The question you opened
Thirteen digits, twenty eight seconds
On 18 June 1980 the computing department at Imperial College picked two random 13 digit numbers.
How long did Shakuntala Devi take to multiply them in her head?
28 seconds.
She gave 18,947,668,177,995,426,462,773,730 in 28 seconds, and the 1982 Guinness Book of Records carried the result.
On 18 June 1980 the Department of Computing at Imperial College London chose two 13 digit numbers at random, 7,686,369,774,870 and 2,465,099,745,779, and asked Shakuntala Devi to multiply them. She answered 18,947,668,177,995,426,462,773,730 in 28 seconds. Guinness recorded it, and the entry appeared in the 1982 edition. The number of single digit multiplications in a straightforward long multiplication of two 13 digit numbers is 169, plus the additions and carries to combine them.
At 28 seconds, that is over six operations a second including the bookkeeping, which is why the writer Steven Smith called the result hard to believe. Devi did not describe herself as doing long multiplication quickly. Her 1977 book Figuring: The Joy of Numbers sets out methods that trade steps for structure: splitting numbers near round anchors, using differences of squares, working from complements, and holding partial results as small named chunks rather than as digits.
The same ideas run through every card in this hub, several orders of magnitude smaller. Two cautions belong with any story of this kind. Calculating prodigies are studied precisely because they are unusual, and the methods that make 47 times 53 easy do not scale into 13 digits for most people however much they are practised. And the record itself was slow to be certified: the certificate was only issued in 2020, seven years after Devi died.
What transfers is the direction of the effort. Speed came from choosing a better route through the arithmetic rather than from running the standard route faster. That is the only part of this story that a person can actually copy, and it is available on the first question they meet.
Technique: Structure beats brute force
Source checked 2026-09-06