Some Mano-Clock models are graduated in inches or feet, to make them suitable for use with different tanks.
Here you can use the tank chart calculator to convert to US gallons.

Select the tank type and enter the height (H) in
inches and the total volume in US gallons.

Round horizontal tank

Round vertical tank

Box-shaped tank
Height (H) in inch.
Max 99 in.
Total volyme in US gal.