115mm vs 125mm Angle Grinder: Which Size Should You Choose?

Choose by cutting depth, access and discs

A 115mm vs 125mm angle grinder choice is mainly about cutting depth, access in tight spaces and the discs you keep in stock. Choose 125mm for broader coverage and deeper cuts; choose 115mm when compact handling matters more.

115mm vs 125mm: The Key Differences

The disc diameter is the real difference; the motor, weight and speed depend on the tool you choose.

115mm vs 125mm angle grinder comparison: cutting depth, grinding face, access and discs to stock — Compare 115mm and 125mm angle grinders by cutting depth, access and disc choice.

Is a 115mm grinder always lighter?

No. Weight depends on the tool body, motor and power source. M7’s QB-7245M (115mm) and QB-7215M (125mm) both weigh 1.8 kg (4.0 lb), with the same length, motor and spindle; the main difference is the disc they take.

Cutting Depth, Disc Coverage and Access

A 125mm disc reaches deeper and covers more surface; a 115mm disc is easier to place in confined work.

Cutting depth: A 125mm disc has a 5mm larger radius than a 115mm disc, which can provide more usable cutting depth on box section, plate and pipe. The exact depth depends on the grinder’s flange and guard, and on disc wear, so check the manufacturer’s stated cutting depth.
Disc coverage: A larger disc covers more surface per pass when grinding flat areas and long welds.
Access: A smaller disc and guard are easier to work into corners, frames and inside fabrications.

How deep can a 125mm angle grinder cut?

Less than half the disc diameter, because the flange, spindle and guard stop the cut before the centre of the disc. A worn disc cuts shallower than a new one. If regular work needs more depth than a 125mm grinder gives, a larger grinder is the better tool.

Which Size Is Best for Your Job?

Match the size to the job you do most often, not the occasional one.

When is 125mm the better choice?

When the work is mostly cutting section and grinding long welds.

Cutting box section, plate and pipe, where depth matters
Grinding back welds over long runs, where a larger face saves passes
Mixed metalwork when the workshop wants one grinder size to cover most cutting jobs

Need greater cutting depth for metalwork? View the M7 QB-7215M 125mm air angle grinder.

When is 115mm the better choice?

When access and control matter more than depth.

Working in tight corners, frames and enclosed fabrications
Detailed finishing and dressing smaller welds
Workshops that already stock 115mm discs

Need a compact grinder for detailed work? View the M7 QB-7245M 115mm air angle grinder.

M7 115mm and 125mm Air Grinder Comparison

M7’s 100° lever-throttle air angle grinders show the 115mm vs 125mm choice with everything else held equal. All figures are taken from the M7 catalogue (air angle grinder section, p.64), checked on 29 October 2026.

Specification	QB-7245 (S/M)	QB-7215 (S/M)
Disc size	115mm (4-1/2″)	125mm (5″)
Spindle	5/8″-11 / M14 x P2.0	5/8″-11 / M14 x P2.0
Free speed	11,000 RPM	11,000 RPM
Motor	1.3 HP	1.3 HP
Air pressure	90 PSI (6.3 bar)	90 PSI (6.3 bar)
Average air consumption	4.2 CFM (119 L/min)	4.2 CFM (119 L/min)
Air inlet / hose	1/4″ inlet, 3/8″ hose	1/4″ inlet, 3/8″ hose
Overall length	232mm (9-1/8″)	232mm (9-1/8″)
Net weight	1.8 kg (4.0 lb)	1.8 kg (4.0 lb)
Sound pressure	90.0 dBA	90.0 dBA
Vibration	1.2 m/s²	1.2 m/s²

Both models have a 100° head, a quick-release guard for easy angle adjustment, a spindle lock, and air-cooled spiral gears with double bearings. Both are listed in S and M versions, with the spindle given as 5/8″-11 / M14 x P2.0; confirm the version that suits your discs with your distributor. Discs are sold separately.

What does this tell a buyer?

Pick the disc size for the work, then pick the tool. When the body, speed and air demand are the same, the decision comes down to cutting depth, disc coverage, access and which discs you keep in stock.

Can You Use a 115mm Disc on a 125mm Grinder?

Only if the grinder’s manual allows it, and never the other way round. HSE Northern Ireland’s guidance sets the rules that decide this:

Every disc is marked with its maximum speed, and the grinder’s spindle speed must never exceed it.
A disc should only be mounted on the type of machine it was designed for.
The guard should expose only the part of the disc needed for the work, and it must stay in place.

In practice:

Smaller disc on a larger grinder: The disc must fit the spindle and flanges, carry a speed rating at or above the grinder’s free speed, and still be properly covered by the guard. If the manual does not allow it, don’t.
Larger disc on a smaller grinder: Never. The guard cannot cover it, and fitting it usually means removing the guard.

HSE’s guidance on abrasive wheels (HSG17) also covers training for people who mount discs.

115mm vs 125mm: Which M7 Grinder Should You Buy?

Choose the size first, then the power source that fits your workplace.

Your situation	M7 option	Power source
Compact, detailed work in a workshop with compressed air	QB-7245M (115mm)	Air, 90 PSI (6.3 bar)
Cutting and weld grinding in a workshop with compressed air	QB-7215M (125mm)	Air, 90 PSI (6.3 bar)
Cutting and grinding away from an air line	125mm cordless models	18V battery

Air or cordless?

Choose air if the workshop already runs compressed air; choose cordless if the work moves around the site. The air models need 90 PSI (6.3 bar) and an average of 4.2 CFM (119 L/min). Among the M7 models discussed here, 115mm is available as an air grinder; the cordless models are 125mm.

Working away from an air line? Compare M7’s 125mm 18V cordless angle grinders.

Sources and Specification Notes

Specifications come from the M7 catalogue, checked on 29 October 2026; safety rules come from UK health and safety guidance.

QB-7245 (S/M) and QB-7215 (S/M) specifications: M7 catalogue, air angle grinder section (p.64), checked on 29 October 2026. Product pages: QB-7245M, QB-7215M.
Disc speed, mounting and guard rules: HSE Northern Ireland, Abrasive wheels information sheet.
Training, mounting and portable grinders: HSE, Safety in the use of abrasive wheels (HSG17).
Cutting depth is not listed for these models; check the manufacturer’s stated figure before relying on a depth for your work.

Frequently Asked Questions

Is 4-1/2 inch the same as 115mm?

Yes. A 4-1/2″ disc measures about 114mm and is sold as 115mm in metric markets such as the UK. A 5″ disc is sold as 125mm.

Which is better, a 115mm or a 125mm angle grinder?

Neither is better for every job. A 125mm grinder gives more usable cutting depth and a larger grinding face, while a 115mm grinder suits tight, detailed work. Choose the size that matches the work you do most often.

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

Only if the grinder’s manual allows it, the disc fits the spindle and flanges, its maximum speed is at or above the grinder’s free speed, and the guard still covers it properly. Never fit a disc larger than the grinder is designed for.

Is a 115mm angle grinder lighter than a 125mm one?

Not necessarily. Weight depends on the tool body, motor and power source. M7’s QB-7245M (115mm) and QB-7215M (125mm) both weigh 1.8 kg (4.0 lb).

What disc speed rating do I need?

The disc’s marked maximum speed must be at or above the grinder’s free speed. Both QB-7245M and QB-7215M run at 11,000 RPM, so any disc fitted must be rated for at least 11,000 RPM.