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Note 17 - Regulatory Capital - Capital Ratios (Details) | Dec. 31, 2017 | Dec. 31, 2016
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Leverage capital, adequately capitalized, ratio | 4.00% | 4.00%
Tier 1 risk based capital, adequately capitalized, ratio | 6.00% | 6.00%
Common equity tier 1 capital, adequately capitalized, ratio | 4.50% | 4.50%
Total risk based capital, adequately capitalized, ratio | 8.00% | 8.00%
Leverage capital, adequately capitalized plus capital conservation buffer, ratio | 4.00% | 4.00%
Tier 1 risk based capital, adequately capitalized plus capital conservation buffer, ratio | 8.50% | 8.50%
Common equity tier 1 capital, adequately capitalized plus capital conservation buffer, ratio | 7.00% | 7.00%
Total risk based capital, adequately capitalized plus capital conservation buffer, ratio | 10.50% | 10.50%
Middlefield Banking Company [Member] |  |
Leverage capital, actual, ratio | 9.47% | 9.29%
Tier 1 risk based capital, actual, ratio | 10.88% | 13.03%
Common equity tier 1 capital, actual, ratio | 10.88% | 13.03%
Total risk based capital, actual, ratio | 11.64% | 14.25%
Leverage capital, well-capitalized, ratio (bank only) | 5.00% | 5.00%
Tier 1 risk based capital, well-capitalized, ratio (bank only) | 8.00% | 8.00%
Common equity tier 1 capital, well-capitalized, ratio (bank only) | 6.50% | 6.50%
Total risk based capital, well-capitalized, ratio (bank only) | 10.00% | 10.00%
Middlefield Banc Corp [Member] |  |
Leverage capital, actual, ratio | 10.20% | 9.27%
Tier 1 risk based capital, actual, ratio | 11.64% | 13.07%
Common equity tier 1 capital, actual, ratio | 10.79% | 13.07%
Total risk based capital, actual, ratio | 12.41% | 15.75%

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