The number of times by which income increases as a result of increase in investment is called investment multiplier. Investment multiplier shows a relationship between initial increment in investment and the resulting increment in national income.
K‌=‌

∆Y

∆I

where K‌=‌ multiplier ‌
∆Y‌=‌ change in income ‌
∆I‌=‌ change in investment ‌
Relation of multiplier with MPC :
‌K=‌

∆Y

∆I

‌K=‌

∆Y

∆C

(Dividing this equation by ∆Y we get)
K‌=‌

∆Y

∆Y−∆C

‌=‌

‌ ∆Y ∆Y

∆Y ∆Y −‌ ∆C ∆Y

=‌

1

1−MPC

There is a direct relation between K and MPC. If MPC is high, K will also be high but if MPC is low K will also be low.