The direction of a crack tells you more than its width. That single idea will save you more money than anything else on this site.
Cracks That Usually Need Nothing
Fine vertical cracks in a poured concrete wall, often within a few feet of a corner or an opening, are commonly shrinkage from the concrete curing. They typically appear in the first year or two and then stop. If one is narrower than the edge of a coin, has no offset between the two sides, and has not changed, it is usually not structural.
Hairline cracking across a slab surface is frequently the same story.
They can still let water through, and sealing a leaking crack is worth doing on its own terms — but that is a moisture decision, not a structural one.
Cracks That Indicate Settlement
Diagonal cracks running out from the upper corners of door and window frames. That is where a wall racks first when the ground under one end drops, which is why the pattern is so consistent.
Cracks that are wider at one end than the other. The taper points toward the side that has moved.
Stair-step cracking through brick or block that follows the mortar joints, particularly if it is concentrated at one corner of the house.
The Ones to Take Seriously
Horizontal cracks in a block wall, usually somewhere in the middle third of its height. This is not settlement — it means the wall is being pushed inward by saturated soil behind it. Put a straightedge or plumb line against the wall: if the middle sits further into the room than the top and bottom, it is bowing, and it needs anchors or carbon fiber rather than piers.
Any crack with offset, where one side sits proud of the other.
What Matters Most
Whether it is moving. A crack that has been identical for fifteen years is telling you something quite different from one that has widened since spring, even if they look the same today.
If you are unsure, mark the ends of the crack with a pencil line and the date, and photograph it. Six months of evidence is worth more than any single measurement.