This wide black Iaido style belt is used by Iaido practitioners but is also popular among Aikido practitioners who prefer a thin and wide belt under the Hakama. This high-class model (hand-made in Japan) has a very soft cotton layer on the inside which diminishes the belt's thickness on its entire length.

This type of Obi is usually tied three times around your waist and fixed by either a knot or by simply folding the ends inside the belt. 
Size calculation: 3x hips size + 50 cm (+/- 20 cm depending on how you tie the belt)

Please note that this belt can only be worn with a Hakama, tied without Hakama it will not fit properly.
This belt with exactly the same specifications also exits in white: Wide White Belt (Iaiobi).
Because this is not a classic belt, the Aikikai logo cannot be added.

Why this type of belt in Aikido?
As the title says, this kind of Obi is typically worn for Iaido practice. As it is tied 3 times around the waste and because of its width it sits very firmly in order to carry a sword. Some Aikido practitioners prefer this type of belt because the knot being flatly tight avoids the bulge in the front of the center and allows the Hakama to fall straight. Moreover, it keeps the Hakama better in place than an ordinary Obi and helps stabilize the hips.