Choosing the right screw length
Diameter gets all the attention, but length is what actually breaks things. A screw 1 mm too long in a laptop can press through a board; one 1 mm too short can shear the moment the lid is stressed.
The minimum bite
As a rule of thumb, the threaded part should go in at least one screw diameter deep into steel, one and a half into aluminium, and two into plastic.
So an M3 into an aluminium frame wants about 4.5 mm of thread inside the hole, on top of whatever thickness it is clamping.
Too long is worse than too short
In a blind hole a long screw bottoms out. It feels tight, but nothing is actually clamped, and in plastic the boss cracks.
In electronics a long screw can touch a board, a battery pouch or a ribbon cable. This is a common cause of a device that worked before a repair and does not after.
Shortening a screw properly
Thread a nut on past the cut line first, cut with a rotary tool or side cutters, dress the end with a file, then wind the nut off — it reforms the damaged first thread as it comes.
Never grind a screw held in the hole you plan to use. Metal dust in a mechanism is worse than the wrong screw.
Where length is measured from
Pan, button, cheese, socket cap and hex heads: measure from under the head to the tip.
Countersunk and flat heads: measure the whole screw, head included, because the head sinks into the material.
Common questions
Is a slightly shorter screw safe?
Usually, if it still has a full diameter of thread engaged. Half a millimetre short is rarely a problem; two millimetres short often is.
Can I just use a washer to take up extra length?
It works mechanically, but in a flush or recessed spot the washer stops the head seating. In electronics, keep to the correct length instead.
What length is standard for laptop screws?
Most bottom-cover screws land between 2.5 mm and 6 mm, and the same laptop often uses two or three different lengths in different positions — which is why laying them out in the shape of the case matters.
Keep reading
Tools and screws for this job
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Metric screw assortment kit
One box covering the sizes in this guide
Precision driver and bit set
The right bit is what stops rounded heads
Digital caliper
Measure before you buy anything