Need to make sure you’re choosing the right size hex bolts? Here’s what most buyers ask us when they’re learning how to measure hex bolt sizes:
How do I measure a hex bolt’s length? Measure from under the head to the tip of the threads. (If it has a flat head, measure the entire length.)
What about diameter and head size? Use calipers or a ruler to measure across the shank for diameter, and across the flats of the head to find the right wrench size.
How do I check the threads? Count the threads within one inch, or use a thread gauge to find the correct TPI (threads per inch).
If you’d like to go deeper, we’ll walk you through every measurement step-by-step, provide useful tips, and share a hex bolt sizes table (plus a link to our full Bolt Size Chart where you can download the complete version).
Ready to order? Explore our hex bolt collection — hand-packed and shipped fast from our Pennsylvania warehouse.
Before you start measuring, make sure you’ve got:
Calipers: Your most accurate option for measuring bolt diameter, head size, and length. Digital calipers give instant readings, while vernier styles work just as well with a steady hand.
Thread Gauge: Used to measure the threads per inch (TPI), helping you match the bolt to the right nut or tapped hole.
Ruler or Tape Measure (in a pinch): Ideal for checking the overall length of larger bolts. It’s less precise than calipers, but perfect for quick measurements or confirming size ranges.
Tip: If you’re working on a professional or load-bearing project, accuracy is everything. Use calipers and a thread gauge together to guarantee a perfect match.
It helps to know what each part of a hex bolt does and which dimensions matter most. Use the guide below to identify each section and the correct measurement point.
| Hex Bolt Part | Where to Measure | What It Tells You |
|---|---|---|
| Head | Measure across the flats (flat side to flat side). | Determines wrench or socket size. |
| Shank | Measure the smooth, unthreaded portion just below the head. | Used for overall bolt length or grip length. |
| Threads | Measure the distance between two peaks with a thread gauge or count threads per inch (TPI). | Ensures compatibility with matching nuts or tapped holes. |
| Point | Measure from under the head to the very tip of the point. | Defines total bolt length. |
| Diameter | Measure across the shank (not the threads). | Determines bolt size classification. |
When measuring a hex bolt’s length, you’re finding the distance from under the head to the tip of the bolt. That total length determines how deep the bolt will sit once installed. Too short, and it won’t hold; too long, and it may bottom out or protrude.
Here’s how to measure bolt length:
Lay the bolt flat on a surface or hold it upright.
Place the end of your calipers or ruler directly beneath the head, not on top of it.
Measure straight to the end of the threads or point — whichever style the bolt has.
Record the measurement in inches or millimeters and round to the nearest standard size.
Tip: If you’re measuring a bolt with a flat or countersunk head, measure from the top of the head to the tip instead. The head becomes part of the fastened depth for that style.
Common hex bolt lengths range from ½ inch to 36 inches, covering everything from small home repairs to heavy-duty construction projects.
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The diameter tells you the thickness of the bolt’s shank (the solid, unthreaded part just below the head). This is the number you’ll match to nuts, pre-drilled holes, or threaded inserts. Getting it right means a snug fit without wobble or resistance.
Here’s how to measure hex bolt diameter:
Hold the bolt steady and place your calipers (or ruler) across the shank, just above the threads.
Make sure you’re measuring perpendicular to the bolt, not at an angle.
Tighten the caliper jaws until they touch both sides of the shank, then note the reading.
Round to the nearest standard size (¼", ⅜", ½", etc.) to identify your bolt diameter.
Tip: Don’t measure across the outer threads — that can make the diameter appear larger than it really is. Always measure the solid section of the bolt.
Common U.S. hex bolt diameters range from 3/16 inch up to 1½ inches, with The Fastener Depot stocking the most frequently used sizes: ¼", ⅜", ½", ⅝", and ¾".
Threads per inch or TPI tells you how many threads are cut into one inch of the bolt’s shank. This number is crucial: it determines whether the bolt will properly mate with a nut or a tapped hole. A mismatch here leads to stripped threads or loose connections.
Pro Thread Tip: Hot-dip galvanized (HDG) nuts are only compatible with HDG bolts. Their threads are intentionally oversized to account for the buildup of galvanization. Because of this, HDG nuts won’t properly fit zinc-plated or stainless steel bolts, a common but significant mistake when mixing materials.
Here’s how to measure bolt TPI:
If you have a thread gauge, line up the teeth of the gauge against the bolt’s threads until you find the one that fits perfectly.
If you don’t have a gauge, use a ruler: mark a one-inch section along the bolt’s threads and count how many peaks fall within that inch.
Write down the number, that’s your TPI. Common examples are 13 TPI for coarse and 20 TPI for fine ½" bolts.
Tip: When in doubt, always double-check the TPI on your matching nut or hole. Even a one-thread difference will prevent the bolt from seating correctly.
The head size is measured across the flats of the hex and tells you what size wrench or socket you’ll need to tighten or loosen the bolt. It’s not the same as the bolt’s diameter, though the two are related. Measuring correctly helps you avoid rounding off edges or using the wrong tool.
Here’s how to measure hex bolt head size:
Place your calipers (or a ruler) flat across two opposite sides of the bolt head — not from corner to corner.
Tighten the calipers gently until they touch both flats.
Record that distance; this is the across-flats (AF) measurement.
Match the reading to your wrench or socket size. For example, a ½" bolt typically takes a ¾" wrench.
Tip: Always measure the flats rather than the corners. Corner-to-corner readings will appear larger and can lead to tool slippage.
Common hex head sizes range from 5/16 inch up to 1-1/8 inches, depending on the bolt’s diameter.
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Now that you know how to measure each part of a hex bolt, it helps to see how common sizes compare. Use this quick reference chart to get a general idea of standard diameters, lengths, thread counts, and head sizes before you buy.
Note: All measurements in inches, based on U.S. standard hex bolts.
| Nominal Size | Typical Length Range | Body Diameter | Threads Per Inch (TPI) | Head Size (Across Flats) |
|---|---|---|---|---|
| 1/4" | 1/2" – 4" | 0.25" | 20 (fine) / 13 (coarse) | 7/16" |
| 5/16" | 1/2" – 5" | 0.31" | 18 / 11 | 1/2" |
| 3/8" | 3/4" – 6" | 0.38" | 16 / 10 | 9/16" |
| 1/2" | 1" – 8" | 0.50" | 13 / 20 | 3/4" |
| 5/8" | 1" – 10" | 0.63" | 11 / 18 | 15/16" |
| 3/4" | 1-1/2" – 12" | 0.75" | 10 / 16 | 1-1/8" |
For more detailed measurements, including head height, width across corners, and thread length over 6 inches, visit our Full Bolt Size Chart Guide or download the complete reference sheet
How do I make sure my measurements are accurate?
Use steady, even pressure when measuring, especially with calipers. Too much pressure can slightly compress softer metals, while too little can give a loose reading.
Do I need special tools to measure hex bolts?
Not necessarily. Calipers and a thread gauge will give you the most precise results, but a simple ruler or tape measure works for general sizing.
Should I measure the bolt straight on or at an angle?
Always measure straight on, keeping your tool perpendicular to the bolt. Even a small tilt can change your reading by a fraction of an inch.
What’s the difference between coarse and fine threads?
Coarse-thread bolts (like ½"-13) have fewer, deeper threads per inch and are more common in construction and wood applications. Fine-thread bolts (like ½"-20) have more threads per inch, offering a tighter hold for machinery and metalwork.
How can I tell if my bolt matches the right nut or hole?
Match both the diameter and TPI (threads per inch). If you’re unsure, use a thread gauge or test-fit the bolt in the nut — it should screw in smoothly by hand without force.
Are hot-dip galvanized (HDG) nuts compatible with zinc or stainless steel bolts?
No. HDG nuts are designed with slightly oversized threads to accommodate the thick zinc coating applied during galvanization. This means they only fit correctly with HDG bolts. Mixing HDG nuts with zinc-plated or stainless steel bolts can cause poor fit, cross-threading, or failure under load.
Quick rule: Always match HDG nuts with HDG bolts to ensure proper threading and performance.
If you’re working on other projects or just want to understand sizing across different fasteners, these quick guides can help:
Wood Screws Guide: Learn how wood screw diameters and lengths compare to bolt sizing.
Screw Size for Decks: Find the right screw length and material for your next decking project.
Types of Lag Bolts, Sizes & TPI Thread Guide: Understand lag bolt sizing and thread differences for heavy-duty applications.
Each guide includes simple visuals and sizing examples to make fastener selection easier, no matter what you’re building.
As a trusted family-owned business, The Fastener Depot is devoted to offering high-grade fasteners alongside expert guidance, facilitating the selection of the perfect match for your projects.
We provide free shipping on orders exceeding $50 and maintain a hassle-free 30-day return policy to make sourcing the best materials straightforward. Reach out to The Fastener Depot today to find the ideal fastener for your needs. Get in touch.
