diff --git a/src/lib.rs b/src/lib.rs
index d73b240..d821b7b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -23,6 +23,9 @@ pub fn run(input: &str) -> Result<String, String> {
 		Err(ParseError::ExpectedUnit) => {
 			return Err("Expected a unit".to_string());
 		}
+		Err(ParseError::AmbiguousUnit(unit, prefix1, prefix2)) => {
+			return Err(format!("Ambiguous unit '{unit}', use either '{prefix1} {unit}' or '{prefix2} {unit}'"));
+		}
 	};
 
 	if non_metric.len() == 0 {
@@ -110,6 +113,7 @@ mod test {
 		assert_eq!(run("1 tf"), Err("Unknown unit: tf".to_string()));
 		assert_eq!(run("1"), Err("Expected a unit".to_string()));
 		assert_eq!(run(""), Err("Expected quantity or quantities to convert".to_string()));
+		assert_eq!(run("1 fl oz"), Err("Ambiguous unit 'fl oz', use either 'imp fl oz' or 'US fl oz'".to_string()));
 		assert_eq!(run("6 ft 1 lbs"), Err("Incompatible units: feet, pounds".to_string()));
 		assert_eq!(run("0 °F 0 °F"), Err("Cannot sum together temperatures".to_string()));
 	}
diff --git a/src/parse.rs b/src/parse.rs
index 53ad3cd..18ded43 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -11,6 +11,7 @@ pub enum ParseError {
 	UnexpectedUnit(String),
 	UnknownUnit(String),
 	ExpectedUnit,
+	AmbiguousUnit(String, &'static str, &'static str),
 }
 
 pub fn parse(input: &str) -> Result<Vec<NonMetricQuantity>, ParseError> {
@@ -280,6 +281,27 @@ fn parse_unit(input: String) -> Result<NonMetric, ParseError> {
 		"us gallons" => Ok(NonMetric::USGallon),
 		"us gal" => Ok(NonMetric::USGallon),
 
+		// Ambiguous units
+		"fluid ounce" => Err(ParseError::AmbiguousUnit(input, "imperial", "US")),
+		"fluid ounces" => Err(ParseError::AmbiguousUnit(input, "imperial", "US")),
+		"fl oz" => Err(ParseError::AmbiguousUnit(input, "imp", "US")),
+		"fl. oz." => Err(ParseError::AmbiguousUnit(input, "imp", "US")),
+		"oz. fl." => Err(ParseError::AmbiguousUnit(input, "imp", "US")),
+
+		"pint" => Err(ParseError::AmbiguousUnit(input, "imperial", "US")),
+		"pints" => Err(ParseError::AmbiguousUnit(input, "imperial", "US")),
+		"pt" => Err(ParseError::AmbiguousUnit(input, "imp", "US")),
+		"p" => Err(ParseError::AmbiguousUnit(input, "imp", "US")),
+
+		"quart" => Err(ParseError::AmbiguousUnit(input, "imperial", "US")),
+		"quarts" => Err(ParseError::AmbiguousUnit(input, "imperial", "US")),
+		"qt" => Err(ParseError::AmbiguousUnit(input, "imp", "US")),
+
+		"gallon" => Err(ParseError::AmbiguousUnit(input, "imperial", "US")),
+		"gallons" => Err(ParseError::AmbiguousUnit(input, "imperial", "US")),
+		"gal" => Err(ParseError::AmbiguousUnit(input, "imp", "US")),
+
+		// Unknown unit
 		_ => Err(ParseError::UnknownUnit(input)),
 	}
 }
@@ -374,6 +396,7 @@ mod test {
 		assert_eq!(parse("ft"), Err(ParseError::UnexpectedUnit("ft".to_string())));
 		assert_eq!(parse("5 tf"), Err(ParseError::UnknownUnit("tf".to_string())));
 		assert_eq!(parse("12"), Err(ParseError::ExpectedUnit));
+		assert_eq!(parse("1 gallon"), Err(ParseError::AmbiguousUnit("gallon".to_string(), "imperial", "US")));
 	}
 
 	#[test]
@@ -608,8 +631,113 @@ mod test {
 		assert_eq!(parse_unit("us gallon".to_string()), Ok(NonMetric::USGallon));
 		assert_eq!(parse_unit("us gallons".to_string()), Ok(NonMetric::USGallon));
 		assert_eq!(parse_unit("us gal".to_string()), Ok(NonMetric::USGallon));
+	}
 
-		// Unknown unit
+	#[test]
+	fn ambiguous_units() {
+		assert_eq!(
+			parse_unit("fluid ounce".to_string()),
+			Err(ParseError::AmbiguousUnit("fluid ounce".to_string(), "imperial", "US"))
+		);
+		assert_eq!(
+			parse_unit("fluid ounces".to_string()),
+			Err(ParseError::AmbiguousUnit("fluid ounces".to_string(), "imperial", "US"))
+		);
+		assert_eq!(
+			parse_unit("fl oz".to_string()),
+			Err(ParseError::AmbiguousUnit("fl oz".to_string(), "imp", "US"))
+		);
+		assert_eq!(
+			parse_unit("fl. oz.".to_string()),
+			Err(ParseError::AmbiguousUnit("fl. oz.".to_string(), "imp", "US"))
+		);
+		assert_eq!(
+			parse_unit("oz. fl.".to_string()),
+			Err(ParseError::AmbiguousUnit("oz. fl.".to_string(), "imp", "US"))
+		);
+
+		assert_eq!(
+			parse_unit("pint".to_string()),
+			Err(ParseError::AmbiguousUnit("pint".to_string(), "imperial", "US"))
+		);
+		assert_eq!(
+			parse_unit("pints".to_string()),
+			Err(ParseError::AmbiguousUnit("pints".to_string(), "imperial", "US"))
+		);
+		assert_eq!(
+			parse_unit("pt".to_string()),
+			Err(ParseError::AmbiguousUnit("pt".to_string(), "imp", "US"))
+		);
+		assert_eq!(
+			parse_unit("p".to_string()),
+			Err(ParseError::AmbiguousUnit("p".to_string(), "imp", "US"))
+		);
+
+		assert_eq!(
+			parse_unit("quart".to_string()),
+			Err(ParseError::AmbiguousUnit("quart".to_string(), "imperial", "US"))
+		);
+		assert_eq!(
+			parse_unit("quarts".to_string()),
+			Err(ParseError::AmbiguousUnit("quarts".to_string(), "imperial", "US"))
+		);
+		assert_eq!(
+			parse_unit("qt".to_string()),
+			Err(ParseError::AmbiguousUnit("qt".to_string(), "imp", "US"))
+		);
+
+		assert_eq!(
+			parse_unit("gallon".to_string()),
+			Err(ParseError::AmbiguousUnit("gallon".to_string(), "imperial", "US"))
+		);
+		assert_eq!(
+			parse_unit("gallons".to_string()),
+			Err(ParseError::AmbiguousUnit("gallons".to_string(), "imperial", "US"))
+		);
+		assert_eq!(
+			parse_unit("gal".to_string()),
+			Err(ParseError::AmbiguousUnit("gal".to_string(), "imp", "US"))
+		);
+	}
+
+	#[test]
+	fn ambiguous_unit_suggestions() {
+		let ambiguous_units = [
+			"fluid ounce",
+			"fluid ounces",
+			"fl oz",
+			"fl. oz.",
+			"oz. fl.",
+
+			"pint",
+			"pints",
+			"pt",
+			"p",
+
+			"quart",
+			"quarts",
+			"qt",
+
+			"gallon",
+			"gallons",
+			"gal",
+		];
+
+		for unit in ambiguous_units {
+			let parsed = parse_unit(unit.to_string());
+			if let Err(ParseError::AmbiguousUnit(unit, prefix1, prefix2)) = parsed {
+				let suggestion1 = format!("{prefix1} {unit}");
+				let suggestion2 = format!("{prefix2} {unit}");
+				assert!(parse_unit(suggestion1).is_ok());
+				assert!(parse_unit(suggestion2).is_ok());
+			} else {
+				unreachable!();
+			}
+		}
+	}
+
+	#[test]
+	fn unknown_unit() {
 		assert_eq!(parse_unit("hutenosa".to_string()), Err(ParseError::UnknownUnit("hutenosa".to_string())));
 	}