You must calculate zakat when it is due, if it is due on the 20th of Ramadan the amount of money you have on that day is what you have to pay zakat on. 

Remember you cannot trick Allah, if you are trying to get rid of your wealth or extra items before the 20th to avoid paying zakat on it remember that Allah is the all hearing and all knowing, you cannot hide anything from Him. If something happens beyond your control and you suffer great losses before calculating your zakat and you will take it into account. Example you have £50,000 in your bank at the start of Ramadan, and during Ramadan you suffered losses and ended up with £10,000 in your bank by the 20th Ramadan , your zakat will be on  the minimum amount.

These are legal issues in Islam so you should go to your local masjid or Islamic centre to get the amount which applies to your locality and country. 

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