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An online paper format calculator helps designers, layout specialists, and print professionals quickly convert standard A3, A4, and A5 sizes into pixels. You can set your own DPI value, from 1 to 1000, and the tool will calculate the exact width and height for the selected format. That is especially useful when preparing print layouts, creating PDFs, or calculating graphic elements.
Just pick the format you need, enter the DPI (96 by default, which is the standard screen value), and solve a simple arithmetic expression to protect against bots. The result appears instantly. The service is mobile-friendly and works without reloading the page — all in clean PHP with a modern interface.
The tasks I set for myself were:
- Convert A3, A4, and A5 paper sizes into pixels.
- DPI had to be taken into account. DPI is dots per inch: the higher the value, the more pixels you get in the final size. I set 96 by default (for example, Paint.NET uses this value by default).
- There had to be a bot-check captcha (simple, written quickly and pragmatically).
- Scroll the screen down to the calculation results.
PHP was chosen because the captcha requires storing the correct answer between requests through sessions, and pure client-side JavaScript cannot do that without a server — any user could easily see the correct answer in the page source and bypass the protection.
Strictly speaking, I could have written this in any other language I know as well: Assembler, ASP.NET, C++, C#, and a few others.
For example, here is a compact piece of Linux x86-64 Assembler code that asks for the DPI, calculates the width and height of A4 in pixels, and prints the result. For simplicity, we use integer fixed-point arithmetic (scaling):
; Fragment: DPI input → A4 pixel calculation → output
; Compilation: nasm -f elf64 calc.asm && ld calc.o -o calc
section .data
msg_dpi db "Enter DPI (1-1000): ", 0
msg_w db "A4 width: ", 0
msg_h db "A4 height: ", 0
newline db 10, 0
format db "%d", 0
out_buf db " ", 0 ; number buffer (8 bytes)
section .bss
dpi resd 1
width resd 1
height resd 1
section .text
extern printf, scanf ; using standard C functions (for convenience)
global main
main:
; --- DPI input ---
push rbp
mov rbp, rsp
sub rsp, 16
mov rdi, msg_dpi
call printf
mov rdi, format
mov rsi, dpi
call scanf
; --- A4 width calculation = (210 / 25.4) * DPI ---
; Using fixed point: 210 / 25.4 = 2100 / 254 = 8.2677...
; Multiply DPI * 2100, then divide by 254.
mov eax, [dpi]
imul eax, 2100 ; DPI * 2100
xor edx, edx
mov ecx, 254
div ecx ; eax = width in pixels
mov [width], eax
; --- A4 height calculation = (297 / 25.4) * DPI ---
mov eax, [dpi]
imul eax, 2970 ; 297 * 10 = 2970
mov ecx, 254
xor edx, edx
div ecx
mov [height], eax
; --- output width ---
mov rdi, msg_w
call printf
mov eax, [width]
mov rsi, out_buf
call int_to_str ; convert number to string (not shown, but assumed)
mov rdi, out_buf
call printf
mov rdi, newline
call printf
; --- output height ---
mov rdi, msg_h
call printf
mov eax, [height]
... same idea ...
leave
ret
; (the int_to_str function is omitted for brevity, but it is easy to add)Source code
Here is the source for our PHP / HTML5 / CSS3 calculator. There is no need to worry about icons. It is 2026 now, and all browsers handle standard Unicode icons just fine, so converting them to HEX codes is not necessary:
<?php
session_start();
// Generate CAPTCHA
function generate_captcha() {
$num1 = rand(1, 9);
$num2 = rand(1, 9);
$op = rand(0, 1) ? '+' : '-';
if ($op == '-' && $num1 < $num2) {
list($num1, $num2) = [$num2, $num1];
}
$expression = "$num1 $op $num2";
$result = ($op == '+') ? $num1 + $num2 : $num1 - $num2;
$_SESSION['captcha'] = $result;
return $expression;
}
// Get POST parameters
$format = $_POST['format'] ?? 'A4';
$dpi = isset($_POST['dpi']) ? (int)$_POST['dpi'] : 96;
$captcha_input = $_POST['captcha_input'] ?? '';
$captcha_ok = false;
$result = null;
$error = '';
$should_scroll = false;
// Validate CAPTCHA on submit
if ($_SERVER['REQUEST_METHOD'] === 'POST') {
if (isset($_POST['calculate'])) {
if ((int)$captcha_input === ($_SESSION['captcha'] ?? 0)) {
$captcha_ok = true;
$should_scroll = true;
if ($dpi < 1) $dpi = 1;
if ($dpi > 1000) $dpi = 1000;
$sizes = [
'A3' => ['width' => 297, 'height' => 420],
'A4' => ['width' => 210, 'height' => 297],
'A5' => ['width' => 148, 'height' => 210],
];
$size = $sizes[$format] ?? $sizes['A4'];
$width_px = round(($size['width'] / 25.4) * $dpi);
$height_px = round(($size['height'] / 25.4) * $dpi);
$result = ['width' => $width_px, 'height' => $height_px, 'dpi' => $dpi, 'format' => $format];
} else {
$error = 'Incorrect CAPTCHA answer. Please try again.';
}
}
$captcha_expr = generate_captcha();
} else {
$captcha_expr = generate_captcha();
}
?>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover">
<title>DPI Calculator: A4, A5, A3 sizes in pixels</title>
<meta name="description" content="Convert A4, A5, A3 paper sizes to pixels based on DPI. Simple online calculator with CAPTCHA, perfect for designers and print professionals.">
<style>
* {
margin: 0;
padding: 0;
box-sizing: border-box;
}
body {
font-family: 'Segoe UI', Roboto, system-ui, -apple-system, sans-serif;
background: linear-gradient(145deg, #f0f4fa 0%, #e2e8f0 100%);
min-height: 100vh;
padding: 2rem 1rem;
display: flex;
justify-content: center;
align-items: center;
}
.card {
max-width: 650px;
width: 100%;
background: rgba(255,255,255,0.96);
backdrop-filter: blur(2px);
border-radius: 2rem;
box-shadow: 0 25px 45px rgba(0,0,0,0.1), 0 0 0 1px rgba(255,255,255,0.5);
overflow: hidden;
transition: transform 0.2s;
}
.card:hover {
transform: scale(1.01);
}
.header {
background: #1e2a3a;
padding: 1.5rem 2rem;
color: white;
}
.header h1 {
font-weight: 600;
font-size: 1.8rem;
letter-spacing: -0.3px;
}
.header p {
opacity: 0.8;
margin-top: 0.5rem;
font-size: 0.95rem;
}
.content {
padding: 2rem;
}
.seo-text {
background: #f8fafc;
padding: 1.2rem;
border-radius: 1.2rem;
margin-bottom: 2rem;
border-left: 5px solid #2c7da0;
font-size: 0.95rem;
line-height: 1.5;
color: #1e2a3a;
}
.seo-text p {
margin-bottom: 0.8rem;
}
.seo-text p:last-child {
margin-bottom: 0;
}
.form-group {
margin-bottom: 1.5rem;
}
label {
font-weight: 600;
display: block;
margin-bottom: 0.6rem;
color: #0f172a;
}
.radio-group {
display: flex;
flex-wrap: wrap;
gap: 1.5rem;
background: #f1f5f9;
padding: 1rem;
border-radius: 1.2rem;
}
.radio-group label {
display: inline-flex;
align-items: center;
gap: 0.5rem;
font-weight: normal;
cursor: pointer;
}
input[type="number"] {
width: 100%;
padding: 0.8rem 1rem;
border: 2px solid #cbd5e1;
border-radius: 1.5rem;
font-size: 1rem;
transition: all 0.2s;
background: white;
}
input[type="number"]:focus {
border-color: #2c7da0;
outline: none;
box-shadow: 0 0 0 3px rgba(44,125,160,0.2);
}
.captcha-box {
background: #eef2ff;
border-radius: 1.5rem;
padding: 1rem 1.5rem;
display: flex;
align-items: center;
flex-wrap: wrap;
gap: 1rem;
margin-bottom: 1.8rem;
}
.captcha-expression {
font-size: 2rem;
font-weight: 700;
font-family: monospace;
background: white;
padding: 0.3rem 1.2rem;
border-radius: 3rem;
border: 2px solid #2c7da0;
}
.captcha-input {
flex: 1;
min-width: 100px;
padding: 0.7rem;
font-size: 1.2rem;
text-align: center;
border: 2px solid #cbd5e1;
border-radius: 2rem;
}
button {
background: #2c7da0;
color: white;
border: none;
width: 100%;
padding: 1rem;
font-size: 1.2rem;
font-weight: 600;
border-radius: 3rem;
cursor: pointer;
transition: all 0.2s;
box-shadow: 0 4px 6px rgba(0,0,0,0.1);
}
button:hover {
background: #1f5e7a;
transform: translateY(-2px);
}
.result {
margin-top: 2rem;
background: #e6f7f0;
padding: 1.2rem;
border-radius: 1.2rem;
border-left: 6px solid #2c7da0;
}
.result h3 {
margin-bottom: 0.8rem;
color: #0f3b2c;
}
.result p {
font-size: 1.1rem;
margin: 0.4rem 0;
font-weight: 500;
}
.error {
background: #fee2e2;
color: #b91c1c;
padding: 0.8rem;
border-radius: 1rem;
margin-top: 1rem;
text-align: center;
}
footer {
text-align: center;
padding: 1rem;
font-size: 0.8rem;
color: #64748b;
border-top: 1px solid #e2e8f0;
margin-top: 1.5rem;
}
@media (max-width: 550px) {
.card {
border-radius: 1.2rem;
}
.content {
padding: 1.5rem;
}
.radio-group {
gap: 0.8rem;
}
.captcha-expression {
font-size: 1.5rem;
padding: 0.2rem 1rem;
}
}
</style>
</head>
<body>
<div class="card">
<div class="header">
<h1>📐 A3, A4, A5 Pixel Size Calculator (DPI based)</h1>
<p>Convert millimeters to pixels using DPI (dots per inch)</p>
</div>
<div class="content">
<div class="seo-text">
<p><strong>Online paper size calculator</strong> helps designers, layout artists, and print professionals quickly convert standard A3, A4, A5 sizes into pixels. Enter your desired DPI (1–1000), and the tool will calculate the exact width and height for the selected format. This is especially useful when preparing print layouts, creating PDFs, or calculating graphic elements.</p>
<p>Simply select the paper size, enter DPI (default 96, standard screen resolution), and solve a simple arithmetic expression to prevent bots. The result appears instantly. The service is fully mobile‑friendly and works without page reloads — pure PHP with a modern interface.</p>
</div>
<form method="post" id="calcForm">
<div class="form-group">
<label>📄 Choose paper size:</label>
<div class="radio-group">
<label><input type="radio" name="format" value="A3" <?= (isset($format) && $format === 'A3') ? 'checked' : '' ?>> A3 (297×420 mm)</label>
<label><input type="radio" name="format" value="A4" <?= (!isset($format) || $format === 'A4') ? 'checked' : '' ?>> A4 (210×297 mm)</label>
<label><input type="radio" name="format" value="A5" <?= (isset($format) && $format === 'A5') ? 'checked' : '' ?>> A5 (148×210 mm)</label>
</div>
</div>
<div class="form-group">
<label>🖨️ DPI (dots per inch) — from 1 to 1000:</label>
<input type="number" name="dpi" value="<?= htmlspecialchars($dpi) ?>" step="1" min="1" max="1000" required>
</div>
<div class="captcha-box">
<span class="captcha-expression"><?= $captcha_expr ?> = ?</span>
<input type="text" name="captcha_input" class="captcha-input" placeholder="Answer" required autocomplete="off">
</div>
<button type="submit" name="calculate">Calculate pixel dimensions</button>
</form>
<?php if ($error): ?>
<div class="error">⚠️ <?= htmlspecialchars($error) ?></div>
<?php endif; ?>
<?php if ($result && $captcha_ok): ?>
<div class="result" id="resultBlock">
<h3>📏 Result for <?= htmlspecialchars($result['format']) ?> (DPI = <?= $result['dpi'] ?>):</h3>
<p>📐 Width: <strong><?= number_format($result['width'], 0, ',', ' ') ?> px</strong></p>
<p>📐 Height: <strong><?= number_format($result['height'], 0, ',', ' ') ?> px</strong></p>
<p style="font-size:0.85rem; margin-top:0.8rem;">* Formula: (mm / 25.4) × DPI</p>
</div>
<?php endif; ?>
<footer>
Accuracy up to whole pixel. Based on ISO 216 international standard.
</footer>
</div>
</div>
<?php if ($should_scroll && $result && $captcha_ok): ?>
<script>
(function() {
function scrollToResult() {
var resultBlock = document.getElementById('resultBlock');
if (resultBlock) {
setTimeout(function() {
resultBlock.scrollIntoView({ behavior: 'smooth', block: 'start' });
var offset = 80;
var elementPosition = resultBlock.getBoundingClientRect().top + window.pageYOffset;
window.scrollTo({ top: elementPosition - offset, behavior: 'smooth' });
}, 100);
}
}
if (document.readyState === 'loading') {
document.addEventListener('DOMContentLoaded', scrollToResult);
} else {
scrollToResult();
}
})();
</script>
<?php endif; ?>
</body>
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I’m Ethan Carter, an American developer and technical writer with more than 20 years of experience in systems and application programming. My core specialty is low-level development in Assembler: 22 years of hands-on work, including deep experience in code optimization, CPU architecture, and performance-critical solutions. I also hold a PhD in Assembler and have spent more than 18 years working with ASP.NET, building enterprise web systems, APIs, and scalable backend solutions.
In addition, I have 9 years of experience in C++ and C#, along with 7 years of hands-on microcontroller programming in Assembler. Thanks to this mix of academic background and practical engineering experience, I can write about software architecture, low-level optimization, and modern development in a way that makes complex technical topics clear for a professional audience.
