This washer suits the newer model inlet valves which is removed by turning the valve cap nut 1/4 of a turn-NOT unscrewing the nut like the older Hushflow valve model. For Armitage Shanks and Ideal Standard valves. Size 32mm (1-1/4"). You will know if this is the correct washer as there will be ridges pressed into the domed part of the washer formed by it being inside the valve.
Please note if your old washer doesn't have 18 little ridges pressed into the rubber, then you need the diaphragm washer for the older Hushflow valve-these are not interchangeable between the 2 types of valve. Please see my other listing for the Hushflow valve washer.