If a US nickel (21.21mm x 1.95mm) by volume were liquid, how many milliliters would that be.?

Presuming your 21.21 mm is a diameter measurement, we can find the volume of the coin in mm³, first. Once we have that, then we can move on:

21.21 mm diameter makes for a 10.605 mm radius:

V = πr²h

V = π(10.605)²(1.95)

V = 219.30874875π mm³

1 cm³ is 1 ml, so now we can convert mm³ to cm³:

219.30874875π mm³ (1/10 cm/mm)³ = 0.2193 cm³ (rounded to 4 DP)

Now that we have this in cm³, convert to ml:

0.2193 cm³ (1 ml/cm³) = 0.2193 ml