If a launch, a sale, or a campaign is coming up, you may be wondering whether your app will hold up when everyone arrives at once. Load testing lets you check that before the big day by sending a crowd of simulated users to your app or website and measuring how fast your product responds, how often requests fail, and how hard your servers work.
As one of several kinds of performance testing, load testing answers the question most teams ask when traffic is about to jump: will we stay up? If the system can’t keep up, customers are usually the first to find out. When Nintendo Switch 2 preorders opened in April 2025, shoppers at Target, Walmart, and Best Buy reported checkout errors, address verification failures, and payment glitches. A month earlier, OpenAI put temporary limits on ChatGPT’s new image tool because demand outpaced its computing power. Both failures had the same cause: more visitors in one moment than the systems were built for.
Below are the three measurements every load test watches, where load testing ends and stress testing begins, and how to tell whether your product needs one at all. Our real-world example is a mobile game whose server started failing requests when 1,000 players joined within 10 seconds.
What Does Load Testing Measure?
Load testing copies what real visitors do, only at a much larger scale. A tool creates hundreds or thousands of pretend shoppers who log in, browse, and buy all at once. Testers call them concurrent users, meaning people on the system at the same moment.
While that crowd is active, the test tracks three things:
- Response time: how long each person waits for the app to react after a tap or a click. A good test checks the slowest responses too, because a healthy average can hide the visitors left staring at a loading screen.
- Error rate: the share of requests that fail. A request is any single message your app sends to the server, such as one to load a page or save an order. Under too much pressure, some of them come back as errors instead of results.
- Resource use: how hard your servers are working, measured by processing power (CPU) and short-term memory (RAM). Once the processor or the memory runs close to full, everything else starts to slow down.
These measurements only become useful once you compare them with a target. Before testing, the team agrees on limits such as “no request takes longer than 3 seconds with 5,000 users online.” The test then shows whether the system meets the target and, if not, the point where performance starts to slip.
With an app, slowdowns can start on the phone itself or on the server behind it. When we run mobile application testing, we check both for weak spots that affect memory use, stability, and load resilience, meaning how well everything holds up as traffic climbs.
Load Testing vs Stress Testing: What's the Difference?
People often confuse load testing and stress testing because both use the same tools. Each type of test answers a different question. Load testing checks whether everything works well at the traffic you expect, up to your usual peak. Stress testing keeps adding users past that point until the system gives way, so you learn where the limit sits and what fails first.
Both definitions match the performance testing guide from the International Software Testing Qualifications Board (ISTQB), which certifies testers worldwide. ISTQB also describes two related types of testing: spike testing and endurance testing, which many teams call soak testing.
Load
Can we handle our busiest normal day?
Climbs to the expected peak and holds there
Slow pages and failed requests at normal peak traffic
Stress
Where is our breaking point?
Keeps rising past that peak
The real limit, and whether the system fails gently or crashes
Spike
What happens when a crowd arrives all at once?
Jumps suddenly, then drops back
Slow recovery, and systems that can’t add computing power fast enough
Soak
Do we stay healthy over many hours?
Holds steady for a long time
Memory that slowly fills up, and connections that are opened but never closed
A system can pass a gradual load test and still break down when a crowd shows up in seconds, which is why spike testing is worth running on its own. For anything that operates around the clock, our guide to soak testing that finds memory leaks covers how long each run needs to last.
What Our Real Load Testing Caught on a Mobile Game Server
We load tested Couple Up!, a mobile game styled as a reality dating show, when its audience began to grow. The game’s studio wanted to know whether the server would stay quick and stable during a much bigger rush.
Our engineers used Apache JMeter, a free load testing tool, to run four rounds of simulated players:
- 1 player added in 1 second
- 100 players added in 1 second
- 1,000 players added in 10 seconds
- 10,000 players added over about 17 minutes
Every request had to finish within 3 seconds, however many players joined.
The 10-second round, which worked like a small spike test, broke that limit badly. Most requests took longer than 3 seconds, and the slowest needed 27. One in five failed outright with a server error, which in a game can mean a player loses saved progress.
The biggest round went better. When 10,000 players arrived gradually over about 17 minutes, most requests came back on time, although response times kept spiking above 6 seconds. The speed of arrival can matter as much as the number of users, which is why launches and flash sales carry more risk than steady growth. Big-budget studios plan for that risk too. For example, before Battlefield 6 launched in October 2025, EA added login queues because the team expected many players to log in at once.
Alongside the Couple Up! load test, we looked for what was causing the slowdowns. Our DevOps engineer, who specializes in hosting, checked how the server was set up. Meanwhile, our senior Python developer reviewed the server’s code. Together, the two specialists gave the studio a step-by-step plan, from quick, low-cost fixes to bigger design changes.
Getting your own game ready for launch? Follow the same load testing steps in our mobile game testing checklist.
When Do You Need Load Testing?
Run a load test whenever your traffic or your system is about to change in a big way. Typical moments include:
- A product launch or a big new release
- A sale or a seasonal promotion
- A marketing campaign, TV spot, or influencer post that sends a wave of visitors
- A move to new hosting or a major change to how the system is built
- Steady growth that has brought you close to the traffic you last tested for
At the extreme end, Shopify’s network peaked at 489 million requests per minute over Black Friday and Cyber Monday 2025. Most stores will never see traffic on that scale, but a sale works the same way at any size: a crowd arrives in a short window, and the checkout has to hold. If you run an online store, our Black Friday testing service prepares your site for those peak hours.
Load testing isn’t only for products that haven’t launched yet. Couple Up! was already on the market and growing when we tested the game. Load testing usually runs against a copy of the live system, so real customers aren’t affected. The right moment is before your next busy day.
However, not every product needs thousands of simulated users. For the Elsewhen website, we skipped the high-load run on purpose. Elsewhen is a digital product studio, and business-to-business websites rarely get sudden crowds of visitors.
Instead, we checked loading speed and found the real problem. On mobile, some pages took more than 10 seconds to show the main content, while people expect 2 to 3 seconds at most.
As a rule of thumb, start with speed checks on individual pages if your traffic is small and steady. If you expect crowds, even occasionally, a load test is the safer bet.
What Load Testing Tools Do Teams Use?
Load testing tools create the simulated crowd. Every pretend user follows a script describing what a real customer does, such as opening the app, searching, and paying. The tool then runs thousands of those scripts at once. Our engineers choose the right tool for your product, write the scripts, and run the tests.
These are the tools we work with, and what each is good at:
- JMeter is free software from the Apache Software Foundation with a visual interface, and it handles a wide range of website and app tests. We used it on the Couple Up! project.
- k6 comes from Grafana Labs, and its JavaScript tests slot neatly into the automated checks developers run on every release.
- Gatling works well for large, code-based test suites and produces detailed reports after each run.
- Locust runs tests written in Python, a natural fit when your product’s own code uses the same language.
- LoadRunner is a paid tool with a long history in large companies that run complex systems.
- Grafana is a dashboard rather than a load generator, and it lets us watch results live while a test runs.
We choose among these tools based on your product, the software your developers already use, and what needs testing. For a deeper comparison, see our guide to choosing an API performance testing tool. If you’re more concerned about how your product runs on phones, which is separate from how the server handles traffic, our roundup of mobile app performance testing tools covers that side.
How QAwerk Approaches Load Testing
You don’t need an in-house performance testing team to get ready for a traffic spike. QAwerk can join a project at any stage, before launch or during growth. We start with an estimate of the effort, so the scope is clear before work begins. Then we agree on performance targets, build tests around real user behavior, and deliver a report that puts the most urgent fixes first. Because our team includes DevOps engineers and developers alongside testers, the report explains both what went wrong and what to change.
Have a busy date on the calendar? Contact our QA team to schedule load testing before that date arrives.
FAQ
What is load testing in software testing?
Load testing in software testing is a rehearsal for your busiest day. A tool sends thousands of virtual visitors to a website or app at the same time. While that crowd browses, testers check that pages stay fast, logins and payments go through, and the servers have enough power. The results show how much traffic the system can handle before slowing down, so problems get fixed before customers notice.
Is load testing the same as performance testing?
No, although load testing and performance testing are closely related. Performance testing is the broad category that covers speed, stability, and capacity. Load testing is one part of that category and checks how a system behaves at the number of users it’s expected to serve, including normal peaks. Stress, spike, soak, scalability, and volume testing belong to the same family, and each one checks a different kind of risk.
How many users should a load test simulate?
Base the number on your own traffic data. Take the highest traffic your analytics have recorded, then add the extra visitors you expect from the upcoming launch, sale, or campaign. A common approach is to run the test at that expected peak and again slightly above it for a safety margin. A separate spike test is worth running too, with thousands of users joining within seconds.
Can you load test an app that is already live?
Yes, and load testing is often most useful after launch, when growth pushes a product past the traffic it was built for. The safest option is a test environment built to match the real one, so customers never notice the extra load. When that isn’t possible, testers can target the live system during quiet hours, raise traffic gradually, and stop the moment response times or errors cross an agreed limit.
How often should you run a load test?
Load testing works best as a regular habit. Plan a full-scale test ahead of every event expected to bring unusual traffic, and repeat it after major changes to hosting or system design. Teams that ship updates often can also add a small automated load test to each release, which catches a new slowdown on the day it appears.
See how we helped Couple Up!
perform load testing
and significantly improve
server performance
on multiple devices
