This interface utilization calculator turns two readings of an interface byte counter into the average bandwidth and the percentage of the link in use, the same maths monitoring tools such as MRTG, Cacti, LibreNMS or Zabbix do for you. It runs entirely in your browser: nothing you type is sent to our server.
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Table of Contents
Getting the counter values
# SNMP (64-bit counters; replace the index 2 with your interface)
snmpget -v2c -c public 192.0.2.1 IF-MIB::ifHCInOctets.2 IF-MIB::ifHCOutOctets.2 IF-MIB::ifHighSpeed.2
# Linux, no SNMP needed
cat /sys/class/net/eth0/statistics/rx_bytes /sys/class/net/eth0/statistics/tx_bytesRead the counter twice, a known number of seconds apart, and enter both values. Full-duplex links have separate capacity in each direction, so calculate inbound and outbound separately.
The formula
Utilization % = (counter2 − counter1) × 8 ÷ seconds ÷ interface speed in bits per second × 100. If the second reading is lower, the counter wrapped: the calculator adds 232 or 264.
Why 64-bit counters matter
A 32-bit octet counter (ifInOctets) holds about 4.29 GB. At 1 Gbps it wraps every 34.4 seconds, so a 5-minute poll cannot tell how many times it wrapped. Use the 64-bit ifHCInOctets / ifHCOutOctets (SNMP v2c or v3) on anything faster than about 100 Mbps.
How we checked it
We verified the maths on 11-12 October 2026: 3.75 GB in 300 s on a 1 Gbps link gives 100 Mbps and 10.00%, a wrapped 32-bit reading is corrected, and the 32-bit wrap time at 1 Gbps is 34.4 s.
Frequently asked questions
How do I calculate bandwidth from SNMP counters?
Subtract the first octet reading from the second, multiply by 8 to get bits, and divide by the seconds between readings. Divide by the interface speed to get utilisation.
Why does my graph show impossible spikes?
A 32-bit counter wrapped more than once between polls, or the device rebooted and reset its counters. Switch to the 64-bit ifHC counters and poll more often.
Which OID gives the interface speed?
ifHighSpeed (1.3.6.1.2.1.31.1.1.1.15) gives the speed in Mbps. The older ifSpeed is capped at 4.29 Gbps.