Apart from many ecological advantages, mangroves in front of a coastal defence may lower the construction and maintenance costs of the defence. Although mangroves have hardly any reducing effect on water levels (and on tsunami impact) mangroves may significantly reduce wave attack on a coastal dike, and in this way reduce the cost of a revetment and lower the design height of a levee because of reduced run]up and overtopping height. This paper describes a design tool for the required width of the mangrove belt, as well as some guidance for planting the mangroves. Design graphs are presented to determine the required mangrove belt as a function of deep water wave action and mangrove density.