Independent comparisons. We may earn from shop links. More info

50 mm or 70 mm smoke alarm magnet

Updated: October 2026

You are holding a battery alarm and two pack sizes, and the question is which disc will meet the flat back. A 50 mm or 70 mm smoke alarm magnet is a size decision first and a price decision second. On our [smoke alarm magnet comparison](/hub/fire-protection/smoke-alarm-magnet) the 70 mm option is the ECENCE ten-pack at about £11.99, and the 50 mm options are the ten-pack at about £10.99 and the three-piece at about £5.99. The IME pad has no diameter, so it is not a third vote. Peter F. checked amazon.co.uk on 20 October 2026 and did not measure your alarm in a lab. A purchase through our links can earn a commission at no extra cost to you.

You are holding a battery alarm and two pack sizes, and the question is which disc will meet the flat back. A 50 mm or 70 mm smoke alarm magnet is a size decision first and a price decision second.

Our top picks

ECENCE ECENCE 70 mm magnetic mount, 10 pieces

Best overall

ECENCE ECENCE 70 mm magnetic mount, 10 pieces
9.5/10about £11.99
View offer
ECENCE ECENCE 50 mm magnetic mount, 10 pieces

Best value

ECENCE ECENCE 50 mm magnetic mount, 10 pieces
9.3/10about £10.99
View offer
ECENCE ECENCE 50 mm magnetic mount, 3 pieces

Rank 3

ECENCE ECENCE 50 mm magnetic mount, 3 pieces
7.8/10about £5.99
View offer
On this page
  1. 1.Measure the flat back
  2. 2.When 50 mm is the match
  3. 3.When 70 mm is the match
  4. 4.The pad with no diameter
  5. 5.Which disc to buy

Measure the flat back

Lay a ruler across the flat area the plate will touch. A rim, a battery hatch, or ribs make that patch smaller than the outer case. The glue only works on a continuous face. Measure the case and the disc can overhang while only the middle sticks. The kept ECENCE titles state 50 mm or 70 mm. A table of 5 cm and 7 cm bases sits on another listing, B01FUFN1PK, which is not in this comparison. Safelincs says bases at least 70 mm across or 60 mm along each edge. Bellman & Symfon sells a kit for alarms at least 70 mm. Ignore a stray 7 mm line that contradicts the 2.8 inch size on that page. Whether to buy a plate at all is the smoke alarm magnet buying guide.

When 50 mm is the match

Choose 50 mm when the flat back is small and the plate can sit without bridging ribs. Both listings claim 500 g. B07G18J8DY is ten plates at about £10.99, about £1.10 each, with 4.6 from 1,842 reviews. B07G1BF811 is three plates at about £5.99, about £2 each, with 4.5 from 1,245 reviews. The ten-pack is for several small alarms. The three-piece is the short run. A review called 50 mm unsuitable for larger and interlinked alarms. The two pages share a photo and one Best Sellers Rank line, so pack size is the title. A correct 50 mm disc on Artex still fails.
ECENCE ECENCE 70 mm magnetic mount, 10 pieces

Testsieger

ECENCE ECENCE 70 mm magnetic mount, 10 pieces
9.5/10about £11.99
View offer

When 70 mm is the match

Choose 70 mm when the flat back is about 70 mm or more, or about 60 mm along each edge as Safelincs prints for its own kit, and the face is flat. B01FUFN6MS is ten plates at about £11.99, about £1.20 each, with 4.6 from 2,047 reviews. The bullets claim 500 g, the same figure as the 50 mm packs. You pay about £1 more than the 50 mm ten-pack, about 10p a plate. That pays off when the back needs the larger contact, and is wasted on a mini alarm. Only six of the first 50 reviews are from the United Kingdom. Do not import 550 g from B01FUFN1PK. The bullets on B01FUFN6MS stay at 500 g.

The pad with no diameter

The IME three-piece, B0DCG9MFZ9, is about £5.99 and measures 5 × 5 × 1 cm, at 90 g. A square of 5 cm is not a 50 mm disc, and the page never calls it 50 mm. There is no load in grams, so it is not a vote on either side. The line is 3.8 from 56 ratings. Several say the alarm fell within a week, and a UK review says the pad is not the 3M brand named in the bullets. Stacking two 50 mm plates does not make 70 mm. You add thickness, not a wider glue patch.

Which disc to buy

Buy the diameter that matches the flat back, or use the screws. The 70 mm ten-pack is the larger face, at about £1 more than the 50 mm ten-pack. The 50 mm ten-pack is for several small alarms, and the three-piece is the short run. The IME pad is neither vote. If the paint will not take adhesive, both diameters are the wrong tool.

Common questions

Do I measure the case or the flat back?
Measure the flat back, the area that will touch the plate. The outer case is often wider, and ribs or a hatch reduce the glue contact. A 90 mm case with a 65 mm flat centre is a 70 mm question. A mini alarm with a flat centre around 40 mm is the other way round. The ruler decides, not the case.
Is 70 mm always the safer plate?
No. It is the better match when the flat back is about 70 mm or meets the edge rule Safelincs prints for its own kit. On a small alarm the extra metal overhangs and does not add grip. It is no safer on a rough ceiling, where the adhesive fails. Both sizes claim 500 g.
Why do both sizes claim 500 g?
Because that is what the bullets say on B01FUFN6MS, B07G18J8DY, and B07G1BF811. The kept 70 mm page does not claim 550 g. A different listing, B01FUFN1PK, puts 550 g in a table cell. Leave it there. A matching claim does not make the discs interchangeable. Diameter still has to meet the flat back.
What is the IME pad's diameter?
The listing does not state one. It gives 5 × 5 × 1 cm and 90 g, at about £5.99, with 3.8 from 56 reviews. A square of 5 cm is not a 50 mm disc and not a 70 mm disc. There is no numeric load to set beside the ECENCE 500 g bullets. Consider it only after you have measured that square against your alarm, and read the fall reports in the same sitting.
Is the 70 mm ten-pack worth about £1 more than the 50 mm ten-pack?
Yes, when the flat back needs about 70 mm of contact. You pay about £11.99 instead of about £10.99, about 10p more a plate, for the larger disc. No, when 50 mm already covers a small alarm. That extra pound is a size gap. Neither price fixes a ceiling the adhesive cannot wet.

Related pages

Best magnetic smoke alarm mount

How to fit a magnetic smoke alarm mount

Smoke alarm magnet

Smoke alarm magnet buying guide: plates, not alarms

Smoke alarm magnet on a textured ceiling will fail

Sources

Public UK sources from the research digest, retrieved 20 October 2026. Listing prices, stars, and review counts are the amazon.co.uk figures stored with this comparison.

  1. Fire Kills — detection (retrieved 20 October 2026)
  2. GOV.UK — fire safety in the home (retrieved 20 October 2026)
  3. GOV.UK — smoke and carbon monoxide alarms, landlord booklet (retrieved 20 October 2026)
  4. legislation.gov.uk — Smoke and Carbon Monoxide Alarm (England) Regulations 2015 (retrieved 20 October 2026)
  5. Scottish Government — fire and smoke alarms in Scottish homes (retrieved 20 October 2026)
  6. Scottish Government — tolerable standard, fire and carbon monoxide detection (retrieved 20 October 2026)
  7. Safelincs — magnetic alarm mounting kit (retrieved 20 October 2026)
  8. Bellman & Symfon UK — magnetic mount kit (retrieved 20 October 2026)