821 US customary cups in imperial gallons
Conversion
821 US customary cups is equivalent to 42.726594098775 imperial gallons.[1]
Conversion formula
How to convert 821 US customary cups to imperial gallons?
We know (by definition) that:
We can set up a proportion to solve for the number of imperial gallons.
Now, we cross multiply to solve for our unknown :
Conclusion:

Conversion in the opposite direction
The inverse of the conversion factor is that 1 imperial gallon is equal to 0.0234046270500337 times 821 US customary cups.
It can also be expressed as: 821 US customary cups is equal to imperial gallons.
Approximation
An approximate numerical result would be: eight hundred and twenty-one US customary cups is about forty-two point seven two imperial gallons, or alternatively, a imperial gallon is about zero point zero two times eight hundred and twenty-one US customary cups.
Footnotes
[1] The precision is 15 significant digits (fourteen digits to the right of the decimal point).
Results may contain small errors due to the use of floating point arithmetic.