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In Dashboard > Object Gateway > Buckets:
Display the user bucket quota usage: the current size, % used and number of objects for the bucket.

It seems the usage info is already available:

$ radosgw-admin bucket stats --uid testid --bucket bucket
[
    {
        "bucket": "bucket",
        "num_shards": 11,
        "tenant": "",
        "zonegroup": "b5f43b50-abd5-4b39-9b45-5b529997ea5b",
        "placement_rule": "default-placement",
        "explicit_placement": {
            "data_pool": "",
            "data_extra_pool": "",
            "index_pool": "" 
        },
        "id": "702aaf09-0a49-49a7-8da0-629e9f905d40.4137.1",
        "marker": "702aaf09-0a49-49a7-8da0-629e9f905d40.4137.1",
        "index_type": "Normal",
        "owner": "testid",
        "ver": "0#2,1#3,2#1,3#1,4#3,5#3,6#3,7#1,8#1,9#1,10#1",
        "master_ver": "0#0,1#0,2#0,3#0,4#0,5#0,6#0,7#0,8#0,9#0,10#0",
        "mtime": "2020-04-08T16:03:53.198989Z",
        "creation_time": "2020-04-08T16:03:50.051813Z",
        "max_marker": "0#,1#,2#,3#,4#,5#,6#,7#,8#,9#,10#",
        "usage": {
            "rgw.main": {
                "size": 12697,
                "size_actual": 40960,
                "size_utilized": 12697,
                "size_kb": 13,
                "size_kb_actual": 40,
                "size_kb_utilized": 13,
                "num_objects": 9
            }
        },
        "bucket_quota": {
            "enabled": false,
            "check_on_raw": false,
            "max_size": -1,
            "max_size_kb": 0,
            "max_objects": -1
        }
    }
]