diff --git a/src/index.ts b/src/index.ts
index a9e2189..706cd7f 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -7,10 +7,10 @@ import {
   parseFormSafe,
 } from './parsers';
 import {
-  BoolAsString,
-  CheckboxAsString,
-  IntAsString,
-  NumAsString,
+  boolAsString,
+  checkboxAsString,
+  intAsString,
+  numAsString,
 } from './schemas';
 
 export {
@@ -20,10 +20,9 @@ export {
   parseQuerySafe,
   parseForm,
   parseFormSafe,
-  BoolAsString,
-  CheckboxAsString,
-  IntAsString,
-  NumAsString,
+  boolAsString,
+  intAsString,
+  numAsString,
 };
 
 export const zx = {
@@ -33,8 +32,8 @@ export const zx = {
   parseQuerySafe,
   parseForm,
   parseFormSafe,
-  BoolAsString,
-  CheckboxAsString,
-  IntAsString,
-  NumAsString,
+  boolAsString,
+  checkboxAsString,
+  intAsString,
+  numAsString,
 };
diff --git a/src/parsers.test.ts b/src/parsers.test.ts
index 81a01d0..525884b 100644
--- a/src/parsers.test.ts
+++ b/src/parsers.test.ts
@@ -11,7 +11,7 @@ describe('parseParams', () => {
   type Result = { id: string; age: number };
   const params: Params = { id: 'id1', age: '10' };
   const paramsResult = { id: 'id1', age: 10 };
-  const objectSchema = { id: z.string(), age: zx.IntAsString };
+  const objectSchema = { id: z.string(), age: zx.intAsString() };
   const zodSchema = z.object(objectSchema);
 
   test('parses params using an object', () => {
@@ -41,13 +41,13 @@ describe('parseParamsSafe', () => {
   type Result = { id: string; age: number };
   const params: Params = { id: 'id1', age: '10' };
   const paramsResult = { id: 'id1', age: 10 };
-  const objectSchema = { id: z.string(), age: zx.IntAsString };
+  const objectSchema = { id: z.string(), age: zx.intAsString() };
   const zodSchema = z.object(objectSchema);
 
   test('parses params using an object', () => {
     const result = zx.parseParamsSafe(params, {
       id: z.string(),
-      age: zx.IntAsString,
+      age: zx.intAsString(),
     });
     expect(result.success).toBe(true);
     if (result.success !== true) throw new Error('Parsing failed');
@@ -88,7 +88,7 @@ describe('parseQuery', () => {
   const queryResult = { id: 'id1', age: 10 };
   const objectSchema = {
     id: z.string(),
-    age: zx.IntAsString,
+    age: zx.intAsString(),
     friends: z.array(z.string()).optional(),
   };
   const zodSchema = z.object(objectSchema);
@@ -96,7 +96,7 @@ describe('parseQuery', () => {
   test('parses URLSearchParams using an object', () => {
     const result = zx.parseQuery(search, {
       id: z.string(),
-      age: zx.IntAsString,
+      age: zx.intAsString(),
       friends: z.array(z.string()).optional(),
     });
     expect(result).toStrictEqual(queryResult);
@@ -115,7 +115,7 @@ describe('parseQuery', () => {
     search.append('friends', 'friend2');
     const result = zx.parseQuery(search, {
       id: z.string(),
-      age: zx.IntAsString,
+      age: zx.intAsString(),
       friends: z.array(z.string()).optional(),
     });
     expect(result).toStrictEqual({
@@ -141,7 +141,7 @@ describe('parseQuery', () => {
     const request = new Request(`http://example.com?${search.toString()}`);
     const result = zx.parseQuery(request, {
       id: z.string(),
-      age: zx.IntAsString,
+      age: zx.intAsString(),
       friends: z.array(z.string()).optional(),
     });
     expect(result).toStrictEqual(queryResult);
@@ -173,7 +173,7 @@ describe('parseQuery', () => {
       search,
       {
         id: z.string(),
-        age: zx.IntAsString,
+        age: zx.intAsString(),
         friends: z.array(z.string()).optional(),
       },
       { parser: customArrayParser }
@@ -205,7 +205,7 @@ describe('parseQuerySafe', () => {
   const queryResult = { id: 'id1', age: 10 };
   const zodSchema = z.object({
     id: z.string(),
-    age: zx.IntAsString,
+    age: zx.intAsString(),
     friends: z.array(z.string()).optional(),
   });
 
@@ -213,7 +213,7 @@ describe('parseQuerySafe', () => {
     const search = new URLSearchParams({ id: 'id1', age: '10' });
     const result = zx.parseQuerySafe(search, {
       id: z.string(),
-      age: zx.IntAsString,
+      age: zx.intAsString(),
       friends: z.array(z.string()).optional(),
     });
     expect(result.success).toBe(true);
@@ -237,7 +237,7 @@ describe('parseQuerySafe', () => {
     search.append('friends', 'friend2');
     const result = zx.parseQuerySafe(search, {
       id: z.string(),
-      age: zx.IntAsString,
+      age: zx.intAsString(),
       friends: z.array(z.string()).optional(),
     });
     expect(result.success).toBe(true);
@@ -268,7 +268,7 @@ describe('parseQuerySafe', () => {
     const request = new Request(`http://example.com?${search.toString()}`);
     const result = zx.parseQuerySafe(request, {
       id: z.string(),
-      age: zx.IntAsString,
+      age: zx.intAsString(),
       friends: z.array(z.string()).optional(),
     });
     expect(result.success).toBe(true);
@@ -316,8 +316,8 @@ describe('parseForm', () => {
   const formResult = { id: 'id1', age: 10, consent: true };
   const objectSchema = {
     id: z.string(),
-    age: zx.IntAsString,
-    consent: zx.CheckboxAsString,
+    age: zx.intAsString(),
+    consent: zx.checkboxAsString(),
     friends: z.array(z.string()).optional(),
     image: z.instanceof(NodeOnDiskFile).optional(),
   };
@@ -379,8 +379,8 @@ describe('parseForm', () => {
     });
     const result = await zx.parseForm(request, {
       id: z.string(),
-      age: zx.IntAsString,
-      consent: zx.CheckboxAsString,
+      age: zx.intAsString(),
+      consent: zx.checkboxAsString(),
       friends: z.array(z.string()).optional(),
       image: z.instanceof(NodeOnDiskFile).optional(),
     });
@@ -454,8 +454,8 @@ describe('parseFormSafe', () => {
   const formResult = { id: 'id1', age: 10, consent: true };
   const zodSchema = z.object({
     id: z.string(),
-    age: zx.IntAsString,
-    consent: zx.CheckboxAsString,
+    age: zx.intAsString(),
+    consent: zx.checkboxAsString(),
     friends: z.array(z.string()).optional(),
     image: z.instanceof(NodeOnDiskFile).optional(),
   });
@@ -465,8 +465,8 @@ describe('parseFormSafe', () => {
     const request = createFormRequest();
     const result = await zx.parseFormSafe(request, {
       id: z.string(),
-      age: zx.IntAsString,
-      consent: zx.CheckboxAsString,
+      age: zx.intAsString(),
+      consent: zx.checkboxAsString(),
       friends: z.array(z.string()).optional(),
       image: z.instanceof(NodeOnDiskFile).optional(),
     });
diff --git a/src/schemas.test.ts b/src/schemas.test.ts
index 35fff22..780bb79 100644
--- a/src/schemas.test.ts
+++ b/src/schemas.test.ts
@@ -1,73 +1,76 @@
 import { zx } from './';
 
-describe('BoolAsString', () => {
+describe('boolAsString', () => {
   test('parses true as string', () => {
-    expect(zx.BoolAsString.parse('true')).toBe(true);
+    expect(zx.boolAsString().parse('true')).toBe(true);
   });
   test('parses false as string', () => {
-    expect(zx.BoolAsString.parse('false')).toBe(false);
+    expect(zx.boolAsString().parse('false')).toBe(false);
   });
   test('throws on non-boolean string', () => {
-    expect(() => zx.BoolAsString.parse('hello')).toThrowError();
+    expect(() => zx.boolAsString().parse('hello')).toThrowError();
   });
 });
 
-describe('CheckboxAsString', () => {
+describe('checkboxAsString', () => {
   test('parses "on" as boolean', () => {
-    expect(zx.CheckboxAsString.parse('on')).toBe(true);
+    expect(zx.checkboxAsString().parse('on')).toBe(true);
+  });
+  test('parses "true" as boolean', () => {
+    expect(zx.checkboxAsString({ trueValue: 'true' }).parse('true')).toBe(true);
   });
   test('parses undefined as boolean', () => {
-    expect(zx.CheckboxAsString.parse(undefined)).toBe(false);
+    expect(zx.checkboxAsString().parse(undefined)).toBe(false);
   });
   test('throws on non-"on" string', () => {
-    expect(() => zx.CheckboxAsString.parse('hello')).toThrowError();
+    expect(() => zx.checkboxAsString().parse('hello')).toThrowError();
   });
 });
 
-describe('IntAsString', () => {
+describe('intAsString', () => {
   test('parses int as string', () => {
-    expect(zx.IntAsString.parse('3')).toBe(3);
+    expect(zx.intAsString().parse('3')).toBe(3);
   });
   test('parses int as string with leading 0', () => {
-    expect(zx.IntAsString.parse('03')).toBe(3);
+    expect(zx.intAsString().parse('03')).toBe(3);
   });
   test('parses negative int as string', () => {
-    expect(zx.IntAsString.parse('-3')).toBe(-3);
+    expect(zx.intAsString().parse('-3')).toBe(-3);
   });
   test('throws on int as number', () => {
-    expect(() => zx.IntAsString.parse(3)).toThrowError();
+    expect(() => zx.intAsString().parse(3)).toThrowError();
   });
   test('throws on float', () => {
-    expect(() => zx.IntAsString.parse(3.14)).toThrowError();
+    expect(() => zx.intAsString().parse(3.14)).toThrowError();
   });
   test('throws on string float', () => {
-    expect(() => zx.IntAsString.parse('3.14')).toThrowError();
+    expect(() => zx.intAsString().parse('3.14')).toThrowError();
   });
   test('throws on non-int string', () => {
-    expect(() => zx.IntAsString.parse('a3')).toThrowError();
+    expect(() => zx.intAsString().parse('a3')).toThrowError();
   });
 });
 
-describe('NumAsString', () => {
+describe('numAsString', () => {
   test('parses number with decimal as string', () => {
-    expect(zx.NumAsString.parse('3.14')).toBe(3.14);
+    expect(zx.numAsString().parse('3.14')).toBe(3.14);
   });
   test('parses number with decimal as string with leading 0', () => {
-    expect(zx.NumAsString.parse('03.14')).toBe(3.14);
+    expect(zx.numAsString().parse('03.14')).toBe(3.14);
   });
   test('parses negative number with decimal as string', () => {
-    expect(zx.NumAsString.parse('-3.14')).toBe(-3.14);
+    expect(zx.numAsString().parse('-3.14')).toBe(-3.14);
   });
   test('parses int as string', () => {
-    expect(zx.NumAsString.parse('3')).toBe(3);
+    expect(zx.numAsString().parse('3')).toBe(3);
   });
   test('parses int as string with leading 0', () => {
-    expect(zx.NumAsString.parse('03')).toBe(3);
+    expect(zx.numAsString().parse('03')).toBe(3);
   });
   test('parses negative int as string', () => {
-    expect(zx.NumAsString.parse('-3')).toBe(-3);
+    expect(zx.numAsString().parse('-3')).toBe(-3);
   });
   test('throws on non-number string', () => {
-    expect(() => zx.NumAsString.parse('a3')).toThrowError();
+    expect(() => zx.numAsString().parse('a3')).toThrowError();
   });
 });
diff --git a/src/schemas.ts b/src/schemas.ts
index 3c66848..aaf31dd 100644
--- a/src/schemas.ts
+++ b/src/schemas.ts
@@ -1,4 +1,5 @@
 import { z } from 'zod';
+import type { errorUtil } from 'zod/lib/helpers/errorUtil';
 
 /**
  * Zod schema to parse strings that are booleans.
@@ -8,37 +9,54 @@ import { z } from 'zod';
  * BoolAsString.parse('true') -> true
  * ```
  */
-export const BoolAsString = z
-  .string()
-  .regex(/^(true|false)$/, 'Must be a boolean string ("true" or "false")')
-  .transform((value) => value === 'true');
+export const boolAsString = (
+  message:
+    | errorUtil.ErrMessage
+    | undefined = 'Must be a boolean string ("true" or "false")'
+) =>
+  z
+    .string()
+    .regex(/^(true|false)$/, message)
+    .transform((value) => value === 'true');
 
 /**
  * Zod schema to parse checkbox formdata.
  * Use to parse <input type="checkbox" /> values.
  * @example
  * ```ts
- * CheckboxAsString.parse('on') -> true
- * CheckboxAsString.parse(undefined) -> false
+ * checkboxAsString().parse('on') -> true
+ * checkboxAsString().parse(undefined) -> false
  * ```
  */
-export const CheckboxAsString = z
-  .literal('on')
-  .optional()
-  .transform((value) => value === 'on');
+export const checkboxAsString = ({
+  trueValue = 'on',
+  ...params
+}: {
+  trueValue?: string;
+} & Parameters<typeof z.union>[1] = {}) =>
+  z.union(
+    [
+      z.literal(trueValue).transform(() => true),
+      z.literal(undefined).transform(() => false),
+    ],
+    params
+  );
 
 /**
  * Zod schema to parse strings that are integers.
  * Use to parse  <input type="number" /> values.
  * @example
  * ```ts
- * IntAsString.parse('3') -> 3
+ * intAsString.parse('3') -> 3
  * ```
  */
-export const IntAsString = z
-  .string()
-  .regex(/^-?\d+$/, 'Must be an integer string')
-  .transform((val) => parseInt(val, 10));
+export const intAsString = (
+  message: errorUtil.ErrMessage | undefined = 'Must be an integer string'
+) =>
+  z
+    .string()
+    .regex(/^-?\d+$/, message)
+    .transform((val) => parseInt(val, 10));
 
 /**
  * Zod schema to parse strings that are numbers.
@@ -48,7 +66,10 @@ export const IntAsString = z
  * NumAsString.parse('3.14') -> 3.14
  * ```
  */
-export const NumAsString = z
-  .string()
-  .regex(/^-?\d*\.?\d+$/, 'Must be a number string')
-  .transform(Number);
+export const numAsString = (
+  message: errorUtil.ErrMessage | undefined = 'Must be a number string'
+) =>
+  z
+    .string()
+    .regex(/^-?\d*\.?\d+$/, message)
+    .transform(Number);