A bstract The inclusive $$ b\overline{b} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>b</mml:mi><mml:mover><mml:mi>b</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math> - and $$ c\overline{c} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>c</mml:mi><mml:mover><mml:mi>c</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math> -dijet production cross-sections in the forward region of pp collisions are measured using a data sample collected with the LHCb detector at a centre-of-mass energy of 13 TeV in 2016. The data sample corresponds to an integrated luminosity of 1.6 fb − 1 . Differential cross-sections are measured as a function of the transverse momentum and of the pseudorapidity of the leading jet, of the rapidity difference between the jets, and of the dijet invariant mass. A fiducial region for the measurement is defined by requiring that the two jets originating from the two b or c quarks are emitted with transverse momentum greater than 20 GeV/ c , pseudorapidity in the range 2 . 2 &lt; η &lt; 4 . 2, and with a difference in the azimuthal angle between the two jets greater than 1.5. The integrated $$ b\overline{b} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>b</mml:mi><mml:mover><mml:mi>b</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math> -dijet cross-section is measured to be 53 . 0 ± 9 . 7 nb, and the total $$ c\overline{c} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>c</mml:mi><mml:mover><mml:mi>c</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math> -dijet cross-section is measured to be 73 ± 16 nb. The ratio between $$ c\overline{c} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>c</mml:mi><mml:mover><mml:mi>c</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math> - and $$ b\overline{b} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>b</mml:mi><mml:mover><mml:mi>b</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math> -dijet cross-sections is also measured and found to be 1 . 37 ± 0 . 27. The results are in agreement with theoretical predictions at next-to-leading order.