The First Computer Was the ENIAC, Completed in 1946

The ENIAC (Electronic Numerical Integrator and Computer) was the first general-purpose electronic computer, finished in February 1946 at the University of Pennsylvania. It was built by J. Presper Eckert and John Mauchly during World War II to calculate artillery firing tables for the U.S. Army. Before ENIAC, "computer" meant a person who did math by hand — the machine took that job and did it faster than any human could.

ENIAC was enormous: it weighed 30 tons, occupied 1,800 square feet of floor space, and used 18,000 vacuum tubes to process information. It consumed 150 kilowatts of electricity and generated so much heat that the room it occupied had to be air-conditioned. Despite its size and power hunger, ENIAC could perform 5,000 additions per second — a speed that seemed miraculous at the time.

The machine was not the only early computer, but it was the first to work reliably and solve real problems. Other experimental machines existed before it, but ENIAC proved that electronic computing could be practical and useful beyond theory.

Key Takeaways

  • ENIAC, completed in 1946, was the first general-purpose electronic computer and could perform 5,000 calculations per second.
  • Before ENIAC, the word "computer" referred to a person doing math by hand, not a machine.
  • ENIAC weighed 30 tons and used 18,000 vacuum tubes, making it impractical for most purposes but revolutionary for military and scientific work.
  • Earlier experimental machines existed, but ENIAC was the first to operate reliably and solve actual problems on demand.
  • The invention of the transistor in 1947 and later the integrated circuit made computers smaller, faster, and affordable enough for widespread use.

Earlier Machines That Led to ENIAC

ENIAC did not appear from nowhere. Mechanical and electromechanical computers existed before it. The Analytical Engine, designed by Charles Babbage in the 1830s, was never built during his lifetime but contained the core ideas of modern computing: a processor, memory, and the ability to follow instructions in sequence. Babbage's work remained largely unknown until decades after his death.

In the 1930s and 1940s, several researchers built machines that moved closer to electronic computing. Konrad Zuse in Germany built the Z3 in 1941, which used electromechanical relays and could perform calculations automatically. In the United States, researchers at Bell Labs and other institutions were experimenting with vacuum tubes as switches. These earlier efforts proved that electronic switching could work, but they were not yet general-purpose computers that could solve many different kinds of problems.

ENIAC built on this foundation. Eckert and Mauchly took the vacuum tube technology that others had tested and scaled it up to create a machine that could be reprogrammed to solve different problems without rewiring it by hand.

How ENIAC Worked and Why It Mattered

ENIAC used vacuum tubes as electronic switches instead of mechanical relays or gears. A vacuum tube is a glass bulb with electrodes inside; when electricity flows through it in the right way, it acts as a switch that turns on and off billions of times per second. This switching speed is what made ENIAC fast compared to mechanical machines.

The machine was programmed by physically plugging cables into a plugboard and flipping switches — a process that took days or weeks. Once set up, ENIAC could run calculations without human intervention. It stored numbers in its vacuum tubes and could add, subtract, multiply, divide, and perform more complex operations. For the first time, a machine could solve problems that would take a human mathematician weeks or months to work through by hand.

ENIAC mattered because it proved that large-scale electronic computing was possible and useful. The military funded it because calculating artillery tables by hand was slow and error-prone. Scientists and engineers saw when ready that such a machine could solve problems in physics, engineering, and mathematics that had been out of reach. Within a few years, universities and government agencies wanted their own computers.

The Invention of the Transistor Changed Everything

ENIAC's vacuum tubes were reliable enough to work, but they were power-hungry, generated enormous heat, and eventually burned out and needed replacement. In December 1947, just months after ENIAC was completed, researchers at Bell Labs invented the transistor — a tiny device that could do the same job as a vacuum tube but used far less power and generated far less heat.

The transistor was smaller, cheaper to manufacture, and more reliable than vacuum tubes. Over the next decade, computers built with transistors replaced those built with tubes. Transistor-based computers were fast enough to be useful for business and science, small enough to fit in a room instead of filling a building, and affordable enough that companies and universities could buy them.

The transistor made the computer revolution possible. Without it, computers would have remained expensive, room-sized machines used only by governments and large institutions. With it, computers became smaller, cheaper, and more powerful with each passing year.

From Room-Sized Machines to Personal Computers

In the 1950s and 1960s, computers were still large and expensive. IBM, founded in 1911 as a maker of punch-card machines, became the dominant computer manufacturer. Their machines cost hundreds of thousands of dollars and were used by banks, insurance companies, and government agencies. Programming them still required specialized knowledge and took weeks of setup.

The integrated circuit, invented in 1958, put thousands of transistors on a single chip of silicon. This made computers even smaller and cheaper. By the 1970s, computers small enough and cheap enough for individual use became possible. The Apple II, released in 1977, was one of the first computers sold to consumers. The IBM Personal Computer, released in 1981, brought computing to offices and homes across the world.

Each step — from ENIAC's vacuum tubes to transistors to integrated circuits — made computers faster, smaller, and more affordable. The computer you use today contains billions of transistors on chips smaller than your fingernail, a direct descendant of the technology that replaced ENIAC's vacuum tubes.

Why ENIAC Gets the Credit

ENIAC is called the first computer because it was the first machine that combined several key features: it used electronics (not mechanics), it could be reprogrammed to solve different problems, it was large and powerful enough to solve real problems, and it actually worked reliably enough to be used. Earlier machines had some of these features but not all of them together.

Some historians point to other machines as "first" depending on how you define computer. The Z3, built in Germany in 1941, was programmable and electronic but used relays instead of vacuum tubes and was not as fast. The Colossus, built in Britain during World War II to break German codes, was electronic and fast but was not general-purpose — it could only solve one type of problem. ENIAC was the first to do everything well enough to be called a true general-purpose electronic computer.

The credit also belongs to the people who built it. Eckert and Mauchly are usually named as the inventors, but ENIAC required a team of engineers, technicians, and mathematicians. Betty Snyder, Jean Jennings, and other women programmed ENIAC and figured out how to make it work reliably — work that was essential but often overlooked in early histories of computing.

What Happened to ENIAC After 1946

ENIAC ran continuously at the University of Pennsylvania until 1955, when it was shut down. During those nine years, it solved problems in ballistics, weather prediction, and atomic energy research. It proved that electronic computers could be trusted with important work and that the investment in building them was worthwhile.

After ENIAC was retired, its components were preserved. Parts of it are now in the Smithsonian Institution and the University of Pennsylvania. The machine that once filled a room and weighed 30 tons is remembered as the ancestor of every computer in use today — a reminder that the technology we take for granted was once a room-sized experiment that seemed impossible to many people.

Frequently Asked Questions

Was ENIAC really the first computer ever built?

ENIAC was the first general-purpose electronic computer that worked reliably and solved real problems. Earlier machines existed — some mechanical, some electromechanical — but ENIAC was the first to combine speed, reliability, and the ability to be reprogrammed for different tasks. Some historians argue other machines came first depending on how you define "computer," but ENIAC is the most widely accepted answer.

How long did it take to build ENIAC?

ENIAC took about three years to build, from 1943 to 1946. The project began during World War II when the U.S. Army needed a faster way to calculate artillery tables. After the war ended, the team finished the machine and it was officially dedicated in February 1946.

Could ENIAC do anything a modern computer can do?

ENIAC could perform the same basic operations as a modern computer — addition, subtraction, multiplication, division, and logical decisions — but it was vastly slower and could store far less information. A smartphone today is millions of times faster and can hold billions of times more data. However, the fundamental idea is the same: electronic switches performing calculations under the control of a program.

Why did ENIAC need so much electricity?

ENIAC's 18,000 vacuum tubes each needed electricity to operate, and vacuum tubes are inefficient — they waste most of the energy they consume as heat. A single tube might use as much power as a light bulb. Transistors, invented shortly after ENIAC, used far less power because they are smaller and more efficient. Modern computers use integrated circuits with billions of transistors, but each transistor uses so little power that a modern computer uses less electricity than ENIAC did.

Who actually invented the computer?

ENIAC is credited to J. Presper Eckert and John Mauchly, but the invention of the computer was a team effort spanning decades. Charles Babbage designed the concepts in the 1830s, researchers in the 1930s and 1940s developed vacuum tube switching, and the team at the University of Pennsylvania built ENIAC. Women programmers like Betty Snyder and Jean Jennings were essential to making ENIAC work, though their contributions were often overlooked.