Note 6 - Investments in Joint Ventures - Condensed Income Statement of EXCOR and All Other Joint Ventures (Details) - USD ($)	3 Months Ended		9 Months Ended
	May 31, 2020	May 31, 2019	May 31, 2020	May 31, 2019
Net sales	$ 9,744,823	$ 14,893,117	$ 37,610,006	$ 42,302,573
Gross profit	3,245,721	4,754,764	12,618,519	13,418,984
Net income	(965,221)	1,482,149	427,163	4,380,776
Northern Technologies International Corporation’s share of equity in income from joint ventures	811,787	1,877,410	3,466,581	5,596,788
Northern Technologies International Corporation’s dividends received from joint ventures	89,227	1,795,542	5,648,153	3,822,785
EXCOR [Member]
Northern Technologies International Corporation’s share of equity in income from joint ventures	536,540	1,353,856	2,246,930	4,094,763
Northern Technologies International Corporation’s dividends received from joint ventures		1,673,700	4,675,850	2,517,450
All Other [Member]
Northern Technologies International Corporation’s share of equity in income from joint ventures	275,247	523,554	1,219,651	1,502,025
Northern Technologies International Corporation’s dividends received from joint ventures	89,227	121,842	972,303	1,305,335
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Net sales	18,782,233	27,774,112	68,531,897	86,003,918
Gross profit	8,647,784	12,557,580	31,107,491	38,547,018
Net income	1,610,524	3,797,785	6,920,047	11,412,552
EXCOR [Member]
Net sales	7,753,631	10,933,386	26,102,046	34,895,042
Gross profit	4,478,739	6,129,046	15,066,309	19,118,704
Net income	1,071,899	2,707,342	4,492,254	8,187,467
All Other [Member]
Net sales	11,028,602	16,840,726	42,429,851	51,108,876
Gross profit	4,169,045	6,428,534	16,041,182	19,428,314
Net income	$ 538,625	$ 1,090,443	$ 2,427,793	$ 3,225,085