Please help - bandwidth usage

Hello SNMP Gurus,

 

 I'm trying to track the bandwidth utilization on a Cisco switch using the
ifInOctets and ifOutOctets. What I'm doing is polling the port every 5
minutes and storing the octets and then using the last poll counter and the
current poll counter to figure out the amount of bits per second that has
been through the port. I've tried using a Cisco formula that goes something
like this:

 

-          Get the delta of the ifInOctets (take the two poll results and
subtract them to get the difference)

-          Then times that delta by 8 then by 100

-          Take that result and divide it by the time between then two polls
(300 seconds) times the ifSpeed (100000000).

 

This should give the bits per second. Now it's not 100% accurate but should
be close enough for me. The problem is that I'm not getting anything close
to what I'm expecting. My sample data is returning about 5 bps and a
monitoring program is telling me it's more like 1572864 bps. Any ideas on if
that equation above is wrong?