We need 1 + 2 + 4 + 8 + ... + 2⁶³

Now a₁ = 1, r = 2, n = 64

Each grain weighs 20 mg = 2 × 10^-5 kg = 2 × 10^-8 tonnes.

So the weight is

(1.84467 × 10¹⁹) × (2 × 10^-8) tonnes = 369 billion tonnes.

NOTE 1: There are 1000 kg in one tonne.

NOTE 2: The world annual output of rice today is only 600 million (not billion) tonnes!

NOTE 3: We are using the US/French 'billion' (10⁹) and not the British 'billion' (10¹²). [See Short and Long Scales.]