Note 6 - Investments in Joint Ventures - Condensed Income Statement of EXCOR and All Other Joint Ventures (Details) - USD ($)	12 Months Ended
	Aug. 31, 2019	Aug. 31, 2018
Net sales	$ 114,635,435	$ 120,060,897
Gross profit	51,312,013	53,348,459
Net income	14,688,999	15,300,276
Northern Technologies International Corporation’s share of equity in income of joint ventures	7,225,518	7,527,383
Northern Technologies International Corporation’s dividends received from joint ventures	5,039,041	3,697,503
EXCOR [Member]
Net sales	47,015,841	47,537,949
Gross profit	25,622,261	25,584,666
Net income	10,827,448	11,095,523
Northern Technologies International Corporation’s share of equity in income of joint ventures	5,415,362	5,549,765
Northern Technologies International Corporation’s dividends received from joint ventures	3,345,600	2,357,544
All Other [Member]
Net sales	67,619,594	72,522,948
Gross profit	25,689,752	27,763,793
Net income	3,861,551	4,204,753
Northern Technologies International Corporation’s share of equity in income of joint ventures	1,810,156	1,977,618
Northern Technologies International Corporation’s dividends received from joint ventures	$ 1,693,441	$ 1,339,959