How to choose a utility knife: 9, 18 or 25 mm blade and lock type

If you cut cardboard, tape, sheathing and thin sheets all day, the most versatile utility knife is the one with an 18 mm blade and a wheel or slider lock. The 9 mm blade is the choice for precision cuts, the 25 mm blade for long cuts and tougher materials. In the collection the bodies range from 130 to 210 mm in length: the longer it is, the better the grip; the shorter it is, the easier it fits in a pocket.

The types

• Utility knife with slider lock: you push the blade out and pull it back by sliding a thumb slider, with one hand. It is handy when you change the blade length often and work wearing gloves. In the collection the slider models include SM9, SM18, Interlock 9, Interlock 18 and the FatMax FMHT10338-0.
• Wheel-lock utility knife: you push the blade out and lock it by turning the wheel. Wheel models include the FatMax FMHT10329-0 (18 mm) and FMHT10330-0 (25 mm), the DynaGrip MPO18 and the DeWalt utility knife with 25 mm locking wheel.
• Utility knife with 9 mm blade (for example the Cutter SM9 or the Cutter Interlock 9): narrow blade, for trimming, cutting shapes and precision work.
• Utility knife with 18 mm blade (such as the Cutter Interlock 18 or the FatMax FMHT10329-0): the most common size for general use, on site and in the warehouse.
• Utility knife with 25 mm blade (such as the Cutter FatMax 25 mm): wide, sturdy blade for long straight cuts and heavy work.
• Utility knife with built-in blade snapper: a system that lets you break off the worn blade segment without holding a separate tool. You find it, for example, on the 18 mm wheel Cutter STHT10500-0 and on the 25 mm DeWalt, which also has a lanyard slot.

The series names you will see (FatMax, DynaGrip, Interlock, SM) mean different families in shape, grip and locking system: always compare the data sheet of the individual model.

How to choose by job

Job	What to get	Why
Warehouse and logistics	18 mm slider knife with snap-off blade	You open boxes and cut tape every minute: you need to retract the blade one-handed and change it without tools. If the risk of cuts is high, see the section on safety knives.
Plasterboard fitting	18 or 25 mm knife with a sturdy body	You score the board along the straightedge: the wide blade stays steadier on the line.
Electrical work and sheathing	9 mm or 18 mm knife	You must score the sheath without nicking the conductor: short blade and a steady hand.
Precision work, shapes	9 mm knife	The narrow blade follows curves and fine lines.
Occasional use	Basic 18 mm knife	A simple model with a retractable blade is enough.

What to look at

Blade width. It comes in three sizes: 9, 18 and 25 mm. The narrow one is more precise, the wide one sturdier and more stable on long cuts. It also matters for spares: the spare blade must have the same width.

Blade thickness. In the models of this collection that report it, the blade is 0.4 mm thick (the 9 mm ones) or 0.5 mm (the 18 mm ones). A thicker blade flexes less on hard cuts, a thin one cuts cleaner on soft materials.

Body length. The models in the collection range from about 130 to 210 mm. A long body gives a better grip for forceful cuts; a short one fits in a trouser pocket or tool belt.

Snap-off or retractable blade. All the knives mentioned in this guide are of the Snap-Off type: the blade is divided into segments, and when the edge wears out you snap one off and have a new sharp one. Almost all also have a retractable blade, which goes back into the body after the job; in the data sheets of the STHT10500-0 and STHT10501-1 the "retractable blade" field is set to "No", so check before ordering if it is a requirement for you.

Blade finish. The data show standard carbon steel blades and induction-hardened blades. The standard carbon blade is the one on the SM, DynaGrip and Interlock; the induction one is the one on the FatMax and the two STHT. The data sheets do not say which lasts longer.

Body and grip. The Interlock models have a metal body, heavier than the plastic models. The SM models have an ABS grip; the FatMax and STHT models have a grip with rubber inserts, which improve hold with dirty or wet hands. The 25 mm DeWalt has a steel blade bar and a two-material grip.

Belt clip. In the Stanley data sheets of the models mentioned here the "belt clip" field is set to "No": if you need the knife always within reach, consider a pocket or a lanyard (the DeWalt has a slot for it).

The models mentioned, in numbers

The data are those of the product sheets.

Model	Blade	Lock	Body	Length
SM9 (0-10-150)	9 mm	Slider	Plastic, ABS grip	130 mm
Interlock 9 (0-10-095)	9 mm	Slider	Metal	135 mm
DynaGrip 9 (0-10-409)	9 mm	Slider	Plastic	135 mm
SM18 (0-10-151)	18 mm	Slider	Plastic, ABS grip	165 mm
Interlock 18 (0-10-018)	18 mm	Slider	Metal	165 mm
DynaGrip MPO18 (1-10-418)	18 mm	Wheel	Plastic	165 mm
FatMax FMHT10329-0	18 mm	Wheel	Plastic, rubber inserts	170 mm
FatMax FMHT10338-0	18 mm	Slider	Plastic, rubber inserts	170 mm
STHT10500-0	18 mm	Wheel	Plastic, rubber inserts	160 mm
STHT10501-1	18 mm	Slider	Plastic, rubber inserts	155 mm
FatMax FMHT10330-0	25 mm	Wheel	Plastic, rubber inserts	210 mm
DeWalt DWHT10333-0	25 mm	Wheel	Steel bar, two-material grip	not stated

Safety knives for warehouse and logistics

Where cartons are opened all day and the tool passes from hand to hand, safety utility knives exist: they are designed to reduce accidental cuts, with the blade retracting or staying protected when you are not cutting. How it works varies from model to model, so always read the data sheet.

One thing to know before ordering: in the Stanley data sheets of the knives in this guide the "automatic-retracting blade" field is set to "No". These models have a blade you retract by hand (slider or wheel), which is a different thing. If you want automatic retraction for your department, tell the AMES technicians before ordering. The choice of tool is part of the risk assessment the employer is responsible for (Italian Legislative Decree 81/2008): it is not only a matter of convenience.

Common mistakes

• Extending too much blade. The more the blade sticks out, the more it flexes and the easier it is for it to snap or slip. Only the part you need is enough.
• Carrying on with a worn blade. A blade with no edge makes you push harder and cut crooked: that is how people get hurt. Snap off the segment or change the blade.
• Cutting towards your body. Always cut moving the blade away from you and keep your free hand out of the path.
• Choosing the wide blade for every job. For fine work the 25 mm blade is awkward and not very precise.
• Buying the knife without checking the spare blades. Check straight away the width (9, 18 or 25 mm) and the type of blade it accepts.
• Forgetting to retract the blade. A knife left open in a pocket or toolbox is a risk for you and for whoever picks it up next.

Frequently asked questions

Which utility knife is good for plasterboard?

A model with an 18 mm or 25 mm blade and a sturdy body that stays straight along the straightedge. To score the board and snap it, a wheel-lock knife with a firm lock is a good choice.

What does Snap-Off mean?

That the blade is divided into segments. When the edge wears out, you snap off the tip segment and have a new one, without changing the whole blade.

How do I change the blade of a Snap-Off knife?

Push the blade out, snap off the worn segment with the dedicated system (some models have the snapper built into the body) and, if none are left, replace the whole blade. In the Stanley data sheets of the models mentioned, the blade change is listed as possible without tools.

Is the 9 mm knife less sturdy?

The blade is narrower, so it is better suited to fine work. For long cuts and hard materials it is better to move up to an 18 or 25 mm blade.

Is a metal or plastic knife better?

The metal body of the Interlock models weighs more. A plastic or ABS body is lighter and enough for occasional use. Grip and comfort also depend on a handle with rubber inserts.

Are blades interchangeable between brands?

Do not take it for granted. You need the same width and the same type of blade (snap-off segments), but always check the compatibility stated for your knife.

You will find all the models in the Cutters collection: if you do not know which blade or lock you need, describe the job to the AMES technicians and they will help you choose, also through a quote request.