10.3cm top x 3.9 cm opening. This is a "wide mouth" funnel, canning-style size.
Owning one of these makes your bulk refill life so much simpler! Stainless steel is a highly valuable recyclable material (at end of life) and can be washed consistently in the dishwasher with no fear of plastic waste in your water table. This item is also very durable, when compared to a plastic funnel.