Remington Rand provided the money to finish the UNIVAC. To reduce the financial losses, it cajoled Prudential and Nielsen into canceling their contracts. The first UNIVAC passed its formal acceptance test on March 29-30, 1951 and was turned over to the Census Bureau, which operated it in the factory for nearly a year. A formal dedication ceremony was held on June 14, but coverage in the general press was minimal. The following day, the New York Times ran a tiny, two-sentence article that referred to the UNIVAC as an "eight-foot-tall mathematical genius, designed to meet problems of the United States Census Bureau", but didn't even mention its name.

The central complex of the UNIVAC was about the size of a one-car garage: 14 feet by 8 feet by 8.5 feet high. It housed the mercury memory unit and all the central processing unit circuitry. The outside of the unit was composed of hinged gray metal doors that could be opened to give access to the circuitry racks. In the center of one of the long sides of the unit, there was a clear Plexiglas door to provide access to the center of the system: it was a walk-in computer. The vacuum tubes generated an enormous amount of heat, so a high capacity chilled water and blower air conditioning system was required to cool the unit. In addition to the central complex, there were eight UNISERVO tape drives, an operator console, and a console typewriter/printer. Originally printing was done offline by the UNIPRINTER, which resembled an overgrown typewriter with an attached tape drive. A much-needed 600 line per minute printer (at 130 characters per line) was added in 1954. The comple! te system had 5200 vacuum tubes, weighed 29,000 pounds, and consumed 125 kilowatts of electrical power.

The UNIVAC represented numbers in binary-coded decimal with six bits for each digit. It employed Excess-3 notation where the binary value was three greater than the actual number, so that zero was 000011, one was 000100, two was 000101, and so on. Excess-3 had been used in the Bell Telephone Laboratories Model I Relay Calculator built in 1940. Excess-3 was chosen for the UNIVAC because it simplified the complementing (making negative) of numbers and made the carries come out right for digit-by-digit decimal addition.

The UNIVAC's word size was 72 data bits, which held eleven digits plus a sign, plus one parity bit for each six data bits, giving a total of 84. The mercury delay line memory amounted to 1000 words. Besides numbers, the UNIVAC could represent alphanumeric data (letters of the alphabet and some punctuation marks) using six bits for each character with twelve characters to the word. Codes were assigned for the letters of the alphabet and punctuation marks, such as 010100 for A, 010101 for B, 010110 for C and so on.

The program instructions were six decimal digits (36 bits, excluding parity bits) long, so two instructions fit in each word. The first two digits of an instruction were the function code, the next digit was unused, and the last three gave the memory address. There were 45 different functions. Many of the function codes were mnemonic, that is, they tried to bear some relation to the operation to be performed. For example, A (still indicated by the bit pattern 010100) was the code for addition. Similarly, D was divide, S was subtract, and C meant to copy the contents of the A (accumulator) register into memory. All the functions didn't work out mnemonically: J meant to store the contents of the X register into memory. On the UNIVAC, addition could be done with just one register, in this case the A register, but other operations involved registers which were designated L and X. An add instruction, such as A 0503, would add the value at the stated memory location (503, in o! ur example) to the value in the A register, leaving the result in the A register. The C instruction C 0504 meant to copy (store, in modern terminology) the value in the A register into memory location 504. On the UNIVAC, multiplication and division involved three registers. For example, the P multiplication instruction multiplied the value in the L register by the value in the stated memory location, giving a 22-digit product contained in the registers A and X. Tape input/output used two 60-word buffers designated I (input) and O (output). The input/output instructions provided for both forward and backward reading of tape. The read backward was particularly useful for sorting, where long strings of data were repeatedly written to tape and read back in through successive merges.

The computer had a high-degree of self-checking: all processing was done in duplicate by two sets of circuitry, and the results were compared to be sure they were identical. Donald Marquardt of DuPont recalled: "One of the big advantages of the UNIVAC was in fact the ability to rely on the accuracy of the numbers when they came out.... Now there were some other computers that I used during that same period where I would make two or three runs on the machine and come up with two or three [different] sets of numbers...."

The UNIVAC had the ability to store the control counter value in memory, making it possible for the flow of a program to go to a subprogram and then return to where it was in the main program. While the 72-bit word could accommodate numbers up to 11 digits, scientific calculations quite often involved larger numbers. To take an example from chemistry, Avogadro's number (the number of molecules in a mole of gas) is 6.02 x 10**23 ; computers could represent this in what is called floating-point format, where part of the word contains the value (6.02) and part of the word contains the exponent. Later computers would be designed with electronic circuits to perform calculations on numbers in floating-point format, but the UNIVAC did not have hardware instructions of this sort. Floating-point calculations could, however, be done by means of software subprogram, making it possible for the UNIVAC to do both scientific computation and business data processing.

Early in the design of the UNIVAC system, Eckert and Mauchly had recognized that for the computer to be useful in handling the large volumes of data used in many business applications, such as payroll of inventory control, it would need to have a high speed input/output system. Since punched cards would be slow, the company developed the UNISERVO tape units to be the primary input/output devices for the computer. Each unit was six feet high and three feet wide. The UNISERVO used metal tape: a 1/2-inch wide thin strip of nickel-plated bronze 1200 feet long. These metal tape reels were very heavy: not the sort of thing for an operator to drop on his or her foot! Data was recorded in eight channels on the tape (six for the data value, one parity channel for error checking, and one timing channel) at a density of 128 characters per linear inch of tape. The tape could be moved at 100 inches per second (as compared with 1.875 on today's cassette tape players), giving a nominal! transfer rate of 12,800 characters p er second. Making allowance for the empty space between tape blocks, the actual transfer rate was around 7,200 characters per second.

No punched card devices were provided with the UNIVAC, so the UNITYPER data entry machine was developed. The data entry clerk typed on a keyboard, and the UNITYPER recorded the values on a reel of metal tape. This lack of integration with punched card systems became a marketing handicap. Many prospective customers already had significant investment in tabulating card systems. When IBM entered the computer business, it made sure that it offered computers that fit easily into existing card processing installations. To fill this gap, Eckert-Mauchly developed a stand-alone card-to-tape unit, which could process 100 cards per minute. Since Eckert-Mauchly was an independent company at the time the design of the card-to-tape converter was done, it naturally followed the market and built a machine that handled IBM's 80-column cards. Sometime after the acquisition by Remington Rand, a version to handle 90-column cards was developed.