IGRP metric for a path to destination is calculated by the following rather complex mathematical formula:

IGRP Metric for the path =

[K1 * (B) + (K2 * (B))/(256-(Load)) + K3*(D)] * [K5/((Reliability) + K4)]

where:

K1, K2, K3, K4, K5: all are constants. Default values are: K1=K3=1, K2=K4=K5=0

(B) = 10,000,000 / (Smallest bandwidth in kilobits, along the path)

(Load): Outgoing interface load at this router, measured from integer 1 (0%) to 255 (100%)

(D) = Sum of outgoing interface delays along the path, starting from this router, in micro seconds, then divide by 10

(Reliability): Outgoing interface reliability at this router, measured from integer 1 (0%) to 255 (100%)

When we fill K1 to K5 constants with default values into the formula, it becomes very simple:

IGRP Metric for the path = (B) + (D)

Wait, what about EIGRP metric? EIGRP metric is just equal to the calculated IGRP metric value multiplied by 256.

EIGRP Metric for the path = 256 * (IGRP Metric for the path)

Of course, after the router calculates metric values of all candidate paths to the destination, it choose the path with the smallest metric value, to put in its routing table.