function calcRoll ()
{
	frm = document.forms['calc_form'];
	roll_width = parseFloat(frm.elements['roll_width'].value.replace (',', '.'));
	roll_height = parseFloat(frm.elements['roll_height'].value.replace (',', '.'));

	room_size = parseFloat(frm.elements['room_size'].value.replace (',', '.'));
	room_height = parseFloat(frm.elements['room_height'].value.replace (',', '.'));

	frm.elements['num_roll'].value = '';
	roll_area = roll_width*roll_height;
	room_area = room_size*room_height;
	if (roll_area <= 0 || isNaN(roll_area) || room_area <= 0 || isNaN(room_area))	return;

	frm.elements['num_roll'].value = Math.ceil(room_area/roll_area);
}