How to Actually Measure
Before we talk about what widths mean, let's talk about measuring accurately.
The Quick Method
A business card is about 0.012 inches thick. If you can't slide one into the crack, it's hairline. A credit card is about 0.03 inches. These give rough reference points in a pinch.
I used the business card test for years before getting the gauge. Not precise, but it tells you whether you're in hairline territory.
The Crack Gauge
The gauge has lines of increasing width. You hold it against the crack and find the line that matches. My gauge goes from hairline up to 1/4 inch in 1/32 inch increments.
Simple. Cheap. Removes the guesswork. Best $15 I've spent on house maintenance.
Where to Measure
Measure at the widest point. My cracks vary in width along their length. I note both the maximum width and the general range.
Also note if the crack is wider at top or bottom. That tells you which direction the movement went. Wider at the top usually means settlement below that point.
Hairline: Under 1/16 Inch
Most of my cracks are in this range. The engineer barely glanced at them.
What They Mean
Hairline cracks are almost always from concrete shrinkage during curing. The concrete was under slight tension as it dried. It cracked to relieve that tension. Normal behavior.
My three vertical cracks from window corners all measure 1/32 inch or less. They formed 45 years ago and haven't moved since.
Should You Worry?
No. Hairline cracks in themselves are not structural concerns. They might let some moisture through, but they don't indicate foundation failure.
The only exception: lots of new hairline cracks spreading rapidly would be concerning. A pattern of fresh cracking suggests something is happening. But old, stable hairlines? Ignore them.
What I Do
I note them during my annual inspection to confirm they haven't grown. They never have. Otherwise, I leave them alone.
Minor: 1/16 to 1/8 Inch
My widest stable crack falls in this range.
What They Mean
Slightly more movement than pure shrinkage. Could be more significant shrinkage, minor settlement, or thermal movement. Worth noting but not usually concerning.
My 1/8 inch crack is vertical, at a corner, and has been identical for five years. It's just where my concrete decided to relieve more stress than usual.
Should You Worry?
Not usually, but these deserve monitoring. I mark them and measure annually. If they're stable, they're fine. If they're growing, investigate further.
What I Do
I have pencil marks at the ends and a width measurement at the same spot each year. Five years of identical data. That stability is reassuring.
If this crack ever started growing, I'd call someone. But it hasn't.
Moderate: 1/8 to 1/4 Inch
This is where I start paying real attention.
What They Mean
Cracks in this range suggest more significant movement. Could be settlement, soil pressure, or structural stress beyond normal shrinkage. The cause matters at this point.
I don't have any cracks this wide. If I did, I'd want to understand why.
Should You Worry?
Maybe. Context matters. A 1/4 inch crack that's been stable for 20 years is different from one that appeared recently. A horizontal 1/4 inch crack is more concerning than a vertical one.
This is the range where I'd consider professional evaluation if other factors were also concerning.
What I'd Do
Investigate the cause. Monitor carefully. Look for related symptoms. Consider professional evaluation, especially if the crack is horizontal, diagonal, or accompanied by other issues.
Significant: 1/4 to 1/2 Inch
Professional evaluation territory.
What They Mean
This width indicates substantial structural movement. Settlement, lateral pressure, or other forces have moved the foundation more than normal shrinkage would produce.
Gary's wall had cracks that reached this range where the bow was worst. That's how I knew his situation was serious before the contractor confirmed it.
Should You Worry?
Yes, to the extent that you should have someone qualified look at it. This doesn't necessarily mean your house is falling down. It means something has happened that warrants expert evaluation.
What to Do
Get a structural engineer's assessment. Document with photos and measurements. Monitor for growth while waiting for professional evaluation.
Don't panic, but don't ignore it either.
Severe: Over 1/2 Inch
Call someone promptly.
What They Mean
Very wide cracks indicate major movement. Something significant has happened. The foundation has shifted well beyond what normal causes would produce.
Should You Worry?
Yes. Professional evaluation is not optional for cracks over 1/2 inch. This is serious enough that you need to understand what's happening and whether the situation is stable or progressing.
What to Do
Contact a structural engineer promptly. Avoid stressing the foundation further. If the crack is still actively growing, treat it as potentially urgent.
Width Isn't Everything
The width guide is useful, but it's not the complete picture.
Orientation Matters
A 1/4 inch horizontal crack is more concerning than a 1/4 inch vertical crack. Horizontal indicates lateral pressure and potential wall failure. My thresholds for concern are lower for horizontal cracks.
Stability Matters
A stable crack is less concerning than a growing crack, regardless of width. My 1/8 inch crack that's been the same for five years is less worrying than a new 1/16 inch crack that didn't exist last month.
This is why I measure and track. The trend matters more than the snapshot.
Displacement Matters
A crack where one side has shifted in or out is more serious than a crack of equal width with no offset. Displacement indicates actual structural movement, not just separation.
I run my finger across my cracks to check for offset. Both sides flush is good. One side sticking out is concerning.
Context Matters
The cause of the crack, the type of foundation, the soil conditions, related symptoms. Width alone doesn't tell the whole story. It's one data point among many.
But it's a useful data point. Better than "kind of wide."