What is Sample Size Calculator?
Sample Size Calculator is a free, browser-based tool in the Marketing Tools suite. Compute the required sample size for surveys and A/B tests using Cochran's formula. Pick confidence level, margin of error, expected proportion, and optional finite population size.
The headline benefit: find out how many respondents you need using cochran's formula.
Unlike most online tools that upload your file to a server, process it, and send it back, Sample Size Calculator runs entirely in your browser. Open DevTools → Network while using it and you'll see zero file-upload requests — only static assets (JavaScript, CSS, fonts) load. Your data never leaves your device.
Why use this sample size calculator?
Three reasons EasyFileKit's Sample Size Calculator stands out from the crowd:
- **Private by design** — all processing happens locally via JavaScript and WebAssembly. No server ever sees your input.
- **Instant** — no upload wait, no queue, no server round-trip. Results appear the moment you act.
- **Free & unlimited** — no accounts, no watermarks, no daily caps. Use it as many times as you like.
How to use Sample Size Calculator — step by step
Here's the complete walkthrough. Everything happens instantly in your browser:
- **Step 1.** Pick a confidence level (90%, 95%, or 99%).
- **Step 2.** Set your desired margin of error (e.g. 5% means ±5 percentage points).
- **Step 3.** Set the expected proportion (use 50% for the most conservative sample size).
- **Step 4.** Optionally enter a finite population size for the corrected sample.
That's it. No sign-up, no upload bar, no waiting. If something doesn't work as expected, check the FAQ below.
Common use cases for Sample Size Calculator
People reach for Sample Size Calculator in a few recurring situations:
- When you need the result **now** and can't wait for a server-based tool to upload, queue, and process your file.
- When your file is **private or sensitive** — financial documents, personal photos, medical PDFs — and you don't want it travelling across the internet.
- When you're on a **slow or metered connection** — uploading a 50 MB file just to compress it makes no sense when the same work can happen locally.
- When you've hit the **daily limit or paywall** on another "free" tool site.
Privacy: what actually happens to your data
This is the single most important point about Sample Size Calculator, so it deserves its own section.
When you use this tool, your input is processed by JavaScript running in your browser tab. The code is downloaded once (cached afterwards) and executes locally on your CPU. At no point is your file, your text, or your input data transmitted to any server.
You can verify this yourself in under 30 seconds:
- Open Sample Size Calculator in your browser.
- Press F12 to open DevTools.
- Switch to the Network tab and tick "Disable cache".
- Use the tool — drop a file, type text, whatever the tool needs.
- Watch the Network log. You'll see only static assets (JS, CSS, fonts, icons). No request contains your data.
This isn't a setting you toggle or a promise in a privacy policy — it's how the tool is architecturally built. There is no upload endpoint to call.
Frequently asked questions about Sample Size Calculator
Q: What is Cochran's formula?
A: n₀ = (Z² · p · (1−p)) / E² — where Z is the z-critical value for your confidence level, p is the expected proportion, and E is the desired margin of error. The finite-population correction is n = n₀ / (1 + (n₀−1)/N).
Q: Why use 50% as the default proportion?
A: When p = 0.5, the product p·(1−p) is at its maximum (0.25), giving the largest sample size. Without prior data, this is the most conservative choice — it ensures your sample is large enough regardless of the true proportion.
Q: What's a good margin of error?
A: ±5% is standard for most surveys. ±3% is used for high-precision polling (e.g. election polls). ±10% is acceptable for rough directional feedback. Halving the margin quadruples the required sample size.
Q: When should I use finite population correction?
A: When your population is small (typically fewer than 100,000) and you're sampling a meaningful fraction of it. For example, surveying 500 employees out of 2,000 total — without correction, the formula overestimates the needed sample.
Q: What confidence level should I use?
A: 95% is the default for most surveys. Use 99% for high-stakes research where you need extra certainty, or 90% for quick directional reads.
Sample Size Calculator: EasyFileKit vs server-based tools
Most "free" online tools that do what Sample Size Calculator does follow the same model: you upload your file to their server, they process it with a backend script, then they send the result back. Here's the honest comparison:
| | EasyFileKit | Server-based tools |
|---|---|---|
| **Your file leaves your device?** | Never | Yes, uploaded to a server |
| **Speed** | Instant (no upload) | Slower (upload + queue + download) |
| **Privacy** | Complete | Your file is on someone else's computer |
| **Cost** | Free, unlimited | Often capped or "premium" gated |
| **Works offline** | Yes (PWA) | No |
Server-based tools aren't evil — they exist because some tasks genuinely need heavy backend compute. But for everything Sample Size Calculator does, client-side processing is strictly better for you.
Under the hood: how Sample Size Calculator works
Sample Size Calculator is built with modern browser APIs. Depending on what it does, it may use:
- **Canvas API** — for image manipulation (pixel-level access, filters, resizing).
- **Web Crypto API** — native, hardware-accelerated cryptography (AES-GCM, SHA-256, PBKDF2) for any encryption or hashing.
- **pdf-lib / pdf.js** — fully client-side PDF creation and rendering.
- **MediaRecorder API** — for capturing screen, audio, and video.
- **WebAssembly** — for heavy codecs (image compression, media processing).
All of these run inside your browser's sandbox. They cannot access your filesystem (beyond files you explicitly choose), cannot make network requests with your data, and cannot run persistently in the background.
Pro tips for getting the most out of Sample Size Calculator
- **Bookmark the tool** — it works offline once cached, so you can use it even without a connection.
- **Install EasyFileKit as a PWA** — open the browser menu and choose "Install app" for a standalone window and offline access.
- **Use it on mobile** — every tool is fully responsive and works on phones and tablets, not just desktops.
- **No file size anxiety** — because nothing uploads, you can process large files that server-based tools would reject or charge for.
Try Sample Size Calculator now
The tool is right above this article — scroll up and start using it. No sign-up, no upload, no limits.
If you found Sample Size Calculator useful, explore the rest of the Marketing Tools suite — there are more tools that work the same private, instant, free way. And if you have a question that isn't covered in the FAQ above, the About page has our contact email.