swift-check

SwiftCheck — Browser Speed Test Extension

A free, open-source Chrome extension that tests your internet speed directly in the browser. No websites to visit, no accounts, no tracking.

Why SwiftCheck?

Most speed test tools report speeds in Mbps (megabits). When you download a file, your OS shows MB/s (megabytes). This gap causes constant confusion — people see “10 Mbps” on a speed test and wonder why their downloads only show 1.2 MB/s in the browser.

SwiftCheck reports everything in MB/s — the same unit your file manager, browser downloads, and torrent clients use. What you see is exactly what you get.

Mbps  =  1.25 MB/s   ← SwiftCheck shows 1.25
Mbps =  12.5 MB/s   ← SwiftCheck shows 12.5
Gbps   =  125 MB/s    ← SwiftCheck shows 125

Features

MB/s display — real megabytes, not megabits
Live animated speedometer — needle fluctuates like a real analog meter during the test
Dynamic dial scale — probes your speed first, then sets the ceiling to match your line. A 1 MB/s connection gets a 0–3 scale with full needle travel; a 50 MB/s connection gets 0–100
Hardware-level timing — uses PerformanceResourceTiming (browser network stack) not JS wall clock, so measurements reflect actual bytes-on-wire throughput
Adaptive test size — downloads just enough data to get an accurate reading (~3 seconds worth), not a fixed large file
Persistent results — last test result survives popup close/reopen with a timestamp
Light / dark theme — toggle in the header, preference saved across sessions
Zero tracking — no analytics, no data collection, no accounts. One outbound host: speed.cloudflare.com

How It Works

Measurement method

Warmup fetch (1 KB)      → opens TCP/TLS connection
Probe fetch   (256 KB)   → estimates your speed
Set dial ceiling         → ceiling = probe × 2, rounded to clean step
Main fetch    (adaptive) → sized to ~3 seconds at probed speed
Confirm fetch (same)     → second sample for accuracy
Report median            → of probe + main + confirm

Download throughput is calculated using browser-native timing:

// responseStart → responseEnd is measured by the browser network stack
// at the OS/NIC level — not affected by JS execution overhead
const transferMs = entry.responseEnd - entry.responseStart;
const mbPerSec = entry.transferSize / (transferMs / 1000) / 1_000_000;

Upload uses wall-clock timing against speed.cloudflare.com/__up with small payloads (200 KB → 500 KB → 1 MB) for speed.

Live needle animation

A requestAnimationFrame loop runs at 60fps during the test. The needle position is driven by smooth noise (sum of sine waves at irrational frequencies) layered over the live reading, giving it the feel of a real analog meter reacting to line variance. The loop stops and the needle settles smoothly to the final value when the test completes.

Installation (Developer Mode)

Clone or download this repository
Open Chrome and go to chrome://extensions
Enable Developer mode (top-right toggle)
Click Load unpacked
Select the repository folder

The extension icon will appear in your toolbar.

File Structure

swiftcheck/
├── manifest.json     # Chrome extension manifest (MV3)
├── popup.html        # UI — glassmorphism layout, SVG speedometer, CSS variables
├── popup.js          # All logic — measurement, jitter engine, theme, persistence
└── icons/
    ├── icon16.png
    ├── icon48.png
    └── icon128.png

No build step. No dependencies. No npm. Edit and reload.

Modifying

Changing the color theme

All colors are CSS variables at the top of popup.html:

:root {
  --a: #4ade80; /* primary accent  */
  --a2: #86efac; /* secondary accent */
  --a3: #2dd4bf; /* teal accent */
  --a4: #bef264; /* lime accent */
  --bg: #030a06; /* background */
  /* ... */
}
html.light {
  /* light theme overrides */
}

Change --a / --a2 / --a3 and the orb colors (--orb1 through --orb4) to retheme the entire extension.

Changing the test server

The extension currently uses Cloudflare’s speed test infrastructure. To use a different server, replace the fetch URLs in popup.js:

const BASE = "https://speed.cloudflare.com/__down";
// upload endpoint:
("https://speed.cloudflare.com/__up");

Any server that returns raw bytes on GET and accepts POST bodies will work. Update host_permissions in manifest.json accordingly.

Adjusting dial ceiling steps

The dynamic ceiling snaps to one of these clean values (in MB/s):

[1, 2, 3, 5, 8, 10, 15, 20, 30, 50, 75, 100, 125, 200];

Edit the pickCeiling() function in popup.js to add or change steps.

Adjusting jitter intensity

In popup.js, the startJitter() function controls wobble amplitude:

const amplitude = Math.max(dialCeiling * 0.06, 0.05);

Increase 0.06 for more dramatic needle movement, decrease for calmer behavior. The 0.05 floor keeps a tiny flicker even on near-zero readings.

Permissions

Permission	Why
`storage`	Save last result and theme preference across popup opens
`https://speed.cloudflare.com/*`	Download and upload test traffic

No other permissions. No broad host access.

Privacy

SwiftCheck makes fetch requests only to speed.cloudflare.com during a test. These requests are indistinguishable from normal browser traffic to that domain. No user data, IP addresses, or results are sent anywhere. Everything is computed locally and stored in chrome.storage.local on your own device.

Contributing

Pull requests welcome. A few things to keep in mind:

No build tooling — keep it plain HTML/CSS/JS, loadable directly as an unpacked extension
No external libraries — the point is a self-contained, auditable codebase
MB/s stays MB/s — do not add Mbps display, it defeats the purpose

License

MIT — do whatever you want, just keep the attribution.

Credits

Built with Cloudflare Speed Test infrastructure for the test endpoints.
Fonts: Barlow Condensed, Inter, JetBrains Mono via Google Fonts.