I'd be most grateful for any help regarding a lift bag calculation.

I wish to raise a 20kg grapnel anchor from the sea bed at 20 metres depth in salt water. I am happy with the calculation apart from working out the actual volume of the anchor. I have guesstimated the volume but I would be grateful if anyone has experience of calculating the volume of an anchor or has actual experience of lifting an anchor which they would be happy to share? I intend to use a 20kg lift bag. My guesstimate of the volume is 5 - 10 litres but I may be miles out hence this post.

If the object inSalt waterIf the object in20 m, weighs20 kg and has a volume of5 L.

you will need a lift capacity of15.6 kg(or15.21 L) and the lift bag will use257.15 Lof air.

Salt water20 m, weighs20 kg and has a volume of10 L.

you will need a lift capacity of11.25 kg(or10.97 L) and the lift bag will use252.91 Lof air.

Thanks