Stresix vs Loader.io
Loader.io sends load at a site you already host and reports how it responded. Stresix starts a copy of your app on a server it creates, loads it, and measures that server, so it can say what ran out first and whether a bigger server would help. They answer different questions, and plenty of people could use both.
Where the two differ
| Aspect | Stresix | Loader.io |
|---|---|---|
| What gets the load | A copy of your app, started for you from a repository or Docker image on a dedicated server | Your live site or API, at a URL you host |
| Proving it is yours | Nothing to verify: the app runs on a server created for the run | Upload a token file to the host (DNS verification on paid plans) |
| Free | One Lite run: 500 users for 4 minutes on 2 vCPU / 4 GB | 10,000 clients per test, 1 minute tests, 1 host, 2 URLs |
| Paid | Credit packs, $10 for 5 or $40 for 25. Standard (2,500 users, 10 minutes) is 1 credit, Massive (10,000 users, 15 minutes) is 3. No subscription | Pro at $99.95 a month: 100,000 clients per test, 10 minute tests, 10 URLs per test |
| Requests per test | Up to 10, any method, with headers and bodies | 2 URLs on the free plan, 10 on Pro |
| What you get back | p50, p95, requests a second and errors every second, plus CPU per core, memory, restarts and out-of-memory kills on the server | Response times, success and error counts, timeouts and bandwidth |
| The answer | What ran out first and whether a bigger server would help, including "no change needed" | Graphs and response stats to read yourself |
The short version
If you want to know how your live site behaves when a burst of traffic hits it, Loader.io does that and the free plan is generous. If you want to know how much server your app needs, and why it slows down when it does, you need numbers from the server, and Loader.io does not see the server. Its results are what the load side measured: response times, errors, timeouts and bytes.
Response times say it got slow, not why
When p95 climbs, the cause could be CPU, memory, the database, or one thread maxed out while the rest of the server sits idle. Those need different fixes, and only one of them is fixed by paying for a bigger server.
A real example: we load tested Umami on 4 vCPU / 8 GB. It stopped getting faster at about 290 requests a second while the whole server was around 40% busy and Postgres idled. One Node thread was at 100% of a core. Response times alone would have said "slow", and the obvious move would have been a bigger server, which would not have changed anything. Running one process per core would.
One minute is short
A one minute test shows whether the app survives a burst. Some problems take longer to show up: memory that keeps growing, connection pools and queues that fill up, caches that stop helping once the working set outgrows them. A Standard Stresix run ramps up for seven and a half minutes and then holds the peak for two and a half, so slow failures have time to appear and you can see the point where latency starts to climb.
Live site vs a copy
Loading your live site tests everything between the load and your app, including your CDN, your host's network and any rate limiting in front of it. It also loads production, so real visitors feel it, and you need to be able to put a file on the host to verify it.
Stresix loads a copy on known hardware from a second server on the same private network. Production is never touched and nothing needs verifying, and the result maps to a server size you can buy. The trade off is that it does not test your CDN, your host or the path your visitors take across the internet.
When Loader.io is the better fit
You want to hit the live site through its CDN and host, a short burst is enough, the site is something you cannot run from a repository or Docker image (a managed host you only have file access to, for example), or you need more than 10,000 clients.
When Stresix is the better fit
You have an app in a repository or Docker image and the question is whether the server is big enough, or whether you need to pay for more. Stresix builds and starts the app, ramps up to 2,500 or 10,000 simulated users, measures CPU per core and memory per service, and tells you what ran out first, from $1.60 a run with the first one free.
Frequently asked questions
Is Loader.io still free?
Yes. Its free plan allows 10,000 clients per test, 1 minute tests, 1 target host and 2 URLs per test, according to its pricing page in October 2026. Pro is $99.95 a month.
Are Loader.io clients the same as Stresix users?
Not quite. Loader.io has three test types: clients per test spreads the clients across the whole test, clients per second starts that many every second, and maintain client load keeps that many repeating. Stresix users are simulated users running at the same time, each sending its next request as soon as the last one answers, with no pause.
Can Stresix test my live site?
No. Stresix runs your app from a repository or Docker image on a server it creates, behind a firewall. To test production through your CDN, use Loader.io, k6 or another tool that sends load from outside.
Can I use both?
Yes, and it is a reasonable setup: Stresix to size the server before a launch, and a short Loader.io test against the live site after you deploy, to check the CDN and host in front of it.
Related guides
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