Calculating area of polygons

[SophieHorsley]

I appreciate this topic has been covered before, but having reviewed various questions and tried to follow various suggestions I am still having problems.

I create content in ArcGIS Pro using British National Grid as the coordinate system and then create a hosted feature layer from it for others to use/add to in AGOL. 

Area_Shape field is automatically generated, but as others have commented it doesn't match any of the area measurements when I measure the area of the polygon with the measure tool. 

Can anyone give me step by step instructions for the best (least technical) way for me to get accurate area measurements in AGOL?

[DanPatterson_Retired]

To find out what is what. I would suggest that you calculate area fields using your national grid projection, then another field using a web Mercator projection and finally one which is in GCS WGS84 aka, decimal degrees.

One of those three columns showing the area calculations should match what is being shown in AGOL.  It is undoubtedly a projection issue that is giving rise to the variations.

[DanPatterson_Retired]

From a number of other threads, Web Mercator shouldn't be used for distance or area calculations.  It is an inherent limitation of the projection