Note 11 - Business Segment Information - Segment Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Revenue	$ 464,000	$ 418,000	$ 1,035,000	$ 856,000
Operating loss	(1,702,000)	(2,005,000)	(3,560,000)	(3,706,000)
Interest expense	(89,000)	(747,000)	(182,000)	(1,504,000)
Research and development	(356,000)	(350,000)	(683,000)	(684,000)
Tangible assets	2,944,000		2,944,000		$ 2,388,000
Operating Lease, Right-of-Use Asset	311,000		311,000		341,000
Investment in South Korean joint venture	40,000		40,000		63,000
Intangible assets	2,150,000		2,150,000		2,150,000
Investment in South Korean Joint Venture [Member]
Investment in South Korean joint venture	40,000		40,000		63,000
Corporate, Non-Segment [Member]
Revenue	0	0	7,000	0
Operating loss	(937,000)	(1,129,000)	(1,860,000)	(1,936,000)
Interest expense	(51,000)	(735,000)	(106,000)	(1,479,000)
Research and development	(209,000)	(119,000)	(545,000)	(321,000)
Tangible assets	1,072,000		1,072,000		603,000
Operating Lease, Right-of-Use Asset	185,000		185,000		215,000
Intangible assets	2,150,000		2,150,000		2,150,000
Corporate, Non-Segment [Member] | Investment in South Korean Joint Venture [Member]
Investment in South Korean joint venture	40,000		40,000		63,000
Operating Segments [Member] | Odor-No-More [Member]
Revenue	318,000	299,000	638,000	596,000
Operating loss	(107,000)	(134,000)	(283,000)	(287,000)
Tangible assets	431,000		431,000		624,000
Operating Lease, Right-of-Use Asset	0		0		0
Intangible assets	0		0		0
Operating Segments [Member] | Odor-No-More [Member] | Investment in South Korean Joint Venture [Member]
Investment in South Korean joint venture	0		0		0
Operating Segments [Member] | Clyra Medical [Member]
Tangible assets	1,016,000		1,016,000		1,125,000
Operating Lease, Right-of-Use Asset	0		0		0
Intangible assets	0		0		0
Operating Segments [Member] | Clyra Medical [Member] | Investment in South Korean Joint Venture [Member]
Investment in South Korean joint venture	0		0		0
Operating Segments [Member] | BLEST [Member]
Tangible assets	321,000		321,000		314,000
Operating Lease, Right-of-Use Asset	126,000		126,000		126,000
Intangible assets	0		0		0
Operating Segments [Member] | BLEST [Member] | Investment in South Korean Joint Venture [Member]
Investment in South Korean joint venture	0		0		0
Operating Segments [Member] | BioLargo Water [Member]
Revenue	0	0	9,000	0
Operating loss	(168,000)	(196,000)	(305,000)	(424,000)
Research and development	(138,000)	(137,000)	(257,000)	(317,000)
Tangible assets	151,000		151,000		105,000
Operating Lease, Right-of-Use Asset	0		0		0
Intangible assets	0		0		0
Operating Segments [Member] | BioLargo Water [Member] | Investment in South Korean Joint Venture [Member]
Investment in South Korean joint venture	0		0		0
Operating Segments [Member] | BioLargo Engineering, Science & Technologies, LLC [Member]
Revenue	266,000	160,000	654,000	444,000
Operating loss	(190,000)	(124,000)	(373,000)	(331,000)
Research and development	(123,000)	(85,000)	(228,000)	(166,000)
Operating Segments [Member] | Clyra Segment [Member]
Revenue	0	21,000	114,000	21,000
Operating loss	(300,000)	(422,000)	(739,000)	(728,000)
Interest expense	(38,000)	(12,000)	(76,000)	(25,000)
Research and development	(6,000)	(47,000)	(33,000)	(61,000)
Consolidation, Eliminations [Member]
Revenue	(120,000)	(62,000)	(387,000)	(205,000)
Research and development	120,000	$ 38,000	380,000	$ 181,000
Tangible assets	(47,000)		(47,000)		(42,000)
Operating Lease, Right-of-Use Asset	0		0		0
Intangible assets	0		0		0
Consolidation, Eliminations [Member] | Investment in South Korean Joint Venture [Member]
Investment in South Korean joint venture	$ 0		$ 0		$ 0