Cost a dish, and see what it has to sell for. Put in what each ingredient costs you, add a wastage allowance, set the gross profit you price to, and the calculator gives you the menu price that target needs — ex VAT and inc VAT — then shows the GP you actually end up with once you round that price to something the menu can print.
This is arithmetic, not advice. It costs one plate against one price — it knows nothing about your rent, wages, energy or how many of them you sell. The 65–70% food GP mentioned above is the range operators commonly work to, not a rule, and a dish that misses it can still be the reason people book. Nothing you type leaves the page.
How the arithmetic works
Gross profit on food is worked out on the ex-VAT price, which is the part of the menu price the business keeps. Add up the ingredients, add the wastage allowance on top, and that is your plate cost. Take the plate cost off the ex-VAT price and you have the gross profit in pounds; divide it by the ex-VAT price and you have GP%. Food cost percentage is the same sum the other way up.
The order matters. A 70% GP target on a £4.52 plate needs £15.07 ex VAT — £18.50 once VAT at 20% is added and the price is rounded to something printable. Put £17.50 on the menu instead, which is what the worked example does, and the same plate returns £10.06 and a GP of 69.0%. That one-point gap between the price your target needs and the price you actually charge is why the calculator shows both.
What it does not know
This is dish-level arithmetic. It knows nothing about rent, wages, energy, breakages or how many of the dish you sell, so a strong GP% on one plate does not mean the menu is profitable. The 65–70% food GP mentioned on the page is the band operators commonly work to, not a rule — drinks are usually run higher, and a deliberately low-GP dish can still earn its place if it fills tables. Nothing you type into it leaves your browser.