diff --git a/src/index.ts b/src/index.ts
index 35bb785..da9ebbe 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -337,6 +337,13 @@ export type {
   HorizonProberConfig,
 } from "./horizonProber.js";
 
+// Invoice calculator
+export {
+  calculateSplitAmounts,
+  computeAmounts,
+  formatSplitPercentage,
+} from "./invoice/calculator.js";
+
 // AMM Calculator
 export { estimateSwapOutput, calculatePoolShare } from "./ammCalculator.js";
 
diff --git a/src/invoice/calculator.ts b/src/invoice/calculator.ts
index 629c5ae..658ff31 100644
--- a/src/invoice/calculator.ts
+++ b/src/invoice/calculator.ts
@@ -10,6 +10,7 @@
  */
 
 import { auditSplitRounding, RoundingOverflowError } from "./rounding.js";
+import { SdkError, SdkErrorCode } from "../errors.js";
 import type { SplitLine, AuditedSplitResult } from "../types.js";
 
 // Re-export the error so callers can import it from the calculator.
@@ -66,3 +67,78 @@ export function computeAmounts(
 ): Record<string, bigint> {
   return calculateSplitAmounts(total, splits).amounts;
 }
+
+// ---------------------------------------------------------------------------
+// Formatting helpers
+// ---------------------------------------------------------------------------
+
+/** Basis points per whole unit (1.0 = 10000 bps). */
+const BASIS_POINTS_PER_UNIT = 10000n;
+
+/**
+ * Format a basis-point value as a human-readable percentage string.
+ *
+ * @param basisPoints - Integer value in basis points (0–10000).
+ * @param opts.decimals - Number of decimal places to display (default 2, range 0–4).
+ * @returns Percentage string such as `"33.33%"` or `"100.00%"`.
+ *
+ * @throws {SdkError} When `basisPoints` is outside the 0–10000 range.
+ * @throws {RangeError} When `opts.decimals` is outside the 0–4 range.
+ *
+ * @example
+ * ```ts
+ * formatSplitPercentage(3333n); // "33.33%"
+ * formatSplitPercentage(1n);    // "0.01%"
+ * formatSplitPercentage(3333n, { decimals: 0 }); // "33%"
+ * ```
+ */
+export function formatSplitPercentage(
+  basisPoints: bigint,
+  opts?: { decimals?: number },
+): string {
+  const decimals = opts?.decimals ?? 2;
+
+  if (decimals < 0 || decimals > 4 || !Number.isInteger(decimals)) {
+    throw new RangeError(`decimals must be an integer between 0 and 4, got ${decimals}`);
+  }
+
+  if (basisPoints < 0n || basisPoints > BASIS_POINTS_PER_UNIT) {
+    throw new SdkError(
+      `basisPoints must be between 0 and 10000, got ${basisPoints.toString()}`,
+      SdkErrorCode.INVALID_RECIPIENT,
+      { basisPoints: basisPoints.toString() },
+    );
+  }
+
+  if (decimals === 0) {
+    // Integer percentage, with standard rounding.
+    const scaled = basisPoints + BASIS_POINTS_PER_UNIT / 2n;
+    const whole = scaled / BASIS_POINTS_PER_UNIT;
+    return `${whole.toString()}%`;
+  }
+
+  // Compute whole and fractional parts without floating point.
+  const whole = basisPoints / BASIS_POINTS_PER_UNIT;
+  const remainder = basisPoints % BASIS_POINTS_PER_UNIT;
+
+  // Scale remainder to the requested number of decimal places.
+  // e.g. decimals=2: remainder * 100 / 10000 = remainder / 100
+  const divisor = 10n ** BigInt(4 - decimals);
+  let fractional = remainder / divisor;
+  const remainderMod = remainder % divisor;
+
+  // Round to nearest at the last displayed decimal place.
+  const halfDivisor = divisor / 2n;
+  if (remainderMod >= halfDivisor) {
+    fractional += 1n;
+  }
+
+  // Handle carry-over that turns fractional into a whole unit (e.g. 99.995 → 100.00).
+  const maxFractional = 10n ** BigInt(decimals) - 1n;
+  if (fractional > maxFractional) {
+    return `${(whole + 1n).toString()}.${"0".repeat(decimals)}%`;
+  }
+
+  const fractionalStr = fractional.toString().padStart(decimals, "0");
+  return `${whole.toString()}.${fractionalStr}%`;
+}
diff --git a/test/invoice/formatSplitPercentage.test.ts b/test/invoice/formatSplitPercentage.test.ts
new file mode 100644
index 0000000..747fb67
--- /dev/null
+++ b/test/invoice/formatSplitPercentage.test.ts
@@ -0,0 +1,56 @@
+import { describe, expect, it } from "vitest";
+import { formatSplitPercentage } from "../../src/invoice/calculator.js";
+import { SdkError, SdkErrorCode } from "../../src/errors.js";
+
+describe("formatSplitPercentage", () => {
+  it("formats 3333 basis points as 33.33%", () => {
+    expect(formatSplitPercentage(3333n)).toBe("33.33%");
+  });
+
+  it("formats 10000 basis points as 100.00%", () => {
+    expect(formatSplitPercentage(10000n)).toBe("100.00%");
+  });
+
+  it("formats 1 basis point as 0.01%", () => {
+    expect(formatSplitPercentage(1n)).toBe("0.01%");
+  });
+
+  it("formats 0 basis points as 0.00%", () => {
+    expect(formatSplitPercentage(0n)).toBe("0.00%");
+  });
+
+  it("supports 0 decimals", () => {
+    expect(formatSplitPercentage(3333n, { decimals: 0 })).toBe("33%");
+  });
+
+  it("rounds up at the last displayed decimal place", () => {
+    // 6667 bps = 66.67% exactly, 66.7% with one decimal place (rounds up from 66.66...)
+    expect(formatSplitPercentage(6666n, { decimals: 1 })).toBe("66.7%");
+  });
+
+  it("supports up to 4 decimal places", () => {
+    expect(formatSplitPercentage(3333n, { decimals: 4 })).toBe("33.3300%");
+  });
+
+  it("throws SdkError for negative basis points", () => {
+    expect(() => formatSplitPercentage(-1n)).toThrow(SdkError);
+    expect(() => formatSplitPercentage(-1n)).toThrow(SdkErrorCode.INVALID_RECIPIENT);
+  });
+
+  it("throws SdkError for basis points above 10000", () => {
+    expect(() => formatSplitPercentage(10001n)).toThrow(SdkError);
+    expect(() => formatSplitPercentage(10001n)).toThrow(SdkErrorCode.INVALID_RECIPIENT);
+  });
+
+  it("throws RangeError for decimals below 0", () => {
+    expect(() => formatSplitPercentage(3333n, { decimals: -1 })).toThrow(RangeError);
+  });
+
+  it("throws RangeError for decimals above 4", () => {
+    expect(() => formatSplitPercentage(3333n, { decimals: 5 })).toThrow(RangeError);
+  });
+
+  it("throws RangeError for non-integer decimals", () => {
+    expect(() => formatSplitPercentage(3333n, { decimals: 2.5 })).toThrow(RangeError);
+  });
+});