Outdoor vs indoor: the most important distinction
Every coverage map distinguishes between outdoor and indoor coverage, though some present only one by default. Outdoor coverage means there's sufficient signal when you're outside with a clear view of the sky. Indoor coverage means the signal is strong enough to penetrate a typical building — a significantly harder standard to meet.
A postcode that shows full outdoor coverage might have very poor indoor signal. If you're checking coverage for your home, always look at the indoor figure. CheckCoverage shows both scores side by side so you can compare them directly.
Coverage layers: 4G vs 5G
Most coverage maps include separate layers for 2G, 3G, 4G and 5G (3G is now largely obsolete — see our 3G switch-off guide). The 4G layer is the most useful for everyday assessment: it determines the speed and reliability of your data connection and, through VoLTE, your voice calls. The 5G layer shows where significantly faster data speeds are available, but it's typically a subset of the 4G coverage area.
If a network shows good 4G but patchy 5G at your location, you'll still have a perfectly usable connection — just without 5G speeds. If the 4G coverage is poor, 5G presence in the same area won't help much.
What do coverage scores mean on CheckCoverage?
CheckCoverage converts Ofcom's postcode-level data into a 0–100 score for each metric: outdoor signal, indoor signal, 4G reliability and 5G availability. A score of 100 means Ofcom predicts full coverage from that network at your postcode. A score of 0 means no coverage predicted. Scores in between reflect partial coverage — common in postcode districts that span both urban and rural areas.
The scores are derived from Ofcom's Connected Nations dataset, which is updated annually. They represent the best independent third-party prediction of coverage — neither provided by nor verified by the networks themselves.
Why maps and reality diverge
Coverage maps are predictions, not measurements. A network models signal from each mast location using known transmission power, antenna configuration, terrain elevation data, and standard building loss factors. The model can't account for your specific building's construction, local obstructions, or the orientation of rooms within a property.
In practice, this means coverage maps are more accurate in open, flat areas and less accurate in dense urban areas (where signal reflections and building configurations vary enormously) and in hilly terrain (where line-of-sight assumptions can be very wrong). If a map shows you're on the edge of coverage, real-world experience may differ significantly from the prediction in either direction.
Frequently asked questions
Why does a coverage map say I have signal but my phone says I don't?
Coverage maps show predicted signal based on modelling from known mast locations. They're estimates, not measurements. Real-world factors — terrain, vegetation, building materials, even weather — can reduce actual signal below the predicted level. If you're in a spot the map shows as covered but your phone disagrees, the real-world measurement wins.
What's the difference between 'predicted' and 'measured' coverage?
Predicted coverage is calculated mathematically from a network's mast locations and transmission power. Measured coverage comes from actual drive tests or crowd-sourced data from phones in the field. Ofcom's data is largely prediction-based; tools like OpenSignal use measured crowd-sourced data. Both are useful but have different strengths.
How often are coverage maps updated?
Ofcom publishes updated coverage data annually, typically each spring. Individual network maps may update more frequently as new masts go live, but there's always a lag between a mast being activated and it appearing on a map.