Do you mean 7 metres by 7 metres and to what depth?

Yes do you mean 7 m2 ie 49 square metres or e.g. 2mx3.5m ie 7 square metres?

Whichever simply multiply that by the depth you want to go to e.g. 3 inches is approx 10cm or 0.1m

This will give you the volume of the gravel you require i.e cubic metres

Then get a bag of the gravel, say 25kg and pour it out into a bucket etc for which you know the volume e.g 10 litres.

Then divide the number of cubic metres of gravel you need by the volume of the gravel in the bucket. This will give you the number of say 25kg bags - or multiples of for bulk delivery of gravel you require.

One of those large bags is generally a cubic metre. 850kilos

also a quick phone call to your local builders merchant will give you the answer straight away as they deal with this all the time.