ISP Speed Reality Check
Your ISP sells you a number. Your devices show you a different number. This calculator works out the gap, how big it is, and which bit of the chain (the line, the router, the wifi, the time of day) is doing the damage.
Explain like I'm 5 (what even is this calculator?)
Tell it the speed your ISP advertised, what kind of broadband you have, where the router lives in the house, and what time of day you usually test. It tells you, in plain numbers, how fast you should actually expect things to feel, and what is to blame for the gap.
Check your real-world speed
Fill in the form and press the button to see how your real speed compares to the advertised one.
Estimated wired sync speed:
Estimated speed at the device (over wifi):
Percentage of advertised:
Main bottleneck:
Recommendation:
Prove it
Here is the working from advertised speed down to the figure your device sees. Each step takes a slice off in the order signals actually drop in the real world.
Attenuation rules of thumb come from the ITU G.993.2 (VDSL2) and G.992.5 (ADSL2+) curves; wifi caps reflect typical real-world throughput, which is well below the marketing PHY rate (the IEEE link rate). Peak-hour contention follows the Ofcom voluntary speed code published average for UK fixed-line ISPs.
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Reality check came back disappointing?
If the line itself is the bottleneck, a faster package, or full fibre where it is available, is the real fix. Compare what is on offer at your address.
Compare broadband deals at broadband.co.ukWhy your real speed is almost never the advertised speed
Broadband adverts quote a number that, technically, exists. It is the figure the line could sync at if every variable lined up at once: a short, clean piece of copper, a router parked in the next room from your laptop, a wifi standard newer than your kettle, and a Tuesday afternoon when nobody else is online. That number is real, in the sense that an Olympic 100 metre time is real. It is not, however, the number you experience on a Wednesday evening with three streams running and the router behind a fish tank.
The gap between sold speed and felt speed has four main culprits. Line attenuation, where the copper between your house and the cabinet eats signal, hits FTTC and ADSL hardest and full fibre not at all. Wifi loss eats anything wireless, with the size of the bite set by the router standard and how many walls between you and it. Router standard caps anything wifi can do regardless of how fast the line is. Peak hour contention takes a flat percentage off everyone's evening because the network is busier. This calculator pulls all four apart so you can see which one is actually costing you.
FTTC, FTTP and the difference distance makes
On FTTC, the fibre stops at the green street cabinet and copper takes over from there. That copper is the limiting factor: at around 200 metres a VDSL2 line can sync at the full 80 Mbps. By 500 metres it is closer to 60. By 1.5 kilometres you are lucky to see 25, and by 2 kilometres you are squarely back in ADSL territory. ADSL2+ is the same physics, on older copper, with a softer ceiling. FTTP runs the fibre all the way to the property and the copper limit goes away entirely. If your address has FTTP available and you are still on FTTC, distance is the upgrade conversation.
Cable and fixed wireless are different beasts
DOCSIS 3.0 cable lines typically deliver close to advertised in the morning and noticeably less at peak, because cable has historically been a shared local segment. DOCSIS 3.1 is much better at handling load and tends to keep its evening speeds within a few percent of off-peak. 5G fixed wireless can be excellent or rotten depending on signal strength to the local mast and how many other people are connected to it. 4G FWA is fine for an emergency, but on a busy mast it gives up quickly. None of those technologies suffer from copper attenuation, but contention and signal strength take their place.
Why your wifi is probably the real bottleneck
If you have a 500 Mbps line and your laptop reports 90 Mbps, the line is not the problem. The wifi is. Real-world wifi throughput, the figure that ends up at the device, is roughly half the marketing PHY rate at best, and falls off a cliff with each wall. Wifi 4 caps out around 150 Mbps in practice. Wifi 5 manages 300 to 500 Mbps in the same room. Wifi 6 starts approaching the gigabit line speeds full fibre offers, and 6E and 7 widen that further with the 6 GHz band. If you upgraded your line and not the router, you paid for capacity you cannot reach.
Peak hour contention, and why ads now quote 8pm
Ofcom's voluntary code on broadband speeds requires UK ISPs to publish a realistic peak-time average, measured at 8pm on a weekday, alongside any "up to" figure they advertise. The gap between the two is usually 10 to 20 percent. The reason is contention: the local exchange, the backhaul to the wider internet, and the ISP's own capacity are all sized on an assumption that not everyone is streaming at the same time. When everyone does, those shared bits get squeezed. There is nothing wrong with the line. There is just more demand on the network.
If the calculator says your speed is fine and it does not feel fine
Speed is not the only thing that makes a connection feel fast. Latency, jitter and packet loss can ruin an otherwise quick line, especially for video calls and gaming. If the reality check says you should be getting close to advertised but the experience is still bad, it is worth running a latency impact check and a VPN throughput estimate if you are routing through one. Bandwidth is the easy thing to talk about. Latency is the thing that actually decides whether your call freezes.
Related calculators
Reality-check is one diagnostic. These cover what to do once you know the truth.
Frequently asked questions
Why am I not getting the broadband speed I pay for?
ISPs advertise the maximum sync speed your line can hit in lab conditions. Distance from the cabinet, copper quality, where the router lives, the wifi standard it supports and peak-hour contention all chip pieces off the headline figure.
Does distance from the cabinet really matter?
On FTTC and ADSL, yes, a lot. On FTTP it does not, because the fibre runs all the way to the property.
How much speed do I lose to wifi?
More than most people think. Two walls away on wifi 4 you can lose more than half the line speed. Wifi 6 next to the router barely registers a hit.
What is peak-hour contention?
Shared parts of the network get busier in the evening, so speeds dip 10 to 20 percent at peak compared to off-peak. UK ISPs now have to publish a realistic 8pm average alongside any "up to" figure.
Should I switch ISP if I am getting much less than advertised?
Find out what the bottleneck is first. If wired sync is fine and the wifi is poor, a router upgrade is cheaper than switching. If the line itself is the limit and FTTP is available, that is the upgrade conversation.