As you will have seen on the news, the rate of VAT in the UK is to change from 17.5% to 20%.
The new VAT conversion fraction is 1/6 (much easier to remember than the old one for 17.5%, which was 7/47).
If want to know the net cost of something, then divide by 6 and multiply by 5. For example, something sold for £100.00 has a net value of £83.33 (100/6*5).
If you want to know the gross cost of something, then divide the net value by 5 and multiply by 6. For example, a net value of £25.00 would cost £30.00 including VAT (25/5*6).
It may sound obvious, but changes to the rate of VAT affect the VAT amount, which in turn adjusts the gross value. £20.00 is not 20% of £120.00, it is 1/6 of £120.00.

