21 US gallons in US quarts
Result
The conversion result is: 21 US gallons is equivalent to 84 US quarts.[1]
Conversion formula
How to convert 21 US gallons to US quarts?
We know (by definition) that: 1 usgallon ≈ 4 usquart.
We can set up a proportion to solve for the number of US quarts.
1 usgallon / 21 usgallon ≈ 4 usquart / x usquart
Now, we cross multiply to solve for our unknown x:
x usquart ≈ (21 usgallon / 1 usgallon) * 4 usquart → x usquart ≈ 84 usquart
Conclusion:
21 usgallon ≈ 84 usquart

Conversion in the opposite direction
The inverse of the conversion factor is that 1 US quart is equal to 0.0119047619047619 times 21 US gallons.
It can also be expressed as: 21 US gallons is equal to 1 / 0.0119047619047619 US quarts.
Approximation
An approximate numerical result would be: twenty-one US gallons is about zero US quarts, or alternatively, a US quart is about zero point zero one times twenty-one US gallons.
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.