If you’ve ever stared at a hex bolt order form, trying to figure out why one line says “M10-20” and another says “3/8-1.5” (and wondering if mixing them up will blow up your project), you’re not alone. As a hex bolt supplier, I get this question at least a dozen times a week—from small workshop owners building custom decks to big construction managers sourcing parts for commercial builds. The truth is, metric and imperial hex bolt sizes don’t just use different numbers; they’re built on totally different systems, and mixing them up is a quick way to end up with bolts that don’t fit, or worse, break mid-job. Let’s break this down like we’re chatting over a workbench coffee, no boring engineering jargon required. Hex Bolt
First, let’s get one thing straight: metric bolts are measured in millimeters (mm), and imperial bolts are measured in inches. That’s the base, but if you stop there, you’ll mess up two key parts of a bolt’s size: the diameter and the length. A lot of new buyers mix up the two, thinking “well, M10 is 10mm so that’s almost a half-inch” (it’s actually ~0.39 inches, for the record) but there’s more to it. Let’s start with diameter, because that’s where most conversions go wrong.
Metric bolt diameter is super straightforward: it’s the outer width of the bolt’s shank (the smooth part under the hex head), rounded up to the nearest whole mm. So an M6 bolt is exactly 6mm across the shank, M8 is 8mm, easy peasy. Imperial bolt diameter? That’s measured as a fraction of an inch, but it’s not just a straight conversion. For example, a 1/4” imperial hex bolt is roughly 6.35mm, but wait—metric doesn’t make a 6.35mm bolt. The closest metric is M6, which is 6mm. That’s a huge difference, and it’s why buying M6 thinking it’s the same as 1/4” is a bad call. Let’s list the most common conversions you’ll actually use, not the random rare ones that no one orders: For diameter, 1/4” imperial ≈ M6 metric, 5/16” imperial ≈ M8 metric, 3/8” imperial ≈ M10 metric, 7/16” imperial ≈ M12 metric, and 1/2” imperial ≈ M14 metric. Notice that it’s never a perfect 1:1? That’s intentional—standards bodies like ISO (metric) and ASME B18.2 (imperial) designed separate systems for a reason, mostly to avoid cross-contamination in critical industries like aerospace or construction.
Next up: bolt length, which is another gotcha. Metric length is measured from under the hex head to the end of the bolt, right? Imperial is almost the same—wait, no, hold on. Wait, imperial hex bolt length is also measured from under the head, but some imperial bolts (like lag bolts, but no, we’re talking hex here) don’t have that rule? No, actually, standard hex cap screws (imperial) do measure length under the head, same as metric. But here’s the tricky part: the increment for length changes. Metric bolts come in 1mm increments for most sizes—so an M10-20 is 20mm long, M10-25 is 25, etc. Imperial bolts use 1/16” increments for lengths under 1 inch, and then 1/4” or 1/2” increments after that. So a 3/8”-1.5” imperial bolt is 1.5 inches long. If you try to convert that to metric, 1.5 inches is ~38.1mm. But metric doesn’t sell 38.1mm bolts—they sell 38mm or 40mm. So if you order a 3/8”-1.5” imperial bolt, don’t grab an M10-38mm metric one, because that doesn’t exist. The closest is M10-40mm, and that will stick out way more than you want. That’s the kind of mistake that makes a deck rail wobble or a machine part not seat right.
Then there’s the thread pitch, the part that even seasoned contractors mess up all the time. Thread pitch is how many threads per unit length, right? Metric pitch is measured as mm per thread—so an M10 bolt with a 1.5mm pitch means there’s one thread every 1.5mm along the shank. Imperial pitch is threads per inch (TPI)—so a 3/8” bolt with 16 TPI has 16 threads per inch of shank. This is where size conversions become completely useless if you ignore it. I can’t tell you how many times a guy has come to me saying “I need bolts that fit these holes, I had 3/8” ones and now I need metric” and he’s surprised when the M10 looks right on diameter but won’t thread. Because most standard 3/8” imperial hex bolts are 16 TPI, while the standard M10 metric hex bolt is 1.5mm pitch. Wait, is 1.5mm equal to 16 TPI? Let’s do the math quick: 1 inch is 25.4mm, so 25.4 /16 = 1.5875mm pitch. Oh, so they’re almost the same, but not quite. So a 3/8” bolt will thread into an M10 hole sometimes, but it’s going to strip out way faster than it should, because the pitch is off. That’s a safety hazard I never want to see on a job site.
Now, let’s talk about when you actually need to convert these sizes. Because here’s the thing: most of the time, you don’t. If your project specs say metric, buy metric hex bolts. If it says imperial, buy imperial. But there are legitimate reasons you might need to convert. For example, you’re replacing an old bolt that’s imperial, and the hardware store only has metric on the shelf. Or you’re sourcing parts for a machine that has both old and new components, some sized in metric, some in imperial. Or maybe you’re ordering from an overseas supplier that lists sizes in the other system. Whatever the reason, there’s a right way and a wrong way to do this.
First, the wrong way: just take the diameter number and convert it straight. Like I thought earlier, M10 is 10mm so that’s ~3/8 inch, which is “close enough.” No, that’s not close enough, as we saw with the thread pitch and even the diameter being off by almost a full mm. The right way is to use a conversion chart specifically for hex bolts, not general length or diameter conversions. Because general conversion charts for nuts or other parts are different—hex bolts have specific head sizes too? Wait, yes, another thing I almost forgot: the hex head size. A lot of people don’t realize that the width across the flats (the size of the wrench you need) is different between metric and imperial, even for “equivalent” diameter bolts. Let’s take that same M10 and 3/8” example. The M10 hex head is 16mm across the flats. The 3/8” imperial hex head is 9/16” (which is ~14.29mm). So if you grab a 16mm wrench for your 3/8” bolt, it’s going to be loose, and you’ll strip the bolt head before you even tighten it. That’s another costly mistake. So when converting, you have to check not just diameter and length, but head size too.
Let’s run through a real example that came through my order desk last month. A guy named Jake from a small construction company called, panicking, because he had to replace 50 bolts on a roof truss. The old specs said 5/16”-2” imperial hex bolts, but his usual supplier was out, and the local big box only had metric. He thought he could just convert it and order M8-50mm? No, wait, let’s do this right. 5/16” diameter is ~7.94mm, so the closest metric is M8 (8mm, perfect). Length: 2 inches is ~50.8mm, so the closest metric length is 50mm, not 48 or 52. Now check thread pitch: 5/16” imperial standard is 18 TPI. M8 metric standard is 1.25mm pitch. Convert that to TPI: 25.4 /1.25 = 20.32 TPI. Oh, so almost, but not quite. Wait, Jake said the truss bolts are for a non-critical joint, just holding two pieces of wood together, so a little thread mismatch wasn’t going to be a safety issue. But we still warned him that it might not be as strong, and if it stripped, to come back to us for exact replacements. That’s the kind of practical advice we give as a hex bolt supplier, not just “here’s your bolt.”
Another example: if you’re working on a car, like an old Jeep Wrangler that uses mostly imperial bolts, and you need to replace a suspension bolt that’s 1/2”-3” imperial. Converting that would be: diameter 1/2” = ~12.7mm, so closest metric is M12. Length 3” = ~76.2mm, so metric length is 75mm (since 76 is too close to 75, not 80). Thread pitch for 1/2” imperial standard is 13 TPI, convert that to ~1.95mm pitch, while M12 metric standard is 1.75mm. Again, close enough for a quick replacement on the side of the road, but if you’re doing a full suspension rebuild, you want the exact size to avoid failures.
Wait, is there a time when converting is actually a bad idea? Definitely. For critical applications: aerospace parts, pressure vessels, structural steel for high-rises, medical equipment, any project where a bolt failure could hurt someone. In those cases, never mix sizes, never convert without checking thread pitch and load ratings, and always get bolts that match the exact spec. We always tell our commercial clients to stick to their specified system, but we also stock both metric and imperial hex bolts so they don’t have to hunt around if they run out on a job.
Now, let’s clear up some common myths people have about converting. Myth #1: “1 inch = 25.4mm, so all bolt sizes convert directly.” No, that’s general length, but bolt sizes are standardized, so a 1” imperial bolt doesn’t exist for hex cap screws (the largest standard imperial hex bolt is 3/4”), and the closest metric is M20, which is 20mm, not 25.4. Myth #2: “Thread pitch doesn’t matter for non-structural stuff.” I’ve seen a patio chair that fell apart because someone used a bolt with slightly off pitch—no one got hurt, but it was a hassle to fix. Myth #3: “The hex head size is just a different wrench size, so it doesn’t matter.” If your wrench is the wrong size, you’ll strip the bolt head, and then you have to drill it out, which takes extra time and might damage the part you’re working on.
So what’s the step-by-step process if you actually need to convert a hex bolt size, no guessing? Let’s make this simple, so you can jot it down for when you’re at the supply store or placing an order. First, write down every detail of the bolt you have: diameter, length, thread pitch (count how many threads are in 1 inch for imperial, or measure the mm between two threads for metric), and width across the hex flats (measure that with a caliper if you have one, don’t guess). Second, look up standard bolt conversion charts specifically for hex cap screws—don’t use a general hardware chart, because those include nails, screws, and other stuff. Third, double-check the thread pitch, because that’s the part that catches everyone. Fourth, if you’re using a bolt for critical work, test fit it first before installing it permanently.
As a hex bolt supplier, we’ve made sure our inventory covers both metric and imperial standard sizes, and we always include the conversion details on our product pages so customers don’t have to do the math. We also have a team of folks who can help if you’re not sure—just shoot us a message with the specs you have, and we’ll tell you exactly what size you need, whether you should stick to your system, or if a close convert will work for your project.
Specialty Bolts At the end of the day, converting between metric and imperial hex bolt sizes doesn’t have to be rocket science, but it does take a little attention to detail. The worst thing you can do is rush it and assume numbers are close enough, because that’s when you end up with bolts that don’t fit, or worse, aren’t strong enough for the job. If you’re working on a project, double-check every size, know the difference between diameter, length, thread pitch, and head size, and don’t be afraid to reach out to a supplier who knows their stuff. Whether you need 10 small M5 bolts for a bike repair or 500 large imperial hex bolts for a construction project, we’ve got you covered, and we’re happy to answer any questions about sizing or conversions. Don’t stress about the numbers—we’re here to make sure your bolt order goes right the first time.
Handan Dongchao Hardware Products Co., Ltd.
As one of the most professional hex bolt manufacturers and suppliers in China, we have world-leading production equipment and strong manufacturing capabilities. Please feel free to wholesale bulk high quality hex bolt in stock here and get pricelist from our factory. Customized orders are welcome.
Address: West of Xiaobeizhuang Village, South of Beizhangzhuang Village, Hanshan District, Handan City
E-mail: david@dcscrews.com
WebSite: https://www.dc-screws.com/
