PESEL Validation in Python

1. What is a PESEL number?

PESEL (Powszechny Elektroniczny System Ewidencji Ludnosci) is the 11-digit national identification number assigned to every person registered in Poland — both Polish citizens and foreign residents. It is issued at birth (or upon registration for foreigners) and remains unchanged for life. PESEL appears on:

• National ID cards (dowod osobisty)
• Tax declarations and invoices
• Social security (ZUS) records
• Medical prescriptions and health insurance
• Bank account applications and KYC forms

Unlike many national IDs that are opaque serial numbers, PESEL encodes real demographic data: date of birth, gender, and a serial number — all verifiable through its built-in checksum.

2. PESEL anatomy

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

Century encoding

Because two digits are not enough to represent the full year, PESEL encodes the century by adding an offset to the month value. This allows the system to handle dates from 1800 all the way through 2299:

For example, the PESEL 44051401358 has month digits 05, which falls in the 01–12 range — meaning this person was born in May of a year in the 1900s. Combined with the year digits 44, the full birth date is 14 May 1944.

3. The checksum algorithm

PESEL uses a weighted MOD-10 checksum. Each of the first 10 digits is multiplied by a fixed weight, and the sum of products modulo 10 produces a value that, combined with the check digit, must equal 0 (mod 10).

1. Multiply each of the first 10 digits by its weight: [1, 3, 7, 9, 1, 3, 7, 9, 1, 3]
2. Sum all products
3. Calculate check = (10 − (sum mod 10)) mod 10
4. Compare the result with the 11th digit

Here is a straightforward Python implementation:

def validate_pesel(pesel: str) -> bool:
    """Validate a Polish PESEL number using the MOD-10 checksum.

    Args:
        pesel: An 11-digit PESEL string.

    Returns:
        True if the checksum is valid, False otherwise.
    """
    if len(pesel) != 11 or not pesel.isdigit():
        return False

    digits = [int(c) for c in pesel]
    weights = [1, 3, 7, 9, 1, 3, 7, 9, 1, 3]

    weighted_sum = sum(d * w for d, w in zip(digits, weights))
    check = (10 - (weighted_sum % 10)) % 10

    return check == digits[10]

4. Why manual validation isn't enough

5. Python code example

Using the isvalid-sdk Python SDK or the requests library. Install with pip install isvalid-sdk or pip install requests.

# pesel_validator.py
import os
from isvalid_sdk import IsValidConfig, create_client

iv = create_client(IsValidConfig(api_key=os.environ["ISVALID_API_KEY"]))

# ── Validate a PESEL number ───────────────────────────────────────────────
result = iv.pl.pesel("44051401358")

if not result["valid"]:
    print("Invalid PESEL")
else:
    print(f"Birth date : {result['birthDate']}")
    print(f"Gender     : {result['gender']}")
    print(f"Over 18    : {result['isOver18']}")
    print(f"Over 21    : {result['isOver21']}")

# app.py (Flask)
import os
from flask import Flask, request, jsonify
from isvalid_sdk import IsValidConfig, create_client

app = Flask(__name__)
iv = create_client(IsValidConfig(api_key=os.environ["ISVALID_API_KEY"]))


@app.post("/verify-identity")
def verify_identity():
    data = request.get_json()
    pesel = data.get("pesel")

    if not pesel:
        return jsonify(error="PESEL is required"), 400

    try:
        result = iv.pl.pesel(pesel)
    except Exception:
        return jsonify(error="PESEL validation service unavailable"), 502

    if not result["valid"]:
        return jsonify(error="Invalid PESEL number"), 400

    if not result.get("isOver18"):
        return jsonify(error="Person must be at least 18 years old"), 403

    return jsonify(
        valid=True,
        birthDate=result["birthDate"],
        gender=result["gender"],
        isOver18=result["isOver18"],
        isOver21=result["isOver21"],
    )

6. cURL example

Validate a PESEL number from the command line:

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

If the PESEL is valid, the API returns the decoded birth date, gender, and age thresholds:

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

7. Understanding the response

Valid PESEL

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

Invalid PESEL

{ "valid": false }

8. Edge cases

# Born in 2002 — month digits are 27 (July, 21st century offset)
result = iv.pl.pesel("02271409862")
print(result["birthDate"])  # "2002-07-14"
print(result["gender"])     # "female" (digit 10 is 6, even)

# Checksum might pass, but Feb 30 does not exist
result = iv.pl.pesel("90023012345")
print(result["valid"])  # False — invalid date

# WRONG — loses the leading zero
pesel = int("02271409862")  # 2271409862 — 10 digits, invalid

# CORRECT — preserve as string
pesel = "02271409862"  # 11 digits, ready for validation

# Person turning 18 today
result = iv.pl.pesel("08031112345")

if result["valid"]:
    if result["isOver18"]:
        print("Person is 18 or older — proceed")
    else:
        print("Person is under 18 — access denied")

import os
from concurrent.futures import ThreadPoolExecutor, as_completed
from isvalid_sdk import IsValidConfig, create_client

iv = create_client(IsValidConfig(api_key=os.environ["ISVALID_API_KEY"]))

pesel_numbers = ["44051401358", "02271409862", "90090515836"]

with ThreadPoolExecutor(max_workers=5) as pool:
    futures = {pool.submit(iv.pl.pesel, p): p for p in pesel_numbers}

    for future in as_completed(futures):
        pesel = futures[future]
        result = future.result()
        status = "valid" if result["valid"] else "invalid"
        print(f"{pesel}: {status}")