PESEL Validation in Node.js

1. What is a PESEL number?

PESEL (Powszechny Elektroniczny System Ewidencji Ludnosci) is the Polish national identification number. Every Polish citizen — and every foreign resident registered in Poland — receives a unique 11-digit PESEL at birth or upon registration. It is the primary identifier used across virtually every public and private system in the country:

• Tax filings — used as the taxpayer ID for individuals without a separate NIP
• Banking and financial services — required for opening accounts, applying for loans, and KYC verification
• Healthcare — identifies patients in the NFZ (National Health Fund) system
• E-government — login to ePUAP, mObywatel, and other digital services
• Social security (ZUS) — tracks contributions and pension entitlements
• Education — student records, university enrollment, and exam registration

Unlike many national ID numbers, PESEL is not just an opaque identifier. It encodes two pieces of real-world data: the holder's date of birth and their biological gender. This makes it both a validation target and a data source — but also means that getting the parsing wrong can produce silently incorrect results.

2. PESEL anatomy

A PESEL number is always exactly 11 digits. Each group of digits carries specific meaning:

For example, the PESEL 44051401358 breaks down as: year 44, month 05, day 14, serial 0135, check digit 8.

Century encoding

Since the year portion is only two digits, PESEL uses an offset on the month value to distinguish centuries. This is the most commonly misunderstood part of the format:

This means a PESEL with month value 25 does not represent the 25th month — it represents May (month 5) in the 2000s (offset +20). For example, 05 is May 1900s, while 25 is May 2000s.

Gender encoding

The 10th digit of the PESEL (the last digit of the serial number) determines gender:

• Odd (1, 3, 5, 7, 9) → male
• Even (0, 2, 4, 6, 8) → female

In our example 44051401358, the 10th digit is 5 (odd), indicating a male holder.

3. The checksum algorithm

PESEL uses a weighted MOD-10 checksum. The algorithm is straightforward but the weight pattern is specific and must be applied exactly:

1. Take the first 10 digits of the PESEL
2. Multiply each digit by its corresponding weight: 1, 3, 7, 9, 1, 3, 7, 9, 1, 3
3. Sum all the products
4. Take the last digit of the sum (i.e. sum mod 10)
5. Subtract that value from 10 — if the result is 10, the check digit is 0; otherwise the result is the check digit
6. Compare with the 11th digit of the PESEL

Step-by-step example

Let's validate 44051401358:

Wait — the 11th digit is 8, not 9. That means this example PESEL does not pass the checksum. This is exactly the kind of subtle error that an API handles for you automatically.

Here is a JavaScript implementation of the algorithm:

function validatePeselChecksum(pesel) {
  const d = pesel.split('').map(Number);
  const weights = [1, 3, 7, 9, 1, 3, 7, 9, 1, 3];
  const sum = weights.reduce((acc, w, i) => acc + d[i] * w, 0);
  const check = (10 - (sum % 10)) % 10;
  return check === d[10];
}

4. Why manual validation isn't enough

5. The right solution: one API call

The IsValid PESEL API handles the complete validation and data extraction pipeline in a single GET request:

Get your free API key at isvalid.dev. The free tier includes 100 calls per day with no credit card required.

Full parameter reference and response schema: PESEL Validation API docs →

6. Node.js code example

Using the @isvalid-dev/sdk package or the built-in fetch API (Node.js 18+):

// pesel-validator.mjs
import { createClient } from '@isvalid-dev/sdk';

const iv = createClient({ apiKey: process.env.ISVALID_API_KEY });

// ── Validate a PESEL number ────────────────────────────────────────────────
const result = await iv.pl.pesel('44051401358');

if (!result.valid) {
  console.log('Invalid PESEL');
} else {
  console.log('Birth date:', result.birthDate);  // "1944-05-14"
  console.log('Gender    :', result.gender);      // "male"
  console.log('Over 18?  :', result.isOver18);    // true
}

7. cURL example

Validate a PESEL number directly from the terminal:

curl -H "Authorization: Bearer YOUR_API_KEY" \
  "https://api.isvalid.dev/v0/pl/pesel?value=44051401358"

A successful response includes the extracted birth date, gender, and age flags:

{
  "valid": true,
  "birthDate": "1990-09-05",
  "gender": "male",
  "isOver15": true,
  "isOver18": true,
  "isOver21": true
}

8. Understanding the response

Valid PESEL

{
  "valid": true,
  "birthDate": "1990-09-05",
  "gender": "male",
  "isOver15": true,
  "isOver18": true,
  "isOver21": true
}

Invalid PESEL

{ "valid": false }

9. Edge cases

// A child born on July 10, 2010
const result = await iv.pl.pesel('10271012345');

if (result.valid) {
  console.log('Birth date:', result.birthDate); // "2010-07-10"
  console.log('Over 15?  :', result.isOver15);  // true (as of 2026)
  console.log('Over 18?  :', result.isOver18);  // false
}

const result = await iv.pl.pesel(userPesel);

if (result.valid) {
  if (result.gender !== userDeclaredGender) {
    console.warn('Gender mismatch: PESEL says', result.gender,
                 'but user declared', userDeclaredGender);
    // Flag for manual review
  }
}

const result = await iv.pl.pesel(applicantPesel);

if (!result.valid) {
  return { error: 'Invalid PESEL number' };
}

// Bank account opening — must be 18+
if (!result.isOver18) {
  return { error: 'Applicant must be at least 18 years old' };
}

// Youth savings account — must be 15+ but under 18
if (result.isOver15 && !result.isOver18) {
  return { accountType: 'youth-savings', birthDate: result.birthDate };
}

// Standard adult account
return { accountType: 'standard', birthDate: result.birthDate };