Fax line capacity calculator
Whether your fax lines can carry your busiest hour, or whether senders are getting a busy signal.
Inbound and outbound together — they share the lines.
Dialling, handshake and confirmation. Paid once per fax, not per page.
This tool runs entirely in your browser. Nothing you enter is sent to VitalFax or stored anywhere.
- Peak-hour utilisation
47%
Within capacity for the hour as described.
- Lines needed for this hour
- 1line
1 fewer than you have, on this hour's figures alone.
- Transmission time demanded
- 45minutes in the hour
- Faxes in the peak hour
- 10
Working
(40 pages × 1 min) + (10 faxes × 0.5 min) = 45 min against 96 usable min
Read this with the result
- Capacity is counted at 80% of the clock, not 100%. A line busy every minute has no room for a retry, and planning at full occupancy is how a set of lines that looks adequate produces engaged tones.
- A busy signal leaves no record on your side. If you cannot tell whether this is already happening, that is itself the finding.
About this calculation
A fax line carries one transmission at a time. When every line is busy the sender gets an engaged tone and either retries or gives up, and neither shows up anywhere you would look. This works out how much of your peak hour your lines can actually carry.
Formula
capacity = lines × 60 min × utilisation; demand = peak pages × minutes per page
- Methodology version
- 1.0.0
- Last reviewed
- 2026-10-29
What this does not account for
- A capacity check, not a queueing model. Real arrivals cluster, so a line set that is adequate on average still turns senders away in the worst ten minutes.
- Cloud fax has no line count in this sense: concurrency is the provider’s problem rather than yours. This tool is for the analogue or PRI case.