Foundation Crack Size: How to Measure and Why It Matters

Spotting a crack in your foundation can be unsettling, but not every crack is a disaster. The key is knowing the size of the crack and what that size tells you about your home’s health. In this guide, we’ll walk through simple steps to measure a crack, break down the common size categories, and help you decide when to call a pro.

Measuring Your Foundation Cracks Accurately

First thing, grab a few basic tools: a ruler or a tape measure, a pencil, and a flashlight. Place the ruler right next to the crack and note the widest point. If the crack is too narrow to see the ruler clearly, use a piece of paper to trace its edges and then measure the trace.

A hairline crack is usually less than 1/16 inch (1.5 mm). You’ll need a magnifying glass to see it well. For cracks between 1/16 and 1/8 inch (1.5–3 mm), a regular ruler works fine. Anything wider than 1/8 inch (3 mm) shows up easily and often signals a bigger problem.

Write down the measurement, the direction of the crack (horizontal, vertical, or diagonal), and where you found it (wall, slab, or footing). This information makes it easier for a contractor to give you an accurate estimate later.

What Different Crack Sizes Tell You

Hairline cracks (< 1/16 inch) usually come from normal settling or temperature changes. They’re mostly cosmetic, but keep an eye on them for any growth.

Small cracks (1/16–1/8 inch) can mean the soil around your foundation is shifting a bit. If they’re vertical and spaced evenly, they’re often still part of normal movement. However, if they start to widen quickly, you should get a professional opinion.

Medium cracks (1/8–1/4 inch) often indicate more serious issues like soil pressure, water intrusion, or poor drainage. Horizontal cracks in this range are a red flag because they can point to wall buckling.

Large cracks (> 1/4 inch) are the ones that demand immediate attention. They can compromise structural integrity, allow water into the basement, and lead to mold. A structural engineer should assess these right away.

Besides width, pay attention to how the crack behaves over time. If the gap gets bigger, if crumbs or soil fall out, or if you notice water seeping in, those are signs that repair is needed sooner rather than later.

For most homeowners, a practical rule of thumb is: if a crack is wider than a pencil line (about 1/8 inch) or if it’s changing fast, call a foundation specialist. They’ll check soil conditions, drainage, and may recommend epoxy injection, carbon‑fiber straps, or pier installation depending on the cause.

In the meantime, you can slow down water damage by sealing the crack with a waterproof epoxy designed for foundations. Clean the crack, apply the epoxy according to the instructions, and let it cure. This is a temporary fix, not a substitute for a professional repair, but it helps keep moisture out while you schedule an expert.

Remember, regular inspection is the best defense. Walk around your home twice a year—once after winter and once after summer—and note any new cracks or changes in existing ones. A quick photo and measurement can save you a lot of money down the road.