In IEEE-754 specification, a double precision is presented in 64 bits, with 1 sign bit + 11-bit exponent + 52-bit mantissa representing a 53-bit significant. Converting a decimal to such a floating point number may lost precision. The way to do the conversion is as follows:

• Convert the decimal number $f$ to an integer times non-positive power-of-10, i.e. $d \times 10^{-k}$
• Since the significant has 53 bits only, for best precision, we scale the integer to $[2^{52}, 2^{53})$ by multiplying with $2^n$, i.e. $f = (d\times 2^n) \times 2^{-n} \times 10^{-k}$
• Then round off the number $d \times 2^n \times 10^{-k}$. The rounding is done according to IEEE 754 round half to even rule, i.e.
• if the remainder is less than half of the divisor, then round down
• if the remainder is more then half of the division, then round up
• if the remainder is half of the divisor, then round up for even quotient or round down otherwise
• Then express the number in normalised binary scientific notation and encode to binary

Two examples are given in the above link, which demonstrates the algorithm clearly:

Consider $f = 3.14159$,

• $707423177667543826432 \times 10^{-5} = 7074231776675438$ with remainder 26432, as $% $, thus round down to 7074231776675438
• Thus $f = 1.1001001000011111100111110000000110111000011001101110 \times 2^{-51} \times 2^{52}$
$= 1.1001001000011111100111110000000110111000011001101110 \times 2^1$
• Encode into binary becomes:
• Sign bit = 0
• Exponent bits = 1 + exponent bias (1023) = 1024 = 10000000000
• Mantissa bits = 1001001000011111100111110000000110111000011001101110
• Value in decimal = 3.14158999999999988261834005243144929409027099609375

Consider $f = 1.2345678901234567 \times 10^{22}$

• $12345678901234567000000 \times 2^{-21} = 5886878443352969} with remainder 1355712, as$1355712 > (2^{21} /2), thus round up to 5886878443352970
• Thus $f = 1.0100111010100001010110110010011100111011001110001010 \times 2^{21} \times 2^{52}$
$= 1.0100111010100001010110110010011100111011001110001010 \times 2^{73}$
• Encode into binary becomes:
• Sign bit = 0
• Exponent bits = 73 + exponent bias (1023) = 1096 = 10001001000
• Mantissa bits = 0100111010100001010110110010011100111011001110001010
• Value in decimal = 12345678901234567741440