Faraday Bag Guide: What They Really Block, What They Don’t and How to Test One

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A Faraday bag is a pouch lined with conductive material that blocks or weakens radio signals to whatever is inside. People buy them for three main reasons: to stop keyless car theft, to take a phone off the network, and to store spare electronics in case of an EMP. It works well for some of those jobs and is oversold for others.

This guide explains what the bags really do, what federal guidance says about them, how to test one at home, and where your money is better spent.

What is a Faraday bag and how does it work?

The name comes from the Faraday cage, an enclosure of conductive material that blocks electromagnetic fields. A bag applies the same idea in flexible fabric. Metal-lined layers intercept radio energy before it reaches the device inside, so your phone can’t hear the tower and your key fob can’t reach your car.

The key word is “reduce.” The National Institute of Standards and Technology (NIST) says Faraday containers may weaken a radio signal without eliminating it completely, so a device very close to a cell tower could still connect. A bag is a filter with very small holes, not a magic wall.

Federal guidelines published by CISA describe the bags as the most basic type of shielding, built mainly for small electronics such as cell phones, key fobs, tablets, laptops and handheld radios. The frequencies covered vary by product: some work as low as 10 MHz, and others as high as 18 GHz.

What these bags are good for

Key fobs and relay theft

This is the most practical everyday use. The Associated Press reports that thieves use equipment to pick up a key fob’s signal and relay it to get into a car, and that this can work while the fob sits inside your house. A pouch blocks that signal. AP’s advice includes keeping all fobs, spares too, inside one when you leave the car.

If you only need this, a small pouch is enough. A pouch won’t stop every kind of car theft, but it does close this particular door. If you’re worried about your vehicle and EMPs too, see our guide to EMP protection for your car.

Phone privacy

Law enforcement uses these bags to keep seized phones off the network. A 2012 National Institute of Justice evaluation found that one forensic pouch blocked the devices from attaching to a cell network in every instance. That is one product, though, not a blanket result.

NIST also describes a Purdue University field test in which shielding products didn’t prevent network communication in all cases, with text messages most often getting through. NIST notes some of those products have since improved, and it tells examiners to test their own products before relying on them. You should do the same.

Storing spare electronics for an EMP

Here’s where the calm answer helps. The CISA-published guidelines say small handheld electronics are relatively immune to EMP effects, unless they have long antennas or power cords attached, so Faraday cages are of secondary importance for them. In other words, your flashlight probably isn’t the problem.

Still, the same guidelines list “wrap spare electronics with aluminum foil or put in Faraday containers” as a low-cost protective step. They say one bag should be enough for most lower-level protection needs, and that nesting bags or storing them in a metal container adds more protection. Our EMP protection guide covers the bigger picture, and the guide to the best EMP protection for your home covers the house itself.

How to choose a Faraday bag: 5 checks

1. Look for a tested bag, not just tested fabric. The guidelines say the closure is often the main leak, and advise looking for bags that state the entire bag was tested.
2. Check the frequency range. The guidelines note that most bags cover at least the pre-5G cellular range, plus GPS and RFID frequencies, and many cover Wi-Fi. If you want 5G blocked, confirm it in the listed specs.
3. Read what it’s actually for. Many bags sold as Faraday bags are made for evidence collection and may offer only electrostatic protection.
4. Don’t confuse it with Mylar. A standard Mylar food bag provides little protection, according to the guidelines.
5. Buy the right size. A bag needs to close properly with your device inside. A forced seal is a leaking seal.

How to test one at home

Testing takes about two minutes and costs nothing:

1. Phone test: Seal your phone in the bag and call it from another phone. If it rings, the bag is leaking. Try this in a spot with a strong signal, because a weak-signal area can make a bad bag look good.
2. Key fob test: AP’s advice is simple. Put the fob in the pouch and walk up to your car. If the doors don’t unlock, the signal is being blocked.
3. Radio test: Turn on a small AM/FM radio, seal it inside and listen for the station to fade.

One honest caveat: passing these tests shows the bag blocks those particular signals. It doesn’t prove how the bag would do in a real EMP, because a home test can’t recreate one.

Mistakes to skip

• Leaving your phone on inside it. NIST says a device that can’t reach a network raises its signal strength to maximum, which shortens battery life. If you’re storing it, power it off first.
• Leaving a charging cable in the seal. NIST warns that exposed cables can act as an antenna and let signal in, and an improperly sealed bag can leak.
• Trusting a metal trash can. The guidelines say trash cans don’t usually provide reliable EMP protection unless modified to close the gaps in the lid, handles and base.
• Using a bag alone for your important gear. Bags wear out. Inspect folds and seams from time to time.

Foil, a soft bag or a hard case?

For a tight budget, the guidelines describe a do-it-yourself method: put the small device in a plastic or paper bag, then wrap it in heavy-duty aluminum foil so it is completely covered, with all seams overlapping, using two or more layers if possible. It’s cheap and it works as a stopgap, but it’s a nuisance to open and rewrap.

A bag is better for things you use often, like key fobs. A rigid Faraday container is more durable and stacks well if you’re storing a lot of gear. For most families, a bag or two is plenty.

What a Faraday bag won’t do in an outage

A bag protects a few small devices. It won’t keep the lights on or the food coming. The CISA-published guidelines pair shielding with practical steps, including storing a week of food, water and other supplies, and keeping a battery AM/FM/NOAA radio for emergency alerts. Those matter for far more likely events than an EMP, such as a storm that takes out the power. Our power outage emergency kit guide and emergency radio guide are a good place to start.

For food, the 1-Week Food Supply Ammo Can matches that one-week goal for one person, with 14,860 calories and a shelf life of up to 25 years when stored properly. For small backup power, the Grid Doctor 300 Solar Generator covers phones, radios, lights and laptops, but not large appliances.

If you want bigger backup power with EMP in mind, the Grid Doctor 3300 Solar Generator is a different kind of product. It doesn’t go in a bag. It’s a 2048Wh system with what the maker calls EMP Intercept Technology, designed to help protect connected electronics from surges, lightning events and electromagnetic disturbances. It’s sized for multi-day outages where refrigeration matters.

Frequently asked questions

Do Faraday bags really work?

Good ones reduce radio signals a lot, but results vary by product and seal. NIST says shielding isn’t always complete, so test yours before you trust it.

Will a Faraday bag protect my phone from an EMP?

It can add a layer of protection, and federal guidelines list Faraday containers as a low-cost step for spare electronics. They also say small handheld devices are relatively immune unless attached to long antennas or cords.

Can I use aluminum foil instead?

Yes, for storage. Wrap the device in a plastic or paper bag first, then cover it completely in heavy-duty foil with overlapping seams, using two or more layers if you can.

Should I keep my phone in a Faraday bag all the time?

No. You can’t get calls while it’s shielded, and a phone left on inside burns battery looking for a signal. Use it only when you have a reason to.

The bottom line

A Faraday bag is a useful, cheap tool for key fobs, phone privacy and storing a few spare electronics. It isn’t an EMP plan on its own. Buy one that lists whole-bag testing, test it yourself, then put your real effort into the supplies your family will need in much more likely emergencies.

Sources: CISA / National Coordinating Center for Communications EMP Protection and Resilience Guidelines, NIST SP 800-101 Rev. 1, National Institute of Justice evaluation, Associated Press. Product details are from My Patriot Supply product pages as of October 27, 2026.