New SIM yet 0.63GB available instead of 1GB

[Petalpower]

I have just this second activated my son’s SIM with 1GB data and he’s not used any - his mobile data toggle is switched off - so why is it 0.63GB and not 1GB when I look at the SIM on the app immediately after? His number and services haven't even transferred over yet so even if the toggle for mobile data were switched on, he couldn't have used that data. 

Thanks

[PandJ2020 · Accepted solution]

When activating a SIM you get a pro-rata amount of data until next billing period.  At that refresh it will be 1GB.

(You are charged pro-rata too)

[GD1]

@Petalpower  The data you see will be pro rata based on the billing date.  So if you're part way through the month you'll only get the remaining data for that month.