diff --git a/src/quantity/quantity.rs b/src/quantity/quantity.rs
index e0e090d..10dd277 100644
--- a/src/quantity/quantity.rs
+++ b/src/quantity/quantity.rs
@@ -94,14 +94,12 @@ impl Quantity {
 
 
 	pub fn convert_to(self, other: Quantity) -> Option<Quantity> {
+		if !self.unit.compatible_with(&other.unit) { return None; }
+
 		let fa = self.unit.to_base_factor();
 		let fb = other.unit.to_base_factor();
-		let r = self * fa / fb;
 
-		// If this didn't work, units are incompatible
-		if r.unit != other.unit { return None; };
-
-		return Some(r);
+		return Some(self * fa / fb)
 	}
 }
 
@@ -167,6 +165,33 @@ impl Quantity {
 }
 
 
+impl Quantity {
+	pub fn mul_no_convert(self, other: Self) -> Self {
+		Quantity {
+			scalar: self.scalar * other.scalar,
+			unit: self.unit * other.unit
+		}
+	}
+
+	pub fn mul_assign_no_convert(&mut self, other: Self) {
+		self.scalar *= other.scalar;
+		self.unit *= other.unit;
+	}
+
+	pub fn div_no_convert(self, other: Self) -> Self {
+		Quantity {
+			scalar: self.scalar / other.scalar,
+			unit: self.unit / other.unit
+		}
+	}
+
+	pub fn div_assign_no_convert(&mut self, other: Self) {
+		self.scalar *= other.scalar;
+		self.unit *= other.unit;
+	}
+}
+
+
 impl Neg for Quantity where {
 	type Output = Self;
 
@@ -182,10 +207,10 @@ impl Add for Quantity {
 	type Output = Self;
 
 	fn add(self, other: Self) -> Self::Output {
-		if self.unit != other.unit { panic!() }
+		if !self.unit.compatible_with(&other.unit) { panic!() }
 
 		let mut o = other;
-		if !o.unit.prefixes_match(&self.unit) {
+		if self.unit != o.unit {
 			o = o.convert_to(self.clone()).unwrap();
 		}
 
@@ -198,10 +223,10 @@ impl Add for Quantity {
 
 impl AddAssign for Quantity where {
 	fn add_assign(&mut self, other: Self) {
-		if self.unit != other.unit { panic!() }
+		if !self.unit.compatible_with(&other.unit) { panic!() }
 
 		let mut o = other;
-		if !o.unit.prefixes_match(&self.unit) {
+		if self.unit != o.unit {
 			o = o.convert_to(self.clone()).unwrap();
 		}
 
@@ -213,10 +238,10 @@ impl Sub for Quantity {
 	type Output = Self;
 
 	fn sub(self, other: Self) -> Self::Output {
-		if self.unit != other.unit { panic!() }
+		if !self.unit.compatible_with(&other.unit) { panic!() }
 
 		let mut o = other;
-		if !o.unit.prefixes_match(&self.unit) {
+		if self.unit != o.unit {
 			o = o.convert_to(self.clone()).unwrap();
 		}
 
@@ -229,10 +254,10 @@ impl Sub for Quantity {
 
 impl SubAssign for Quantity where {
 	fn sub_assign(&mut self, other: Self) {
-		if self.unit != other.unit { panic!() }
+		if !self.unit.compatible_with(&other.unit) { panic!() }
 
 		let mut o = other;
-		if !o.unit.prefixes_match(&self.unit) {
+		if self.unit != o.unit {
 			o = o.convert_to(self.clone()).unwrap();
 		}
 
@@ -240,26 +265,38 @@ impl SubAssign for Quantity where {
 	}
 }
 
+
 impl Mul for Quantity {
 	type Output = Self;
 
 	fn mul(self, other: Self) -> Self::Output {
 
-		let f = self.unit.match_prefix_factor(&other.unit);
+		let mut o = other;
+		if self.unit != o.unit {
+			if self.unit.compatible_with(&o.unit) {
+				o = o.convert_to(self.clone()).unwrap()
+			}
+		}
 
 		Quantity {
-			scalar: self.scalar * other.scalar * f.scalar,
-			unit: self.unit * other.unit * f.unit,
+			scalar: self.scalar * o.scalar,
+			unit: self.unit * o.unit
 		}
 	}
 }
 
 impl MulAssign for Quantity where {
 	fn mul_assign(&mut self, other: Self) {
-		let f = self.unit.match_prefix_factor(&other.unit);
 
-		self.scalar *= other.scalar * f.scalar;
-		self.unit *= other.unit * f.unit;
+		let mut o = other;
+		if self.unit != o.unit {
+			if o.unit.compatible_with(&self.unit) {
+				o = o.convert_to(self.clone()).unwrap()
+			}
+		}
+
+		self.scalar *= o.scalar;
+		self.unit *= o.unit;
 	}
 }
 
@@ -267,17 +304,33 @@ impl Div for Quantity {
 	type Output = Self;
 
 	fn div(self, other: Self) -> Self::Output {
+
+		let mut o = other;
+		if self.unit != o.unit {
+			if self.unit.compatible_with(&o.unit) {
+				o = o.convert_to(self.clone()).unwrap()
+			}
+		}
+
 		Quantity {
-			scalar: self.scalar / other.scalar,
-			unit: self.unit / other.unit
+			scalar: self.scalar / o.scalar,
+			unit: self.unit / o.unit
 		}
 	}
 }
 
 impl DivAssign for Quantity where {
 	fn div_assign(&mut self, other: Self) {
-		self.scalar /= other.scalar;
-		self.unit /= other.unit;
+
+		let mut o = other;
+		if self.unit != o.unit {
+			if self.unit.compatible_with(&o.unit) {
+				o = o.convert_to(self.clone()).unwrap()
+			}
+		}
+
+		self.scalar /= o.scalar;
+		self.unit /= o.unit;
 	}
 }
 
diff --git a/src/quantity/unit/freeunit.rs b/src/quantity/unit/freeunit.rs
index 6fd4bcf..8567e86 100644
--- a/src/quantity/unit/freeunit.rs
+++ b/src/quantity/unit/freeunit.rs
@@ -1,4 +1,4 @@
-use std::hash::{Hash, Hasher};
+use std::hash::Hash;
 
 use crate::quantity::Scalar;
 use crate::quantity::Quantity;
@@ -8,26 +8,15 @@ use super::Unit;
 use super::unit_db;
 
 
+#[derive(Hash)]
 #[derive(Debug)]
 #[derive(Copy, Clone)]
+#[derive(PartialEq, Eq)]
 pub struct FreeUnit {
 	pub (in super) base: UnitBase,
 	pub (in super) prefix: Prefix
 }
 
-impl Hash for FreeUnit {
-	fn hash<H: Hasher>(&self, state: &mut H) {
-		self.base.hash(state);
-	}
-}
-
-impl Eq for FreeUnit {}
-impl PartialEq for FreeUnit {
-	fn eq(&self, other: &Self) -> bool {
-		self.base.eq(&other.base)
-	}
-}
-
 
 macro_rules! unpack_base_factor {
 	(
@@ -83,11 +72,6 @@ impl FreeUnit {
 	pub fn set_prefix(&mut self, prefix: Prefix) { self.prefix = prefix; }
 	pub fn get_prefix(&self) -> Prefix { self.prefix }
 
-	pub fn same_with_prefix(&self, other: &FreeUnit) -> bool {
-		self.base.eq(&other.base) && self.prefix.eq(&other.prefix)
-	}
-
-
 	pub fn to_base_factor(&self) -> Quantity {
 
 		let q = unit_db!(self.base, unpack_base_factor);
@@ -96,7 +80,7 @@ impl FreeUnit {
 		let mut p = self.prefix.to_ratio();
 		p.insert_unit(FreeUnit::from_base(self.base), Scalar::new_rational(1f64).unwrap());
 		p.insert_unit(FreeUnit::from_base_prefix(self.base, self.prefix), Scalar::new_rational(-1f64).unwrap());
-		q *= p;
+		q.mul_assign_no_convert(p);
 
 		return q;
 	}
diff --git a/src/quantity/unit/unit.rs b/src/quantity/unit/unit.rs
index 84821e7..25b9e5d 100644
--- a/src/quantity/unit/unit.rs
+++ b/src/quantity/unit/unit.rs
@@ -97,47 +97,12 @@ impl Unit {
 		return n;
 	}
 
-	pub fn prefixes_match(&self, other: &Unit) -> bool {
-		let v = self.get_val();
-		for (u, _) in other.get_val() {
-			let k = v.get_key_value(u);
-
-			if k.is_some() {
-				let k = k.unwrap().0;
-				if !u.same_with_prefix(k) { return false; }
-			}
-		}
-		return true;
+	pub fn compatible_with(&self, other: &Unit) -> bool {
+		let s = self.clone() * self.to_base_factor().unit;
+		let o = other.clone() * other.to_base_factor().unit;
+		return o == s;
 	}
 
-	pub fn match_prefix_factor(&self, other: &Unit) -> Quantity {
-		let mut f = Quantity::new_rational(1f64).unwrap();
-
-		let v = self.get_val();
-		for (ou, op) in other.get_val() {
-			let k = v.get_key_value(ou);
-
-			if k.is_some() {
-				let (su, _) = k.unwrap();
-
-				// Conversion factor ou -> basic
-				let mut p = ou.prefix.to_ratio();
-				p.insert_unit(FreeUnit::from_base(ou.base), Scalar::new_rational(1f64).unwrap());
-				p.insert_unit(FreeUnit::from_base_prefix(ou.base, ou.prefix), Scalar::new_rational(-1f64).unwrap());
-
-				// Conversion factor su -> basic
-				let mut q = su.prefix.to_ratio();
-				q.insert_unit(FreeUnit::from_base(su.base), Scalar::new_rational(1f64).unwrap());
-				q.insert_unit(FreeUnit::from_base_prefix(su.base, su.prefix), Scalar::new_rational(-1f64).unwrap());
-
-				f = f * (p / q).pow(Quantity::from_scalar(op.clone()));
-			}
-		}
-
-		return f;
-	}
-
-
 	pub fn insert(&mut self, u: FreeUnit, p: Scalar) {
 		let v = self.get_val_mut();
 		match v.get_mut(&u) {
@@ -165,15 +130,13 @@ impl Unit {
 
 		for (u, p) in self.get_val().iter() {
 			let b = u.to_base_factor();
-			q *= b.pow(Quantity::from_scalar(p.clone()));
+			q.mul_assign_no_convert(b.pow(Quantity::from_scalar(p.clone())));
 		}
 
 		return q;
 	}
 }
 
-
-
 impl Unit {
 	pub fn from_string(s: &str) -> Option<Quantity> {
 		macro_rules! unpack_fromstring {
@@ -220,8 +183,6 @@ impl Unit {
 }
 
 
-
-
 impl PartialEq for Unit {
 	fn eq(&self, other: &Self) -> bool {
 		let v = self.get_val();
@@ -231,6 +192,15 @@ impl PartialEq for Unit {
 				None => { return false; }
 			};
 		}
+
+		let v = other.get_val();
+		for (u, p) in self.get_val() {
+			match v.get(u) {
+				Some(i) => { if i != p { return false; } },
+				None => { return false; }
+			};
+		}
+
 		return true;
 	}
 }
diff --git a/src/tokens/operator.rs b/src/tokens/operator.rs
index bee9cad..e6cf835 100644
--- a/src/tokens/operator.rs
+++ b/src/tokens/operator.rs
@@ -383,7 +383,7 @@ impl Operator{
 					let j = args[i].as_number();
 					if let Token::Number(v) = j {
 
-						if sum.unit() != v.unit() {
+						if !sum.unit.compatible_with(&v.unit) {
 							return Err(EvalError::IncompatibleUnit);
 						}
 
@@ -456,7 +456,7 @@ impl Operator{
 				if let Token::Number(va) = a {
 					if let Token::Number(vb) = b {
 
-						if va.unit() != vb.unit() {
+						if !vb.unitless() {
 							return Err(EvalError::IncompatibleUnit);
 						}