To add 20% VAT, multiply the net price by 1.20. To remove it, divide the gross price by 1.20.
TL;DR: Adding VAT is net × 1.20. Removing it is gross ÷ 1.20. At a 20% rate the VAT portion is always one sixth of the gross. The VAT calculator does both directions and shows the VAT amount on its own.
Adding VAT to a net price
Say you quote a job at £500 before tax. Multiply by 1.20 and you get £600. The VAT is £100, the difference between the two. If the rate is 5% instead of 20%, you multiply by 1.05.
The rule is the same whatever the rate: net × (1 + rate). Set the rate once in the VAT calculator and it handles the multiplication.
Removing VAT (working backwards)
If you only know the gross price and need the net figure, divide by 1.20. A £600 gross price gives £500 net, leaving £100 of VAT.
There is a shortcut worth remembering. At 20%, the VAT is one sixth of the gross. So £600 ÷ 6 = £100 of VAT straight away. That works because 20% of the net equals one sixth of the gross (the gross is six fifths of the net). It only holds at the 20% rate.
What “reverse VAT” means here
On this tool, “reverse VAT” means the arithmetic above: starting from a VAT-inclusive total and stripping the tax back out. It is not the reverse-charge mechanism used in construction, where the customer accounts for the VAT instead of the supplier. That is a reporting rule, not a sum you work out by hand. If you need the construction reverse charge, check the invoice terms rather than this calculator.
Once you have the net, the VAT, and the gross all visible, you can copy whichever one your invoice needs.