(57) The invention relates to a method and an arrangement for manufacturing a mineral fibre web. In the method mineral fibres are produced with a fiberising apparatus, the produced mineral fibres are collected onto a moving collecting member and a continuous primary fibre web is formed. The primary fibre web is transferred in the first longitudinal direction, and arranged by folding in partially overlapping layers perpendicularly to the first longitudinal direction to form a secondary fibre web. In the method the target surface weight of the secondary fibre web and one of the following selection variables is selected: i) a whole number representing the number of folds in the vertical direction of the secondary fibre web, or ii) surface weight of the primary fibre web to be collected, whereby the production value of the unselected selection variable is determined with the aid of the target surface weight of the secondary fibre web and the selected selection variable, and the manufacturing process is adjusted to obtain the production value.