diff --git a/CLAUDE.md b/CLAUDE.md
index 53c564a..6015c86 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -68,7 +68,7 @@ docs.rs for that exact version.
 ## Error handling and tests
 
 - `thiserror` for typed errors in `immo-core`; `anyhow` only at the `immo-web` boundary.
-- No `unwrap()`, `expect()`, or `panic!()` outside tests and `main`.
+- No `unwrap()` or `panic!()` outside tests and `main`. `expect()` can only be used for cases where the Result will provably be `Ok()` 
 - Every `core` function producing a number the user sees has a known-answer test, with the worked
   example stated in the test name or a comment above it.
 - Test the amortisation schedule against hand-checked values, never against the implementation's
diff --git a/crates/immo-core/Cargo.toml b/crates/immo-core/Cargo.toml
index 351117f..24c2337 100644
--- a/crates/immo-core/Cargo.toml
+++ b/crates/immo-core/Cargo.toml
@@ -5,7 +5,7 @@ edition.workspace = true
 rust-version.workspace = true
 
 [dependencies]
-rust_decimal = "1.42.1"
+rust_decimal = { version = "1.42.1", features = ["maths"] }
 thiserror = "2.0.19"
 
 [lints.clippy]
diff --git a/crates/immo-core/src/loan_offer.rs b/crates/immo-core/src/loan_offer.rs
index 3ac886a..5e5345d 100644
--- a/crates/immo-core/src/loan_offer.rs
+++ b/crates/immo-core/src/loan_offer.rs
@@ -1,4 +1,5 @@
 use rust_decimal::Decimal;
+use rust_decimal::MathematicalOps;
 use thiserror::Error;
 
 use crate::euros::Euros;
@@ -69,11 +70,31 @@ impl LoanOffer {
     pub fn annual_nominal_rate_in_percent(&self) -> Decimal {
         self.annual_nominal_rate * Decimal::ONE_HUNDRED
     }
+
+    pub fn proportional_monthly_interest_rate(&self) -> Decimal {
+        self.annual_nominal_rate / Decimal::from(12)
+    }
+
+    pub fn monthly_repayment(&self) -> Euros {
+        let monthly_rate = self.proportional_monthly_interest_rate();
+        let mut repayment_in_euros = if monthly_rate == Decimal::ZERO {
+            (self.borrowed_capital.as_decimal() / Decimal::from(self.loan_duration_in_months))
+                .round_dp(2)
+        } else {
+            let repayment_numerator = self.borrowed_capital.as_decimal() * monthly_rate;
+            let repayment_denominator = Decimal::ONE
+                - (Decimal::ONE / (Decimal::ONE + monthly_rate).powu(self.loan_duration_in_months));
+            (repayment_numerator / repayment_denominator).round_dp(2)
+        };
+        repayment_in_euros.rescale(2);
+        Euros::from_euros_as_decimal(repayment_in_euros).expect("Borrowed capital and annual rate are constrained on construction, repayment_in_euros cannot exceed 10b, and scale=2 is guaranteed through rescale")
+    }
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::fixtures::loan_offer_fixture::*;
 
     #[test]
     fn creating_loan_offer_with_negative_interest_rate_returns_negative_interest_rate_error() {
@@ -169,6 +190,18 @@ mod tests {
         );
     }
 
+    #[test]
+    fn creating_loan_offer_with_limit_inputs_works() {
+        let ret = LoanOffer::new(
+            MAX_BORROWED_CAPITAL,
+            MAX_ANNUAL_NOMINAL_RATE,
+            MAX_LOAN_DURATION_IN_MONTHS,
+        );
+        assert!(ret.is_ok());
+        // Should not panic - checks that expect assertion is valid
+        ret.unwrap().monthly_repayment();
+    }
+
     #[test]
     fn creating_loan_offer_with_null_interest_rate_works() {
         let ret = LoanOffer::new(Euros::from_cents_as_i64(10), Decimal::ZERO, 15);
@@ -192,4 +225,30 @@ mod tests {
         .unwrap();
         assert_eq!(offer.annual_nominal_rate_in_percent(), Decimal::new(4, 2));
     }
+
+    #[test]
+    fn monthly_repayment_works_for_standard_loan_offer() {
+        let fixture = offre_a();
+        let offer = LoanOffer::new(
+            fixture.borrowed_capital,
+            fixture.annual_nominal_rate,
+            fixture.total_duration_in_months,
+        )
+        .unwrap();
+
+        assert_eq!(offer.monthly_repayment(), fixture.monthly_repayment);
+    }
+
+    #[test]
+    fn monthly_repayment_works_for_zero_annual_nominal_rate() {
+        let fixture = offre_c_ptz();
+        let offer = LoanOffer::new(
+            fixture.borrowed_capital,
+            fixture.annual_nominal_rate,
+            fixture.total_duration_in_months,
+        )
+        .unwrap();
+
+        assert_eq!(offer.monthly_repayment(), fixture.monthly_repayment);
+    }
 }