Add or remove VAT from a price — free, no sign-up, nothing leaves your browser.
Whether a price should have VAT added at all depends on the seller, not just the item — a UK business only has to register for and charge VAT once its taxable turnover passes £90,000 in a rolling 12-month period (a business can also choose to register voluntarily before reaching that point). Below that threshold, a business simply doesn't charge VAT, and there's nothing to add. This is worth knowing if you're ever unsure whether a quoted price should include VAT: it depends on the specific seller's registration status and the goods or services involved, not a blanket rule that applies to every transaction.
Because the percentage is a proportion of the original price, not the VAT-inclusive total — a £2,400 VAT-inclusive price at 20% VAT works out to £2,000 before VAT, not £1,920 (which is what you'd wrongly get by just subtracting 20% of £2,400).
20% standard rate (most goods and services), 5% reduced rate (e.g. domestic energy), and 0% zero rate (e.g. most food, children's clothing, books) — unchanged since January 2011.
No — only businesses registered for VAT do, which is required once taxable turnover passes £90,000 in a rolling 12-month period, though smaller businesses can register voluntarily too.
No. Everything is calculated locally in your browser — nothing you enter is sent to a server.