The aim of the paper is to present the problems associated with the correct calculation of the economic impact of the operation of VAT in agriculture. This applies to flat-rate farmers and farmers who resigned from the lump sum and became active VAT payers. There were analyzed selected publications from the years 2000-2015, addressing the problem of VAT in agriculture and websites containing „calculators” to support the calculation effects of the withdrawal of farmers from the lump sum. It turned out that in spite of the long existence of VAT in agriculture the principles of operation are misinterpreted and wrongly presented the economic impact of the resignation of the status of the flat-rate farmer.