diff --git a/contracts/simple_dex/Cargo.lock b/contracts/simple_dex/Cargo.lock
index 04f35808e2..0ce3811097 100644
--- a/contracts/simple_dex/Cargo.lock
+++ b/contracts/simple_dex/Cargo.lock
@@ -40,6 +40,27 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d468802bab17cbc0cc575e9b053f41e72aa36bfa6b7f55e3529ffa43161b97fa"
 
+[[package]]
+name = "bit-set"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0700ddab506f33b20a03b13996eccd309a48e5ff77d0d95926aa0210fb4e95f1"
+dependencies = [
+ "bit-vec",
+]
+
+[[package]]
+name = "bit-vec"
+version = "0.6.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "349f9b6a179ed607305526ca489b34ad0a41aed5f7980fa90eb03160b69598fb"
+
+[[package]]
+name = "bitflags"
+version = "1.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bef38d45163c2f1dde094a7dfd33ccf595c92905c8f8f4fdc18d06fb1037718a"
+
 [[package]]
 name = "bitvec"
 version = "1.0.1"
@@ -87,6 +108,12 @@ version = "1.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "87c5fdd0166095e1d463fc6cc01aa8ce547ad77a4e84d42eb6762b084e28067e"
 
+[[package]]
+name = "byteorder"
+version = "1.4.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "14c189c53d098945499cdfa7ecc63567cf3886b3332b312a5b4585d8d3a6a610"
+
 [[package]]
 name = "camino"
 version = "1.1.1"
@@ -217,6 +244,21 @@ version = "1.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "90e5c1c8368803113bf0c9584fc495a58b86dc8a29edbf8fe877d21d9507e797"
 
+[[package]]
+name = "fastrand"
+version = "1.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a7a407cfaa3385c4ae6b23e84623d48c2798d06e3e6a1878f7f59f17b3f86499"
+dependencies = [
+ "instant",
+]
+
+[[package]]
+name = "fnv"
+version = "1.0.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3f9eec918d3f24069decb9af1554cad7c880e2da24a9afd88aca000531ab82c1"
+
 [[package]]
 name = "fs2"
 version = "0.4.3"
@@ -484,6 +526,15 @@ dependencies = [
  "synstructure",
 ]
 
+[[package]]
+name = "instant"
+version = "0.1.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7a5bbe824c507c5da5956355e86a746d82e0e1464f65d862cc5e71da70e94b2c"
+dependencies = [
+ "cfg-if",
+]
+
 [[package]]
 name = "itertools"
 version = "0.10.3"
@@ -505,6 +556,12 @@ version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f9b7d56ba4a8344d6be9729995e6b06f928af29998cdf79fe390cbf6b1fee838"
 
+[[package]]
+name = "lazy_static"
+version = "1.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
+
 [[package]]
 name = "libc"
 version = "0.2.132"
@@ -734,6 +791,38 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "proptest"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e0d9cc07f18492d879586c92b485def06bc850da3118075cd45d50e9c95b0e5"
+dependencies = [
+ "bit-set",
+ "bitflags",
+ "byteorder",
+ "lazy_static",
+ "num-traits",
+ "quick-error 2.0.1",
+ "rand",
+ "rand_chacha",
+ "rand_xorshift",
+ "regex-syntax",
+ "rusty-fork",
+ "tempfile",
+]
+
+[[package]]
+name = "quick-error"
+version = "1.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a1d01941d82fa2ab50be1e79e6714289dd7cde78eba4c074bc5a4374f650dfe0"
+
+[[package]]
+name = "quick-error"
+version = "2.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a993555f31e5a609f617c12db6250dedcac1b0a85076912c436e6fc9b2c8e6a3"
+
 [[package]]
 name = "quote"
 version = "1.0.21"
@@ -779,12 +868,57 @@ dependencies = [
  "getrandom",
 ]
 
+[[package]]
+name = "rand_xorshift"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d25bf25ec5ae4a3f1b92f929810509a2f53d7dca2f50b794ff57e3face536c8f"
+dependencies = [
+ "rand_core",
+]
+
+[[package]]
+name = "redox_syscall"
+version = "0.2.16"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fb5a58c1855b4b6819d59012155603f0b22ad30cad752600aadfcb695265519a"
+dependencies = [
+ "bitflags",
+]
+
+[[package]]
+name = "regex-syntax"
+version = "0.6.28"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "456c603be3e8d448b072f410900c09faf164fbce2d480456f50eea6e25f9c848"
+
+[[package]]
+name = "remove_dir_all"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3acd125665422973a33ac9d3dd2df85edad0f4ae9b00dafb1a05e43a9f5ef8e7"
+dependencies = [
+ "winapi",
+]
+
 [[package]]
 name = "rlibc"
 version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fc874b127765f014d792f16763a81245ab80500e2ad921ed4ee9e82481ee08fe"
 
+[[package]]
+name = "rusty-fork"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb3dcc6e454c328bb824492db107ab7c0ae8fcffe4ad210136ef014458c1bc4f"
+dependencies = [
+ "fnv",
+ "quick-error 1.2.3",
+ "tempfile",
+ "wait-timeout",
+]
+
 [[package]]
 name = "ryu"
 version = "1.0.11"
@@ -927,6 +1061,7 @@ dependencies = [
  "ink_storage",
  "openbrush",
  "parity-scale-codec",
+ "proptest",
  "scale-info",
 ]
 
@@ -971,6 +1106,20 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "55937e1799185b12863d447f42597ed69d9928686b8d88a1df17376a097d8369"
 
+[[package]]
+name = "tempfile"
+version = "3.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5cdb1ef4eaeeaddc8fbd371e5017057064af0911902ef36b39801f67cc6d79e4"
+dependencies = [
+ "cfg-if",
+ "fastrand",
+ "libc",
+ "redox_syscall",
+ "remove_dir_all",
+ "winapi",
+]
+
 [[package]]
 name = "thiserror"
 version = "1.0.34"
@@ -1042,6 +1191,15 @@ version = "0.9.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "49874b5167b65d7193b8aba1567f5c7d93d001cafc34600cee003eda787e483f"
 
+[[package]]
+name = "wait-timeout"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9f200f5b12eb75f8c1ed65abd4b2db8a6e1b138a20de009dacee265a2498f3f6"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "wasi"
 version = "0.11.0+wasi-snapshot-preview1"
diff --git a/contracts/simple_dex/Cargo.toml b/contracts/simple_dex/Cargo.toml
index 8318177cca..cfca965111 100644
--- a/contracts/simple_dex/Cargo.toml
+++ b/contracts/simple_dex/Cargo.toml
@@ -21,6 +21,9 @@ openbrush = { git = "https://github.com/Supercolony-net/openbrush-contracts.git"
 access_control = { path = "../access_control", default-features = false, features = ["ink-as-dependency"] }
 game_token = { path = "../game_token", default-features = false, features = ["ink-as-dependency"] }
 
+[dev-dependencies]
+proptest = "1.0"
+
 [lib]
 name = "simple_dex"
 path = "lib.rs"
@@ -47,4 +50,4 @@ std = [
 ink-as-dependency = []
 
 [profile.dev]
-codegen-units = 16
\ No newline at end of file
+codegen-units = 16
diff --git a/contracts/simple_dex/lib.rs b/contracts/simple_dex/lib.rs
index c848b08eb1..d73051912b 100644
--- a/contracts/simple_dex/lib.rs
+++ b/contracts/simple_dex/lib.rs
@@ -375,8 +375,22 @@ mod simple_dex {
             let balance_token_in = self.balance_of(token_in, this)?;
             let balance_token_out = self.balance_of(token_out, this)?;
 
+            Self::_out_given_in(
+                amount_token_in,
+                balance_token_in,
+                balance_token_out,
+                self.swap_fee_percentage,
+            )
+        }
+
+        fn _out_given_in(
+            amount_token_in: Balance,
+            balance_token_in: Balance,
+            balance_token_out: Balance,
+            swap_fee_percentage: Balance,
+        ) -> Result<Balance, DexError> {
             let op0 = amount_token_in
-                .checked_mul(self.swap_fee_percentage)
+                .checked_mul(swap_fee_percentage)
                 .ok_or(DexError::Arithmethic)?;
 
             let op1 = balance_token_in
@@ -395,6 +409,8 @@ mod simple_dex {
 
             balance_token_out
                 .checked_sub(op4)
+                // If the division is not even, leave the 1 unit of dust in the exchange instead of paying it out.
+                .and_then(|result| result.checked_sub(if op3 % op2 > 0 { 1 } else { 0 }))
                 .ok_or(DexError::Arithmethic)
         }
 
@@ -509,4 +525,30 @@ mod simple_dex {
             SimpleDex::new()
         }
     }
+
+    #[cfg(test)]
+    mod test {
+        use proptest::prelude::*;
+
+        use super::*;
+
+        proptest! {
+            #[test]
+            fn rounding_benefits_dex(
+                balance_token_a in 1..1000u128,
+                balance_token_b in 1..1000u128,
+                pay_token_a in 1..1000u128,
+                fee_percentage in 0..10u128
+            ) {
+                let get_token_b =
+                    SimpleDex::_out_given_in(pay_token_a, balance_token_a, balance_token_b, fee_percentage).unwrap();
+                let balance_token_a = balance_token_a + pay_token_a;
+                let balance_token_b = balance_token_b - get_token_b;
+                let get_token_a =
+                    SimpleDex::_out_given_in(get_token_b, balance_token_b, balance_token_a, fee_percentage).unwrap();
+
+                assert!(get_token_a <= pay_token_a);
+            }
+        }
+    }
 }