Twenty-one mithqals: half a mithqal is due on the initial twenty; the extra one does not add another due amount. Total 0.5 mithqal.
Twenty-three mithqals: the extra three do not complete a four-mithqal block, so the total remains 0.5 mithqal.
This is the treatment of an incomplete increment, not a denial of gold nisab or of the basic zakat rate.
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Zakat on Gold: Nisab, Incremental Dues and Jewelry
Twenty-one mithqals: half a mithqal is due on the initial twenty; the extra one does not add another due amount. Total 0.5 mithqal.
Twenty-three mithqals: the extra three do not complete a four-mithqal block, so the total remains 0.5 mithqal.
This is the treatment of an incomplete increment, not a denial of gold nisab or of the basic zakat rate.