Why signal is weak inside buildings and basements

Weak signal indoors happens because the radio waves from a mobile mast lose strength every time they pass through a wall, a floor, metal or coated glass. A basement is worst of all, with soil and concrete all around. The usual fixes are to move towards a window or upper floor, use Wi-Fi calling, or ask your carrier about an approved indoor solution.

How mobile signal gets into a building

Mobile signal is a radio wave sent from a mast that may be hundreds of metres or several kilometres away. Outdoors, it travels in the open and fades with distance. Indoors, it also has to get through the building itself. Each obstacle takes a bite out of the signal.

Signal can enter through:

Windows, especially plain glass.
Walls, losing strength on the way.
Openings like doors and stairwells.
Reflections from nearby buildings.

By the time it reaches the middle of a building or a room below ground, there may be little left.

Which materials block the most?

Some materials barely matter. Others block signal heavily.

Wood, plasterboard and plain glass: usually mild losses.
Brick and stone: moderate losses, more with thick walls.
Concrete, especially reinforced with steel: heavy losses.
Metal: very heavy. Metal roofs, cladding, foil-backed insulation and lift shafts can almost stop signal.
Energy-efficient windows: many have a thin metallic coating to keep heat in, which also reflects radio waves.
Soil and rock: basements and underground car parks are surrounded by them, so signal often cannot get in at all.

That is why a modern, well-insulated office can have worse signal than an old brick house next door.

Why some bands get indoors better than others

Mobile networks use several frequency bands. As a rule, lower frequencies travel further and get through walls better, while higher frequencies carry more data but fade faster and struggle with obstacles.

Low bands, often below 1 GHz, are the ones most likely to reach you indoors.
Mid bands carry much of the everyday 4G and 5G traffic and lose more strength through walls.
Very high 5G bands, called millimetre wave where they are used, barely get through walls or even glass.

So indoors, your phone often switches to a lower band. It might show 4G instead of 5G, or a slower 5G. Our guide on why your phone drops from 5G to 4G covers this in more detail.

How to tell if your weak signal indoors is the building

Check your signal strength in dBm outside the front door.
Walk inside and check again in a few rooms.
Go to the basement or the middle of the building and check once more.
Compare the readings. A big drop as you go inside means the building is the main cause, not the network.

If the signal is weak even outdoors, the problem is coverage in your area, not your walls. Our guide on how to get better mobile signal at home covers both cases.

What helps with weak signal indoors

Find the best spot. Signal can vary a lot between rooms, and even within a room. Windows facing the nearest mast are often best. See how to find the spot with the best signal in your house.

Go up. Upper floors usually have less in the way than ground floors, and basements are usually worst.

Use Wi-Fi calling. If your carrier supports it, calls and texts go over your home internet when mobile signal is poor. This is often the simplest fix. See Wi-Fi calling: when to turn it on.

Ask your carrier about indoor options. Some carriers offer approved indoor units or small cells for homes and businesses with poor coverage. Availability and cost vary.

Be careful with boosters. In many countries, it is illegal to use a mobile signal repeater that the regulator or carrier has not approved, because a badly set up repeater can interfere with the network for others. Check local rules before you buy one.

What does not help

Apps that claim to boost signal. No app can make a radio signal stronger. An app can only measure it and help you find a better spot.
Stickers and antenna patches. These do not improve reception.
Holding the phone higher in the same spot. Moving a few metres helps far more than lifting your arm.

Why GPS fails too

Satellite navigation signals are even fainter than mobile signals and suffer more indoors. If your map location jumps around in a building, that is the same problem in a stronger form, because the satellites are much further away than any mobile mast.

How CellRadar helps

CellRadar shows your live mobile signal in dBm with network type, band and cell ID, so you can compare readings room by room and floor by floor. The signal map puts your own readings on a map, which makes it easy to see where coverage drops. It cannot boost the signal; it helps you find the best spot and know when to call your carrier.

Frequently asked questions

Why do I have signal outside but not inside?

The building's walls, roof, metal and coated windows absorb the signal. Readings that drop sharply as you go inside confirm the building is the cause.

Why is signal worse in new buildings?

New buildings often use more concrete, metal and energy-efficient coated glass, which block radio waves more than older materials.

Does 5G work in basements?

Usually only on lower bands, if at all. Higher 5G bands struggle to get through walls and soil, so phones often fall back to 4G or lose signal underground.

Are signal boosters legal?

It depends on the country. In many places, only repeaters approved by the regulator or your carrier are allowed, so check before buying one.