Generate PromptPay QR (EMVCo) in Pure Python — No External Libraries

I needed to generate PromptPay payment QR codes automatically in a point-of-sale system. I did not want the hassle of integrating a payment gateway because I only needed to display a QR code for customers to scan and transfer money directly to my own number/account number. At first, I planned to drop in a library, but after sitting down and reading the specification, I found that it was just a matter of assembling an EMVCo-standard string and appending a checksum — something I could write in pure Python with no dependencies at all. So I thought I would write it up here.

Structure: TLV (Tag-Length-Value)

A Thai QR payload is a series of fields joined together. Every field has exactly the same format: a 2-digit tag + a 2-digit length + a value of that length. For example, the version field 000201 reads as tag 00, length 02, value 01.

The main fields needed for a PromptPay transfer:

• 00 = payload version (fixed value 01)
• 01 = type — 11 is a static QR (no fixed amount; enter it after scanning), 12 is dynamic (fixed amount)
• 29 = PromptPay information (contains another layer of sub-fields — phone number/national ID number/e-wallet)
• 53 = currency (764 = baht)
• 54 = amount (included for dynamic QR)
• 58 = country (TH)
• 63 = CRC checksum (must always come last)

Tag 29 contains nested sub-fields — 00 = PromptPay Application ID (A000000677010111), followed by the proxy type: 01 = mobile number, 02 = national ID/tax ID number, 03 = e-wallet ID

Mobile Numbers Need Converting First

PromptPay stores phone numbers in an international format. Remove the leading 0 and add the country code 66, making 13 digits in total — 0812345678 becomes 0066812345678.

def format_target(target):
    t = target.replace('-', '').replace(' ', '')
    if len(t) >= 15:          # e-wallet id 15 หลัก
        return '03', t
    if len(t) == 13:          # เลขบัตร ปชช. / เลขผู้เสียภาษี
        return '02', t
    # เบอร์มือถือ -> 0066 + 9 หลักท้าย
    return '01', '0066' + t[-9:]

CRC16 — The Most Common Checksum Mistake

Field 63 is CRC-16/CCITT-FALSE (poly 0x1021, init 0xFFFF), and the part people get wrong is that the CRC must cover the entire string, including tag 63's own tag and length. That means appending 6304 to the payload first, then running CRC over the whole thing. The result is 4 uppercase hex digits, which you then append to the actual payload.

def crc16(data: str) -> str:
    crc = 0xFFFF
    for ch in data.encode('ascii'):
        crc ^= ch << 8
        for _ in range(8):
            crc = ((crc << 1) ^ 0x1021) & 0xFFFF if (crc & 0x8000) else (crc << 1) & 0xFFFF
    return format(crc, '04X')

Putting It All Together

def field(tag, value):
    return f'{tag}{len(value):02d}{value}'

def promptpay_payload(target, amount=None):
    kind, acc = format_target(target)
    merchant = field('00', 'A000000677010111') + field(kind, acc)
    payload = (
        field('00', '01') +
        field('01', '12' if amount else '11') +
        field('29', merchant) +
        field('53', '764') +
        (field('54', f'{amount:.2f}') if amount else '') +
        field('58', 'TH')
    )
    payload += '6304'                 # tag+len ของ CRC ต้องรวมในตอนคำนวณ
    return payload + crc16(payload)

if __name__ == '__main__':
    import sys
    tgt = sys.argv[1]
    amt = float(sys.argv[2]) if len(sys.argv) > 2 else None
    print(promptpay_payload(tgt, amt))

Converting It to an Image

The payload is ready. Render it as a PNG with any library you like. I use segno because it is pure-python and does not require Pillow:

pip install segno

import segno
qr = segno.make(promptpay_payload('0812345678', 100.00), error='m')
qr.save('pay.png', scale=8, border=2)

Testing to Make Sure It Works

Do not assume it is correct — open a banking app on your phone and actually scan it once before putting it into production. This matters because even a single incorrect CRC character will make the app reject it. I compared my own payload against one generated by a standard tool, byte for byte, until they matched before I was confident.

1. Generate the payload -> compare the string with a reference so every character matches
2. Render PNG -> scan it with 1-2 actual banking apps
3. Check that the recipient account name is correct + the amount matches (if dynamic)
4. Then put it into the POS/web system

So PromptPay QR is not as mysterious as I thought. It is just TLV + CRC that I can assemble myself. The advantage of doing it myself is avoiding a dependency that might stop being maintained one day, and understanding every bit I send out. Anyone who wants to integrate it into their own sales system can adapt it as needed. If I have left anything out or made a typo, I will come back and fix it later.