How many tons of sand are there in the deserts?

I have seen that Each grain of sand weights 0.0027 grams

One ton is 907,185 grams,

That is 335,994,400 grains of sand per ton and there are (aproximately) 700,500,000,000,000,000,000 grains of sand on earth (seven quintillion five quadrillion) from what I could found on the internet,

but I don't know for the deserts only

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Not all deserts have sand- many are rocky. No one has measured the sand in all the deserts, so the weight is not known.