How to Test a Faraday Bag: 4 Home Tests That Actually Tell You Something

Short answer: if you want to know how to test a Faraday bag, the most useful home check is the AirTag test: seal a Bluetooth tracker inside and see whether a phone can still find it. The popular "call your phone" test is worth doing, but it has a blind spot. Below are four tests, how to read them, and what none of them can prove.

Before you test: seal it properly

Most failures are closure failures. SANS Internet Storm Center testers found Bluetooth audio still worked with a partially closed bag. Push the device well below the fold line, press the air out, and close it exactly as the maker says, with roll-top bags folded at least twice. Check the inside for holes or worn seams.

Test 1: the AirTag go/no-go test

Matt Blaze, a computer scientist known for security research, measured phone-sized Faraday bags with lab gear and concluded most people don't need precise numbers. His recommended quick test:

1. Drop an Apple AirTag (or similar tracker) in the bag and seal it.
2. From an iPhone, try to play a sound on it or locate it in Find My.
3. If the phone cannot reach it, the bag passes. Beware of cached locations in Find My, which can make a tag look reachable when it is not.

Blaze says this "won't tell you the exact attenuation level" but will tell you if blocking is "sufficient for most practical purposes," and can catch a bag that has been damaged.

Test 2: the phone call test (with a caveat)

1. Put the phone inside with ringer on, seal it, wait about a minute.
2. Call it from another phone. It should go to voicemail without ringing.

The blind spot, in Blaze's words: "you likely don't know how far away the cell tower is, what frequency it's using, or its signal strength." A bag that passes at home might fail near a tower. Repeat the test in a strong-signal spot.

Test 3: Bluetooth and Wi-Fi

Play music from the phone to a Bluetooth speaker, then seal the phone in the bag. The audio should cut out within seconds and stay off. For Wi-Fi, ping or message the device from another one on the same network; it should drop off.

Test 4: key fob and tap cards

Test the thing you bought it for. Seal a keyless car fob in the pouch, walk up to the locked car and try the handle; it should stay locked. Hold a contactless card in its sleeve to a reader; it should not register. Building fobs use lower frequencies that some pouches block poorly, so test those separately.

If it fails

Reseal with an extra fold and retest. If it still fails, stop relying on it. Linings are thin metal-coated fabric, and creases wear through. In Blaze's tests, two commercial pouches performed very well while makeshift solutions, including foil, did not do consistently well.

What no home test proves

• EMP or solar storm protection. A home test shows everyday signals are blocked. It does not measure shielding against a high-altitude EMP, the scenario in our Starfish Prime history. For grid-down storage, see our DIY Faraday cage guide.
• Total invisibility. A blocked phone cannot be reached by a cell-site simulator while sealed (see how to detect a stingray device), but carrier records of where it was before are a separate matter.
• Safety. A sealed phone gets no emergency alerts. Don't bag the phone you rely on in a fire, flood or storm, and turn phones off before long storage so they don't drain hunting for signal.

Documented vs opinion

Documented: the test methods and limits above come from Blaze's measurements and SANS ISC testing. Our opinion: buy a bag you can test, test it the day it arrives, and retest every few months. Untested gear is a guess.

If you are building a kit, our store carries field-tested preparedness and communications gear. Whatever you buy, run these tests on it.

Quick test checklist

• Seal fully, with the device below the fold line.
• AirTag or tracker: phone cannot locate it or play a sound. Pass.
• Call test in a strong-signal spot: no ring. Pass.
• Bluetooth audio cuts out within seconds and stays off. Pass.
• Car fob: locked car stays locked. Contactless card: reader sees nothing. Pass.
• Any ring, sound, unlock or read: reseal and retest. Second failure: replace the bag.

Write the date and result on a strip of tape on the bag. It sounds fussy, but a dated record is the only way to know a bag that worked last year still works today. Retest after travel, after the bag has been crushed in a pack, and whenever the lining shows white creases where the metal coating has cracked.

Sources

• Matt Blaze, "Testing Phone-Sized Faraday Bags": mattblaze.org
• SANS Internet Storm Center, "Signals Defense With Faraday Bags & Flipper Zero": isc.sans.edu
• PrepPro, "How to Test a Faraday Bag" (vendor guide, used for closure and fob steps): preppro.com.au

Frequently asked questions

What is the best way to test a Faraday bag at home?

Security researcher Matt Blaze recommends an AirTag go/no-go test: seal an AirTag in the bag and see whether an iPhone can locate it or make it play a sound. A call test is quicker but less reliable.

Why is the phone call test not reliable?

Your phone may be near a strong tower in one place and a weak one in another. A bag that silences a call at home may not block a stronger signal elsewhere.

Can aluminum foil work as a Faraday bag?

In Blaze's tests, makeshift options like foil did not perform consistently, though foil sometimes worked under ideal conditions that are hard to repeat.

How often should I test a Faraday bag?

Test it when new, after rough use and every few months. Linings wear at folds and seams.

Does a Faraday bag that passes a home test protect against EMP?

Not proven. A home test shows everyday radio signals are blocked; it does not measure shielding against an EMP or solar storm.