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just do the below: program newtestfpjson; uses sysutils , fpjson ; type TJSONFloat4Number = class(TJSONFloatNumber) protected function GetAsString: TJSONStringType; override; end; function TJSONFloat4Number.GetAsString: TJSONStringType; var F: TJSONFloat; begin F := GetAsFloat; Str(F:0:4, Result); // format with your preferences end; var jsObjValues: TJSONObject; rValor: real; cValor: currency; sValor: single; dValor: double; begin SetJSONInstanceType(jitNumberFloat, TJSONFloat4Number); rValor := 1.23456; cValor := 1.23456; sValor := 1.23456; dValor := 1.23456; jsObjValues := TJSONObject.Create; jsObjValues.Add('REAL', rValor); jsObjValues.Add('CURRENCY', cValor); jsObjValues.Add('SINGLE', sValor); jsObjValues.Add('DOUBLE', dValor); WriteLn(jsObjValues.FormatJSON()); ReadLn(); end. And the output: { "REAL" : 1.2346, "CURRENCY" : 1.2346, "SINGLE" : 1.2346, "DOUBLE" : 1.2346 }