Huge difference.

As the name implies, a `double`

has 2x the precision of `float`

^{[1]}. In general a `double`

has 15 decimal digits of precision, while `float`

has 7.

Here’s how the number of digits are calculated:

`double`

has 52 mantissa bits + 1 hidden bit: log(2^{53})÷log(10) = 15.95 digits

`float`

has 23 mantissa bits + 1 hidden bit: log(2^{24})÷log(10) = 7.22 digits

This precision loss could lead to greater truncation errors being accumulated when repeated calculations are done, e.g.

```
float a = 1.f / 81;
float b = 0;
for (int i = 0; i < 729; ++ i)
b += a;
printf("%.7g\n", b); // prints 9.000023
```

while

```
double a = 1.0 / 81;
double b = 0;
for (int i = 0; i < 729; ++ i)
b += a;
printf("%.15g\n", b); // prints 8.99999999999996
```

Also, the maximum value of float is about `3e38`

, but double is about `1.7e308`

, so using `float`

can hit “infinity” (i.e. a special floating-point number) much more easily than `double`

for something simple, e.g. computing the factorial of 60.

During testing, maybe a few test cases contain these huge numbers, which may cause your programs to fail if you use floats.

Of course, sometimes, even `double`

isn’t accurate enough, hence we sometimes have `long double`

^{[1]} (the above example gives 9.000000000000000066 on Mac), but all floating point types suffer from *round-off errors*, so if precision is very important (e.g. money processing) you should use `int`

or a fraction class.

Furthermore, don’t use `+=`

to sum lots of floating point numbers, as the errors accumulate quickly. If you’re using Python, use `fsum`

. Otherwise, try to implement the Kahan summation algorithm.

^{[1]: The C and C++ standards do not specify the representation of float, double and long double. It is possible that all three are implemented as IEEE double-precision. Nevertheless, for most architectures (gcc, MSVC; x86, x64, ARM) float is indeed a IEEE single-precision floating point number (binary32), and double is a IEEE double-precision floating point number (binary64).}