diff --git a/libs/nshmp-lib/calc/maths.util.ts b/libs/nshmp-lib/calc/maths.util.ts
index 5a5a814a37a31d9ba9d39b9c1528918829220393..bd67e9cdc4243cdb822f4b35323b3212f2a144af 100644
--- a/libs/nshmp-lib/calc/maths.util.ts
+++ b/libs/nshmp-lib/calc/maths.util.ts
@@ -26,6 +26,28 @@ const erf = (x: number) => {
   return x < 0.0 ? -erfBase(-x) : erfBase(x);
 };
 
+/**
+ * Interpolate.
+ *
+ * @param xySequence The Xy sequence
+ * @param value The value to interpolate at.
+ */
+const interpolate = (xySequence: XySequence, value: number): number => {
+  const xValues = xySequence.xs;
+  const yValues = xySequence.ys;
+
+  const indexBelow = yValues.findIndex(y => {
+    return y < value;
+  });
+
+  const x0 = xValues[indexBelow - 1];
+  const x1 = xValues[indexBelow];
+  const y0 = yValues[indexBelow - 1];
+  const y1 = yValues[indexBelow];
+  const x = interpolateValue(x0, x1, y0, y1, value);
+  return isNaN(x) ? x : round(x, 6);
+};
+
 /**
  * Round a number to specific format
  *
@@ -65,30 +87,26 @@ const calculateResponseSpectrum = (
   xySequence: XySequence,
   returnPeriod: number
 ): number => {
-  returnPeriod = 1 / returnPeriod;
-  const xValues = xySequence.xs;
-  const yValues = xySequence.ys;
-
-  const afeIndexBelowReturnPeriod = yValues.findIndex(y => {
-    return y < returnPeriod;
-  });
-
-  const x0 = xValues[afeIndexBelowReturnPeriod - 1];
-  const x1 = xValues[afeIndexBelowReturnPeriod];
-  const y0 = yValues[afeIndexBelowReturnPeriod - 1];
-  const y1 = yValues[afeIndexBelowReturnPeriod];
-  const x = returnPeriodInterpolation(x0, x1, y0, y1, returnPeriod);
-  return isNaN(x) ? x : round(x, 6);
+  return interpolate(xySequence, 1 / returnPeriod);
 };
 
-const returnPeriodInterpolation = (
+/**
+ * Interpolate.
+ *
+ * @param x0 X value before value
+ * @param x1 X value after value
+ * @param y0 Y value before value
+ * @param y1 Y value after value
+ * @param value Value to interpolate at
+ */
+const interpolateValue = (
   x0: number,
   x1: number,
   y0: number,
   y1: number,
-  returnPeriod: number
+  value: number
 ): number => {
-  return x0 + (Math.log10(returnPeriod / y0) * (x1 - x0)) / Math.log10(y1 / y0);
+  return x0 + (Math.log10(value / y0) * (x1 - x0)) / Math.log10(y1 / y0);
 };
 
 const erfBase = (x: number) => {
@@ -115,6 +133,7 @@ const erfBase = (x: number) => {
 export const Maths = {
   normalCcdf,
   erf,
+  interpolate,
   round,
   responseSpectrum,
 };