It can be tempting to manually calculate the amount of VAT by taking a price including VAT and multiplying it by 0.25, or 0.12, or 0.06 or whatever the VAT rate is. This is wrong - remember that VAT is a surcharge on the price excluding VAT. Here is the formula to calculate a price including VAT, in this example it is a 25% VAT rate.
500 kr (excl VAT) * 1,25 = 625 kr (incl VAT)
This is because you take 25% of the price excluding VAT and add this as a surcharge. 25% of 500 is 125 kr which is our VAT amount.
And to calculate the non-VAT price, you can take the formula and run it backwards.
625 kr (incl VAT) / 1,25 = 500 kr (excl VAT)