_toipe_results(
+            final_chars_typed_correctly: usize,
+            final_uncorrected_errors: usize,
+            duration: f64,
+        ) -> ToipeResults {
+            let started_at = Instant::now();
+            let seconds = duration.round();
+            let nanoseconds = (duration - seconds) * 1_000_000_000.0;
+            let ended_at = started_at + Duration::new(seconds as u64, nanoseconds as u32);
+            ToipeResults {
+                total_words: 0,
+                total_chars_typed: 0,
+                total_chars_in_text: 0,
+                total_char_errors: 0,
+                final_chars_typed_correctly,
+                final_uncorrected_errors,
+                started_at,
+                ended_at,
+            }
+        }
+
+        let max_ulps = 1;
+        assert_ulps_eq!(
+            get_toipe_results(100, 5, 30.0).wpm(),
+            30.0,
+            max_ulps = max_ulps
+        );
+        assert_ulps_eq!(
+            get_toipe_results(1000, 50, 30.0).wpm(),
+            300.0,
+            max_ulps = max_ulps
+        );
+        assert_ulps_eq!(
+            get_toipe_results(200, 0, 30.0).wpm(),
+            80.0,
+            max_ulps = max_ulps
+        );
+        assert_ulps_eq!(
+            get_toipe_results(200, 30, 30.0).wpm(),
+            20.0,
+            max_ulps = max_ulps
+        );
+        // too many errors - cancels out
+        assert_ulps_eq!(
+            get_toipe_results(200, 40, 30.0).wpm(),
+            0.0,
+            max_ulps = max_ulps
+        );
+        // no negative wpms
+        assert_ulps_eq!(
+            get_toipe_results(200, 50, 30.0).wpm(),
+            0.0,
+            max_ulps = max_ulps
+        );
+        assert_ulps_eq!(
+            get_toipe_results(1, 0, 1.0).wpm(),
+            12.0,
+            max_ulps = max_ulps
+        );
+        // skdlhaslkd won't give you any score!
+        assert_ulps_eq!(
+            get_toipe_results(0, 10, 1.0).wpm(),
+            0.0,
+            max_ulps = max_ulps
+        );
+        assert_ulps_eq!(
+            get_toipe_results(0, 0, 0.01).wpm(),
+            0.0,
+            max_ulps = max_ulps
+        );
+        // we don't consider the case of duration = 0 because that seems impossible
+    }
+}