why the standard deviation is not as the same as online calculator

Question

I need to calculate the standard deviation for these numbrs:

-12
-3
0
-13
8
-6
0
-22
-1
7
-7
1
-2
-13
-4
0
-6
-4
-10
3

I did everything, but still my answer is different from the online standard deviation calculator, even though i tried many online calculator.

I will state my steps, and please help me to find if there is any mistake:

The mean

it is -4.2

The difference from the mean

-7.8
1.2
4.2
-8.8
12.2
-1.8
4.2
-17.8
3.2
11.2
-2.8
5.2
2.2
-8.8
0.2
4.2
-1.8
0.2
-5.8
7.2

Power the difference from the mean

60.84
1.44
17.64
77.44
148.84
3.24
17.64
316.84
10.24
125.44
7.84
27.04
4.84
77.44
0.04
17.64
3.24
0.04
33.64
51.84

Calculate the variance

56.34

Sqrt the variance

7.505997602

it seems that the difference is in the variance,

in order to make it easy for your, I uploaded an excel sheet containing the numbers, maybe that is easier for u

This is the sheet

Answer 1

Test your sum of square $ s = 60.84 + \dots 51.84$. I get it as $s = 1003.20$. Then depending of the standard deviation type you get for the sample std. dev. $$\sigma_s = \sqrt{s/19} = 7.26636$$ or the population std. dev. $$\sigma_p = \sqrt{s/20} = 7.08237$$

Answer 2

There are two ways to calculate variance. First one is biased estimator and other is unbiased estimator of variance. In e-gadgets, probably estimator is unbiased estimator and that's what you are getting inconsistent result. Try dividing variance by $n-1$ , the number of data points instead of $n$ and then take square root. Good Luck!