diff --git a/app/src/util/series.ts b/app/src/util/series.ts
new file mode 100644
index 000000000..02453bc98
--- /dev/null
+++ b/app/src/util/series.ts
@@ -0,0 +1,46 @@
+export function estimate(series: { year: number, value: number }[], year: number): number | null {
+
+  // if it's in the series, just return the value
+
+  const exact = series.find(e => e.year === year);
+  if (exact) {
+    return exact.value;
+  }
+
+  // we need at least two points to interpolate or extrapolate
+
+  if (series.length < 2) {
+    return null;
+  }
+
+  if (year < series[0].year) { // extrapolate backwards
+    let first = series[0]
+    let next = series.find(e => e.year - first.year >= first.year - year)
+    if (!next) {
+      return null
+    }
+    let rate = (next.value - first.value) / (next.year - first.year);
+    let value = first.value - rate * (first.year - year)
+    return value
+  } else if (year > series[series.length - 1].year) { // extrapolate forwards
+    let last = series[series.length - 1]
+    let prev = series.findLast(e => last.year - e.year >= year - last.year)
+    if (!prev) {
+      return null
+    }
+    let rate = (last.value - prev.value) / (last.year - prev.year);
+    let value = last.value + rate * (year - last.year)
+    return value
+  } else { // interpolate
+    let prev = series.findLast(e => e.year < year)
+    let next = series.find(e => e.year > year)
+    if (!prev || !next) {
+      return null
+    }
+    let rate = (next.value - prev.value) / (next.year - prev.year);
+    let value = prev.value + rate * (year - prev.year)
+    return value
+  }
+
+  return null;
+}
\ No newline at end of file
diff --git a/app/tests/series.test.ts b/app/tests/series.test.ts
new file mode 100644
index 000000000..76da43a60
--- /dev/null
+++ b/app/tests/series.test.ts
@@ -0,0 +1,60 @@
+import { describe, it } from "node:test"
+import assert from "node:assert/strict"
+import { estimate } from "@/util/series"
+
+describe("Series", () => {
+  let series0 = [
+    { year: 2000, value: 100 }
+  ];
+
+  let series = [
+    { year: 2000, value: 100 },
+    { year: 2010, value: 200 },
+    { year: 2020, value: 400 }
+  ];
+
+  it("should return exact value for small series on match", () => {
+    let result = estimate(series0, 2000);
+    assert.strictEqual(result, 100);
+  })
+
+  it("should return null for small series on no match", () => {
+    let result = estimate(series0, 2001);
+    assert.strictEqual(result, null);
+  })
+
+  it("should return exact value", () => {
+    let result = estimate(series, 2010);
+    assert.strictEqual(result, 200);
+  })
+
+  it("should return interpolated value at low rate", () => {
+    let result = estimate(series, 2005);
+    assert.strictEqual(result, 150);
+  })
+
+  it("should return interpolated value at high rate", () => {
+    let result = estimate(series, 2015);
+    assert.strictEqual(result, 300);
+  })
+
+  it("should return extrapolated value at low rate", () => {
+    let result = estimate(series, 1995);
+    assert.strictEqual(result, 50);
+  })
+
+  it("should return extrapolated value at high rate", () => {
+    let result = estimate(series, 2025);
+    assert.strictEqual(result, 500);
+  })
+
+  it("should return null if below safe extrapolation range", () => {
+    let result = estimate(series, 1979);
+    assert.strictEqual(result, null);
+  })
+
+  it("should return null if above safe extrapolation range", () => {
+    let result = estimate(series, 2041);
+    assert.strictEqual(result, null);
+  })
+})
\ No newline at end of file