diff --git a/src/custom-types.ts b/src/custom-types.ts
index e0e35a0..979f817 100644
--- a/src/custom-types.ts
+++ b/src/custom-types.ts
@@ -140,3 +140,12 @@ export interface CoordsValues {
   startSubjPixels?: number;
   endSubjPixels?: number;
 }
+
+export type posnumber = number & { readonly _positive: unique symbol };
+
+export function toPositiveNumber(value: number): posnumber {
+  if (value < 0) {
+    throw new Error(`${value} is not a positive number`);
+  }
+  return value as posnumber;
+}
diff --git a/tests/custom-types.test.ts b/tests/custom-types.test.ts
index 10496b7..1cdd697 100644
--- a/tests/custom-types.test.ts
+++ b/tests/custom-types.test.ts
@@ -12,6 +12,8 @@ import {
   textDefaults,
   rectDefaults,
   lineDefaults,
+  toPositiveNumber,
+  posnumber,
 } from '../src/custom-types';
 
 // Test suite for ColorSchemeEnum
@@ -112,3 +114,24 @@ describe('CoordsValues Interface', () => {
     expect(coords.end).toBe(90);
   });
 });
+
+describe('toPositiveNumber', () => {
+  it('should return a posnumber for a positive input', () => {
+    const value = 5;
+    const result: posnumber = toPositiveNumber(value);
+    expect(result).toBe(value); // Check if the returned value is the input itself
+    expect(typeof result).toBe('number'); // Ensure the type is still a number
+  });
+
+  it('should return a posnumber for zero', () => {
+    const value = 0;
+    const result: posnumber = toPositiveNumber(value);
+    expect(result).toBe(value); // Check if the returned value is zero
+    expect(typeof result).toBe('number'); // Ensure the type is still a number
+  });
+
+  it('should throw an error for a negative input', () => {
+    const value = -3;
+    expect(() => toPositiveNumber(value)).toThrow(`${value} is not a positive number`);
+  });
+});