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Two numbers decide more Wi-Fi arguments than any spec sheet: how much power your access point actually radiates, and how much of each channel your SSIDs use up before anyone sends a byte of data. In this blog, we'll walk through our two free Wi-Fi calculators, the EIRP Calculator and the SSID Overhead Calculator, with worked examples and the design fixes they point to.
Wi-Fi Calculators: Why These Two Numbers Matter
EIRP tells you whether an access point (AP) is legal and how far its signal really reaches. SSID overhead tells you how much airtime is gone before users get a turn. Both are simple sums, but they're easy to get wrong on a whiteboard, so we built two free calculators that do the maths in your browser.
The EIRP Calculator turns transmit power, cable loss and antenna gain into EIRP in dBm and milliwatts.
The SSID Overhead Calculator shows the share of a channel that goes on beacons, by band, SSID count, APs on the same channel and minimum data rate.
Neither needs a login or an email address. They sit alongside our Wi-Fi Access Point Estimator, which gives you a first AP count for a site.
EIRP Calculator: What Is Your Access Point Actually Radiating?
EIRP (Equivalent Isotropically Radiated Power) is the transmit power of the radio, minus any cable and connector loss, plus the gain of the antenna. The ACMA (Australian Communications and Media Authority) sets Wi-Fi limits as EIRP, not transmit power, so EIRP is the number that tells you whether a radio is within the rules.
The formula is EIRP (dBm) = transmit power (dBm) − cable loss (dB) + antenna gain (dBi). Because everything is in decibels, it's just addition and subtraction. The calculator does it for you and converts the answer to milliwatts as you type.
Here's where it earns its keep. Say you're putting a directional patch antenna on a warehouse aisle AP, using the lower 5 GHz channels. The radio is set to 17dBm, the cable and connectors lose 1.5dB, and the antenna has 8dBi of gain. That's 23.5dBm, or about 224mW. The limit on those channels in Australia is 200mW (23dBm), so you're just over.
For context, the ACMA's Low Interference Potential Devices (LIPD) class licence allows up to 200mW on the lower 5 GHz channels, up to 4W on the upper 5 GHz channels and in 2.4 GHz, and 250mW (24dBm) indoors in the 6 GHz band. Our What Is EIRP post has the full 5 GHz table.
One thing the calculator doesn't do is multi-chain gain. Modern APs transmit on several radio chains at once (MIMO), which adds up to about 6dB for four chains. The What Is EIRP post walks through that maths with a worked example.
Pro-Tip: When you fit an external antenna, enter its gain in the AP or controller configuration. Enterprise APs set to the Australian country code stay within the limits automatically, but only if they know the antenna gain. Leave it at the default and the AP can't calculate its own EIRP correctly.
And remember that more power isn't automatically better. An AP turned right up can be heard by a phone a long way off, but the phone transmits at much lower power and can't always reply. That one-way link is a common cause of "full bars, nothing works" complaints.
SSID Overhead Calculator: How Much Airtime Do Beacons Use?
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Every SSID on every AP sends a beacon about ten times a second, at the slowest basic data rate so every device can hear it. The SSID Overhead Calculator multiplies your SSID count by the APs heard on the same channel, works out how long each beacon takes to send, and shows the share of the channel that's gone.
The numbers can be startling. Take a 2.4 GHz network with four SSIDs, three APs heard on each channel, a 350-byte beacon and the factory-default 1Mbps minimum rate. Beacons use 35.6% of the channel. Add four more SSIDs and it's over 70%.
The minimum basic rate matters even more than the SSID count. Raise that same network's minimum rate from 1Mbps to 12Mbps and beacon airtime drops from 35.6% to 3.4%, because each beacon is sent about ten times faster.
So does the band change anything? At the same data rate, a beacon takes about the same time in 2.4, 5 or 6 GHz. The difference is which rates you're allowed to use. The slow 802.11b rates only exist in 2.4 GHz, while 5 and 6 GHz start at 6Mbps. The same four SSIDs at 6Mbps in 5 GHz use 6.2%.
6 GHz has one more trick. APs normally use Multiple BSSID there, sending one beacon that lists every SSID instead of one beacon per SSID. The calculator switches to Multiple BSSID when you pick 6 GHz, and the same example drops to 2.0%.
The calculator follows the same method as Revolution Wi-Fi's well-known SSID overhead calculator, and uses the same Low, Medium, High and Very high bands. It doesn't model collisions or retries, so real overhead is a little higher.
Design Fixes: What to Do With the Numbers
If the SSID Overhead Calculator shows High or Very high, the fixes are fewer SSIDs, a higher minimum data rate, or fewer APs on each channel. If the EIRP Calculator shows you're over the limit or running radios flat out, the fix is usually more APs at lower power, not more power.
Cutting SSIDs is the cleanest fix. Most enterprise networks need no more than four to five. Identity PSK (iPSK) lets staff, guest and IoT groups share one SSID, each with its own key and policy, so you keep the separation without the beacons.
Raising the minimum basic rate is the quickest win, but check your devices first. Disabling the 802.11b rates drops any client that can't do anything faster, which can include old scanners, medical devices or building systems. It also shrinks each AP's cell a little, which is usually what you want in a dense design.
Too many APs on one channel is a design problem rather than a setting. Lower transmit power, sensible power caps and RF tuning after install keep co-channel APs from hearing each other across the building.
Pro-Tip: Run both calculators before you change anything, and save the numbers. They make a clear before-and-after story for the change request, and for your boss.
Wi-Fi Design: When a Calculator Isn't Enough
Calculators give you the maths, not the building. How many APs a phone really hears on one channel, how walls and glass attenuate the signal, and where the interference comes from all need measuring on site. That's what a Wi-Fi survey is for.
We've walked into plenty of sites where the plan said four SSIDs and the air said eleven, because every new system quietly added its own. A survey finds those, along with the APs turned up to full power to "fix" a coverage complaint.
For an existing network, our Wi-Fi Assurance Review checks configuration and RF health against good practice, including SSID count, data rates and power settings.
Free Wi-Fi Calculators: FAQs
What Is an EIRP Calculator?
An EIRP calculator works out the power an access point actually radiates, from its transmit power, cable and connector loss and antenna gain. IPTel's free EIRP Calculator gives the answer in dBm and milliwatts, so you can compare it with the ACMA limit for the band you're using.
How Do You Calculate SSID Overhead?
Multiply the number of SSIDs by the APs heard on the same channel and the beacons each sends per second, then by the time one beacon takes at the minimum basic rate. IPTel's free SSID Overhead Calculator does this for 2.4, 5 and 6 GHz, including Multiple BSSID.
What Is the Maximum EIRP for Wi-Fi in Australia?
It depends on the band. Under the ACMA LIPD class licence, 2.4 GHz allows up to 4W EIRP, the lower 5 GHz channels 200mW, the upper 5 GHz channels up to 4W, and 6 GHz 250mW indoors. Enterprise APs apply these limits when set to the Australian country code.
How Many SSIDs Should You Have?
Most enterprise Wi-Fi networks should have no more than four to five SSIDs. Beyond that, beacon overhead starts to take a real share of the airtime, especially in 2.4 GHz at low data rates. iPSK and Cisco ISE let you keep groups separate on fewer SSIDs.
Free Wi-Fi Calculators: Summary
EIRP and SSID overhead are two numbers every Wi-Fi network should be checked against: one keeps you legal and sizes your cells, the other shows how much airtime you're giving away. Our free EIRP Calculator and SSID Overhead Calculator do the maths in seconds, and the fixes they point to (fewer SSIDs, sensible minimum rates and more APs at lower power) cost very little compared with the performance they give back.
Need Help With Wi-Fi Design?
If the calculators show a problem, we can help you fix it properly. Our Wi-Fi survey and design team measures what's really happening on site, and the Wi-Fi Assurance Review gives an existing network a clear health check. For a wider view, try our free Network Assessment. If that's useful, get in touch with the IPTel team or drop us a line at sales@iptel.com.au.