How to Perform API Load Testing Using cURL

APIs are the backbone of modern applications. From processing payments and retrieving user data to powering mobile apps and SaaS platforms, APIs need to perform reliably even when hundreds or thousands of users access them simultaneously.

But how do you know whether your API can handle that traffic?

That’s where API load testing comes in.

If you already use cURL to send HTTP requests and test API endpoints, you have a starting point. However, sending a single request with cURL doesn't tell you how your API will behave under heavy traffic.

In this guide, we'll explain how to use cURL for API testing, why basic cURL requests aren't enough for load testing, and how you can turn a simple API request into a more meaningful performance test.

What Is API Load Testing?

API load testing is the process of sending multiple requests to an API under a controlled workload to understand how it performs under traffic.

It helps developers measure:

• Response time: How quickly the API responds.
• Throughput: How many requests the API processes over time.
• Requests per second (RPS): The rate at which requests reach the API.
• Error rate: The percentage of requests that fail.
• Latency: The time it takes for a request to receive a response.

Load testing helps identify performance bottlenecks before they affect real users.

For example, your API might respond in 100 milliseconds when tested manually. But when 500 users access it simultaneously, response times could increase significantly or requests could start failing.

Without load testing, these issues may remain hidden until your application experiences real traffic.

What Is cURL and How Is It Used for API Testing?

cURL is a command-line tool that allows you to transfer data to and from servers using protocols such as HTTP and HTTPS.

Developers commonly use cURL to test endpoints, inspect responses, send request headers, and submit data to APIs.

Here's a simple example:

curl -X GET "https://api.example.com/users"

This sends a GET request to the specified endpoint.

If your API requires authentication, you might use:

curl -X GET "https://api.example.com/users" \
  -H "Authorization: Bearer YOUR_TOKEN"

You can also send a POST request with JSON data:

curl -X POST "https://api.example.com/users" \
  -H "Content-Type: application/json" \
  -d '{"name":"John","email":"john@example.com"}'

These commands are useful for checking whether an endpoint works correctly.

However, a successful individual request doesn't guarantee that your API will perform well under heavy traffic.

Can You Perform Load Testing Using cURL?

Yes, but there are limitations.

You can use shell scripts to execute repeated cURL requests. For example:

for i in {1..100}; do
  curl -s -o /dev/null \
    -w "HTTP %{http_code} | Time %{time_total}s\n" \
    "https://api.example.com/users"
done

This runs 100 requests sequentially and prints the HTTP status code and total response time for each request.

However, sequential requests are not the same as a realistic concurrent load test.

This script sends one request at a time. It doesn't accurately simulate hundreds of users making requests simultaneously, and it doesn't provide the comprehensive performance analysis required for a proper load test.

You could build more complex scripts to introduce concurrency, but doing so requires additional configuration, error handling, result collection, and performance analysis.

For a more complete test, you need a tool designed to generate controlled traffic and analyze how your API responds.

What Should You Measure During an API Load Test?

When testing an API, focus on the metrics that reveal how it behaves under increasing traffic.

Requests per Second (RPS)

RPS measures how many requests your API receives or processes each second.

Testing different RPS levels helps you understand how the API performs as demand increases.

Response Time and Latency

Response time shows how long requests take to complete.

Pay attention to percentile measurements such as p95 and p99. These help reveal slow responses that average response time might hide.

Error Rate

Monitor HTTP errors, timeouts, and failed requests. A server may appear responsive while silently failing a growing percentage of requests.

Throughput

Throughput measures how much work the system successfully processes over time. It helps you understand whether increasing traffic actually results in more completed requests.

Performance Under Increasing Load

Gradually increase traffic and observe when response times rise sharply or errors become more frequent. This can help identify the system's capacity limits.

A Simpler Way to Load Test cURL Requests

If you already have a working cURL request, you don't necessarily need to build an entire testing script from scratch.

Loadcurl is designed to help developers turn API requests into configurable load tests through a dashboard.

Instead of manually writing and maintaining scripts for every test, you can use a workflow built around your existing API request.

A typical workflow looks like this:

1. Copy your cURL request. Start with the endpoint, headers, authentication, and request body you want to test.
2. Import the request into Loadcurl. Use the request as the starting point for configuring your test.
3. Configure the workload. Set your target request rate, ramp-up, and test duration according to your testing goals.
4. Run the load test. Generate controlled traffic against an API you own or have permission to test.
5. Analyze the results. Review response times, throughput, and errors to understand how the API performs under load.

This approach makes it easier to move from manually checking whether an endpoint works to measuring how it behaves under a defined workload.

When Should You Load Test Your API?

API load testing is especially useful when:

• You're preparing to launch a new application or feature.
• You expect a marketing campaign or product launch to increase traffic.
• You're introducing a new API endpoint or making significant code changes.
• Your application is experiencing slow responses.
• You're evaluating whether your current infrastructure can handle more users.
• You want to establish performance benchmarks before production.

Testing before a major release can help you discover bottlenecks while there's still time to address them.

Always begin with a controlled workload and monitor the target system. Avoid sending high traffic to production systems without appropriate authorization and safeguards.

Final Thoughts

cURL is an excellent tool for testing individual API requests, validating endpoints, and troubleshooting HTTP communication. But checking one request at a time doesn't tell you how an API will behave when traffic increases.

To understand API performance, you need controlled workloads, reliable measurements, and clear reporting.

If you already have a cURL request, the next step is to turn it into a structured load test.

Ready to test your API under load? Start testing your API with Loadcurl.

Frequently Asked Questions

Can cURL be used for API load testing?

Yes. You can run repeated cURL requests using scripts, but more complex load tests require additional work to manage concurrency, traffic levels, and result analysis.

What is the difference between API testing and API load testing?

API testing checks whether an endpoint behaves correctly. API load testing measures how the endpoint performs under a specified level of traffic.

What does RPS mean in API testing?

RPS stands for requests per second. It measures the rate at which requests are sent to or processed by an API.

Why is API load testing important?

It helps identify performance bottlenecks, slow responses, capacity limits, and errors before they affect application users.

Do I need to write scripts to load test an API?

Not necessarily. Some load-testing tools provide dashboard-based configuration, allowing developers to configure workloads without building an entire test script from scratch.

Related guides

• How to Perform Load Testing Using cURL
• What Is API Load Testing?
• How to Calculate RPS for API Load Testing
• What Is p95 and p99 Latency?