115mm vs 125mm Angle Grinder Discs: Which Do You Need?

Short answer: buy the disc that matches your grinder, not the other way round. A 115 mm (4½″) grinder takes 115 mm discs; a 125 mm (5″) grinder takes 125 mm discs. The 125 mm gives you roughly 3–5 mm more cutting depth; the 115 mm is lighter and easier to control in tight spaces. Both use the same 22.23 mm bore, which is why the two sizes are so often confused.

What is the actual difference?

Cutting depth is not simply half the diameter — the guard, flange and spindle housing all eat into usable depth, so treat the figures above as working approximations and check your own machine.

Will a 125mm disc fit a 115mm grinder?

No — and it is not a modification worth attempting. The bore will slide onto the spindle, which is exactly what makes this dangerous: it feels like it fits. It does not, for two reasons:

• The guard will not cover it. A 115 mm guard cannot enclose a 125 mm disc. Removing or forcing the guard leaves the disc exposed and removes the thing that protects you if it shatters.
• Rim speed rises. A larger disc on the same spindle speed means the edge is travelling faster. Exceed the disc’s rated maximum and you are into failure territory.

Going the other way — a 115 mm disc on a 125 mm grinder — is mechanically possible and the guard will cover it, but you lose depth and the guard no longer sits close to the work. Buy the right size.

Does bore size matter?

Both sizes use a 22.23 mm bore as standard in the UK, which fits the spindle flange on virtually every 115 mm and 125 mm angle grinder. You will also see 20 mm bores on some larger discs (230 mm and up) where a reducing ring may be supplied. If you are buying for a standard 4½″ or 5″ grinder, 22.23 mm is the number you want.

Which size for which job?

Tile and porcelain cutting

Either size works for wall tiles. For 20 mm outdoor porcelain slabs the extra depth of a 125 mm disc is genuinely useful. Blades such as the Marcrist CK350S (from £7.00) and the Marcrist FZ550 come in both diameters.

General cutting and grinding

The SX-10 Superior segmented grinding disc (from £7.45) is available in 115 mm and 125 mm. For mixed-material cutting, the Marcrist BF850 SilentMAX spans 115 mm to 350 mm.

Mortar raking and repointing

This is where size matters most, because depth is the whole point. You need to rake to roughly twice the joint width — typically 15–20 mm — and both sizes manage that comfortably. Most raking kit is built to fit both: our Ultimate Repointing Blade (£7.99) and Superior Mortar Raking Blade (£12.95) suit all angle grinders, and the Dust Extraction Cutting Guard is made for 115 mm and 125 mm machines alike.

What about M14?

M14 is the spindle thread, not the disc size — it is the standard on almost all UK 115 mm and 125 mm grinders. Plain discs with a 22.23 mm bore mount on the flange and are held by the lock nut. Accessories that thread directly onto the spindle — core bits, mortar rake pins, some drill bits — are the ones described as “M14 fitting”, such as the Diamond Mortar Rake Pin. A keyless M14 flange lets you change plain discs without a spanner.

So which should you buy?

• Match your grinder. This is the only rule that really matters.
• Buying a grinder and undecided? 115 mm for indoor, overhead and confined work; 125 mm if you regularly cut thicker material such as 20 mm porcelain or paving.
• Check the bore is 22.23 mm.
• Check the max RPM on the disc against your grinder’s no-load speed — smaller discs are rated to spin faster.

Frequently asked questions

Can I put a 125mm disc on a 115mm grinder?

No. The guard cannot cover it and the rim speed exceeds what the pairing is designed for. It is one of the more common causes of disc failure.

Is a 125mm grinder better than a 115mm?

Not better — different. The 125 mm cuts deeper; the 115 mm is lighter and easier to control in confined spaces. Most trades that do a lot of overhead or indoor work prefer 4½″.

What bore size do 115mm and 125mm discs use?

22.23 mm, on both.

How deep will a 115mm disc cut?

Around 28–30 mm in practice once the guard and flange are accounted for — comfortably enough for mortar raking, which typically needs 15–20 mm.

Read next

• Tile cutting blade: your go-to guide — choosing between blades once you know your size.
• How to repoint brickwork — where cutting depth actually determines whether the job lasts.
• What drill bit for porcelain tiles? — the drilling equivalent of this question.

Not sure which fits your machine? Ask us — tell us the make and model and we will confirm the disc size and bore you need.