National income is just the part of GDP that's generated from the payment to factors of production, or all the resources owned by the nation and used in production whether they are located home or abroad, including wage for labor, interest for capital, rent for land, and company saving (all before direct taxation). You can simply calculate NI by subtracting capital depreciation and indirect tax from GDP.
Company saving is company profit less dividends plus capital depreciation. It's what companies save for themself rather than their owners, to renew old equipment, housing and so on as well as to purchase new ones.
Take United States for example, national income also equals to net domestic product plus net earnings from U.S. resources abroad minus indirect business taxes(net of subsidies).
Refer to factors of production.
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