There are several units in common usage. In the US they like "inches of mercury" and "Paschals." Industrial types (especially those dealing in steam) like inches of WC (water column). Scientists like...
By one of:1) Income. Sum up all income in the economy from all sources.2) Expenditure. Sum up all sources of spending in the economy.3) Value-added. Sum up all value-added in the production of...