The ENIAC was the first general-purpose electronic computer, completed in 1946

The ENIAC (Electronic Numerical Integrator and Computer) is widely recognized as the first general-purpose electronic computer. It was completed in November 1946 at the University of Pennsylvania and was built to calculate artillery firing tables for the U.S. Army during World War II. ENIAC weighed 30 tons, occupied 1,800 square feet of floor space, and used 18,000 vacuum tubes to process information — it was roughly the size of a large room.

Before ENIAC, machines existed that could perform calculations, but they were either mechanical (using gears and levers) or designed for a single specific task. ENIAC was the first to use electronic components to solve a wide range of mathematical problems by reprogramming its circuits. It could perform about 5,000 additions per second, which was revolutionary for the time.

The team that built ENIAC was led by John Mauchly and J. Presper Eckert, with support from dozens of engineers and technicians. The machine ran continuously for nearly a decade and proved that electronic computing was practical and powerful enough to solve real-world problems.

Key Takeaways

  • ENIAC, completed in 1946, is recognized as the first general-purpose electronic computer and could be reprogrammed to solve different types of problems.
  • Earlier calculating machines existed, including mechanical devices and special-purpose electronic machines, but they could not be reprogrammed for different tasks.
  • ENIAC used 18,000 vacuum tubes instead of mechanical parts, which allowed it to perform calculations thousands of times faster than previous machines.
  • The machine was enormous by modern standards — it weighed 30 tons and took up as much space as a large room — yet it had less computing power than a modern smartphone.

Mechanical and electromechanical computers came before ENIAC

The history of computing does not start with ENIAC. Mechanical calculating machines existed for centuries. In the 1600s, Blaise Pascal built the Pascaline, a mechanical calculator that could add and subtract. In the 1800s, Charles Babbage designed the Analytical Engine, which is often called the first computer because it could be programmed using punch cards — but it was never fully built during his lifetime.

In the early 1900s, electromechanical machines became common. These used electric motors and relays (switches controlled by electromagnets) to perform calculations faster than purely mechanical devices. The most famous was the IBM Automatic Sequence Controlled Calculator, also called the Mark I, completed in 1944. It was programmable and could solve complex mathematical problems, but it was still based on mechanical and electromechanical parts, not electronics.

The key difference between these earlier machines and ENIAC was speed and flexibility. Electromechanical machines had moving parts that wore out and limited how fast they could operate. ENIAC had no moving parts — it used vacuum tubes, which are electronic switches that turn on and off millions of times per second.

Other early electronic computers built around the same time

ENIAC was not the only electronic computer being developed in the mid-1940s. In Britain, the Colossus was built in secret during World War II to break German military codes. Colossus was operational by 1944, making it technically earlier than ENIAC, but it was designed for one specific task — breaking the Lorenz cipher — and could not be reprogrammed for other purposes. After the war, the machines were dismantled and the project remained classified for decades.

The ACE (Automatic Computing Engine), also built in Britain, was completed in 1950 and is considered the first stored-program electronic computer — meaning the program instructions were stored in the machine's memory rather than being entered manually each time. This was a major step forward because it made computers much faster and easier to reprogram.

In the United States, UNIVAC I (Universal Automatic Computer) was completed in 1951 and was the first commercial electronic computer sold to customers. It was smaller and more practical than ENIAC, though still enormous by modern standards.

Why vacuum tubes made electronic computing possible

The vacuum tube was the crucial technology that made ENIAC and other early electronic computers work. A vacuum tube is a glass bulb with electrodes inside and most of the air removed. When electricity flows through it, the tube acts as a switch — it can turn on and off almost when ready. This is fundamentally different from a mechanical relay, which uses a physical electromagnet to move a metal switch, a process that takes milliseconds.

Because vacuum tubes could switch on and off millions of times per second, electronic computers could perform calculations far faster than electromechanical machines. The downside was that vacuum tubes generated enormous amounts of heat, burned out frequently, and consumed huge amounts of electricity. ENIAC used so much power that it reportedly dimmed the lights in the Philadelphia neighborhood where it was housed.

Vacuum tubes remained the standard for computers until the late 1950s, when transistors — smaller, more reliable, and more efficient electronic switches — began to replace them. Transistors eventually led to integrated circuits and the microprocessors that power modern computers.

The practical limitations of early computers

ENIAC and its contemporaries were not practical for most organizations. They cost hundreds of thousands of dollars (equivalent to millions in today's money), required entire rooms to house, needed constant maintenance, and demanded specialized operators and programmers. Programming was done by hand, often by rewiring circuits or entering instructions one at a time using punch cards or switches.

Despite these limitations, early computers proved their value. ENIAC was used to calculate ballistics tables, design nuclear weapons, and solve complex mathematical problems in physics and engineering. Universities and large corporations began to see computers as tools worth the investment, even at that scale.

It took until the 1960s and 1970s for computers to become smaller, cheaper, and more accessible. The minicomputer revolution of the 1960s brought computers that could fit in a large cabinet instead of a room. The personal computer revolution of the 1970s and 1980s finally brought computing power to individual users and small businesses.

How we define "first computer" matters

The answer to "when was the first computer made" depends on how you define a computer. If you mean the first electronic general-purpose programmable computer, ENIAC in 1946 is the standard answer. If you mean the first electronic computer of any kind, Colossus in 1944 comes first, though it was special-purpose. If you mean the first stored-program electronic computer, the ACE in 1950 holds that title. If you mean the first commercial computer, UNIVAC I in 1951 is the answer.

Historians and computer scientists often point to different machines depending on which capability they consider most important. What matters for understanding computing history is recognizing that the "first computer" was not a single invention but a series of breakthroughs — mechanical calculation, electromechanical automation, electronic switching, and programmability — that built on each other over centuries.

Frequently Asked Questions

Was ENIAC really the first computer ever made?

ENIAC was the first general-purpose electronic computer, meaning it could be reprogrammed to solve different types of problems. Earlier machines like Colossus were electronic but designed for one specific task. Mechanical and electromechanical computers existed before ENIAC but were much slower and could not use electronics.

How much did ENIAC cost to build?

ENIAC cost approximately $500,000 to build, which is roughly equivalent to $8 million in today's money. This enormous expense was justified by the U.S. military because the machine could calculate ballistics tables in hours instead of the weeks it took human mathematicians.

Could ENIAC run multiple programs at the same time?

No. ENIAC could only run one program at a time. Switching between programs required physically rewiring circuits or manually entering new instructions, a process that took hours. Modern computers can run thousands of programs simultaneously through a technique called multitasking.

How long did ENIAC stay in use?

ENIAC operated from 1946 until 1955, when it was shut down. During its nine years of operation, it ran continuously and was used for scientific calculations, weapons design, and mathematical research. By the time it was retired, newer and more efficient computers had already been built.

Why did early computers use so much electricity?

Vacuum tubes, the electronic switches that made early computers work, generated enormous heat and required constant power to operate. ENIAC used about 150 kilowatts of electricity — roughly equivalent to powering 100 modern homes. This waste of energy was one reason transistors, which were far more efficient, eventually replaced vacuum tubes.