{"id":617,"date":"2026-10-26T14:09:52","date_gmt":"2026-10-26T06:09:52","guid":{"rendered":"http:\/\/www.congresollevadoreslleida.com\/blog\/?p=617"},"modified":"2026-10-26T14:09:52","modified_gmt":"2026-10-26T06:09:52","slug":"how-to-convert-between-metric-and-imperial-hex-bolt-sizes-499a-0963a1","status":"publish","type":"post","link":"http:\/\/www.congresollevadoreslleida.com\/blog\/2026\/10\/26\/how-to-convert-between-metric-and-imperial-hex-bolt-sizes-499a-0963a1\/","title":{"rendered":"How to convert between metric and imperial hex bolt sizes?"},"content":{"rendered":"

If you\u2019ve ever stared at a hex bolt order form, trying to figure out why one line says \u201cM10-20\u201d and another says \u201c3\/8-1.5\u201d (and wondering if mixing them up will blow up your project), you\u2019re not alone. As a hex bolt supplier, I get this question at least a dozen times a week\u2014from 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\u2019t just use different numbers; they\u2019re built on totally different systems, and mixing them up is a quick way to end up with bolts that don\u2019t fit, or worse, break mid-job. Let\u2019s break this down like we\u2019re chatting over a workbench coffee, no boring engineering jargon required. Hex Bolt<\/a><\/p>\n

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First, let\u2019s get one thing straight: metric bolts are measured in millimeters (mm), and imperial bolts are measured in inches. That\u2019s the base, but if you stop there, you\u2019ll mess up two key parts of a bolt\u2019s size: the diameter and the length. A lot of new buyers mix up the two, thinking \u201cwell, M10 is 10mm so that\u2019s almost a half-inch\u201d (it\u2019s actually ~0.39 inches, for the record) but there\u2019s more to it. Let\u2019s start with diameter, because that\u2019s where most conversions go wrong.<\/p>\n

Metric bolt diameter is super straightforward: it\u2019s the outer width of the bolt\u2019s 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\u2019s measured as a fraction of an inch, but it\u2019s not just a straight conversion. For example, a 1\/4\u201d imperial hex bolt is roughly 6.35mm, but wait\u2014metric doesn\u2019t make a 6.35mm bolt. The closest metric is M6, which is 6mm. That\u2019s a huge difference, and it\u2019s why buying M6 thinking it\u2019s the same as 1\/4\u201d is a bad call. Let\u2019s list the most common conversions you\u2019ll actually use, not the random rare ones that no one orders: For diameter, 1\/4\u201d imperial \u2248 M6 metric, 5\/16\u201d imperial \u2248 M8 metric, 3\/8\u201d imperial \u2248 M10 metric, 7\/16\u201d imperial \u2248 M12 metric, and 1\/2\u201d imperial \u2248 M14 metric. Notice that it\u2019s never a perfect 1:1? That\u2019s intentional\u2014standards 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.<\/p>\n

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\u2014wait, 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\u2019re talking hex here) don\u2019t have that rule? No, actually, standard hex cap screws (imperial) do measure length under the head, same as metric. But here\u2019s the tricky part: the increment for length changes. Metric bolts come in 1mm increments for most sizes\u2014so an M10-20 is 20mm long, M10-25 is 25, etc. Imperial bolts use 1\/16\u201d increments for lengths under 1 inch, and then 1\/4\u201d or 1\/2\u201d increments after that. So a 3\/8\u201d-1.5\u201d imperial bolt is 1.5 inches long. If you try to convert that to metric, 1.5 inches is ~38.1mm. But metric doesn\u2019t sell 38.1mm bolts\u2014they sell 38mm or 40mm. So if you order a 3\/8\u201d-1.5\u201d imperial bolt, don\u2019t grab an M10-38mm metric one, because that doesn\u2019t exist. The closest is M10-40mm, and that will stick out way more than you want. That\u2019s the kind of mistake that makes a deck rail wobble or a machine part not seat right.<\/p>\n

Then there\u2019s 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\u2014so an M10 bolt with a 1.5mm pitch means there\u2019s one thread every 1.5mm along the shank. Imperial pitch is threads per inch (TPI)\u2014so a 3\/8\u201d 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\u2019t tell you how many times a guy has come to me saying \u201cI need bolts that fit these holes, I had 3\/8\u201d ones and now I need metric\u201d and he\u2019s surprised when the M10 looks right on diameter but won\u2019t thread. Because most standard 3\/8\u201d 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\u2019s do the math quick: 1 inch is 25.4mm, so 25.4 \/16 = 1.5875mm pitch. Oh, so they\u2019re almost the same, but not quite. So a 3\/8\u201d bolt will thread into an M10 hole sometimes, but it\u2019s going to strip out way faster than it should, because the pitch is off. That\u2019s a safety hazard I never want to see on a job site.<\/p>\n

Now, let\u2019s talk about when you actually need to convert these sizes. Because here\u2019s the thing: most of the time, you don\u2019t. 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\u2019re replacing an old bolt that\u2019s imperial, and the hardware store only has metric on the shelf. Or you\u2019re sourcing parts for a machine that has both old and new components, some sized in metric, some in imperial. Or maybe you\u2019re ordering from an overseas supplier that lists sizes in the other system. Whatever the reason, there\u2019s a right way and a wrong way to do this.<\/p>\n

First, the wrong way: just take the diameter number and convert it straight. Like I thought earlier, M10 is 10mm so that\u2019s ~3\/8 inch, which is \u201cclose enough.\u201d No, that\u2019s 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\u2014hex bolts have specific head sizes too? Wait, yes, another thing I almost forgot: the hex head size. A lot of people don\u2019t realize that the width across the flats (the size of the wrench you need) is different between metric and imperial, even for \u201cequivalent\u201d diameter bolts. Let\u2019s take that same M10 and 3\/8\u201d example. The M10 hex head is 16mm across the flats. The 3\/8\u201d imperial hex head is 9\/16\u201d (which is ~14.29mm). So if you grab a 16mm wrench for your 3\/8\u201d bolt, it\u2019s going to be loose, and you\u2019ll strip the bolt head before you even tighten it. That\u2019s another costly mistake. So when converting, you have to check not just diameter and length, but head size too.<\/p>\n

Let\u2019s 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\u201d-2\u201d 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\u2019s do this right. 5\/16\u201d 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\u201d 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\u2019t 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\u2019s the kind of practical advice we give as a hex bolt supplier, not just \u201chere\u2019s your bolt.\u201d<\/p>\n

Another example: if you\u2019re working on a car, like an old Jeep Wrangler that uses mostly imperial bolts, and you need to replace a suspension bolt that\u2019s 1\/2\u201d-3\u201d imperial. Converting that would be: diameter 1\/2\u201d = ~12.7mm, so closest metric is M12. Length 3\u201d = ~76.2mm, so metric length is 75mm (since 76 is too close to 75, not 80). Thread pitch for 1\/2\u201d 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\u2019re doing a full suspension rebuild, you want the exact size to avoid failures.<\/p>\n

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\u2019t have to hunt around if they run out on a job.<\/p>\n

Now, let\u2019s clear up some common myths people have about converting. Myth #1: \u201c1 inch = 25.4mm, so all bolt sizes convert directly.\u201d No, that\u2019s general length, but bolt sizes are standardized, so a 1\u201d imperial bolt doesn\u2019t exist for hex cap screws (the largest standard imperial hex bolt is 3\/4\u201d), and the closest metric is M20, which is 20mm, not 25.4. Myth #2: \u201cThread pitch doesn\u2019t matter for non-structural stuff.\u201d I\u2019ve seen a patio chair that fell apart because someone used a bolt with slightly off pitch\u2014no one got hurt, but it was a hassle to fix. Myth #3: \u201cThe hex head size is just a different wrench size, so it doesn\u2019t matter.\u201d If your wrench is the wrong size, you\u2019ll strip the bolt head, and then you have to drill it out, which takes extra time and might damage the part you\u2019re working on.<\/p>\n

So what\u2019s the step-by-step process if you actually need to convert a hex bolt size, no guessing? Let\u2019s make this simple, so you can jot it down for when you\u2019re 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\u2019t guess). Second, look up standard bolt conversion charts specifically for hex cap screws\u2014don\u2019t use a general hardware chart, because those include nails, screws, and other stuff. Third, double-check the thread pitch, because that\u2019s the part that catches everyone. Fourth, if you\u2019re using a bolt for critical work, test fit it first before installing it permanently.<\/p>\n

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As a hex bolt supplier, we\u2019ve 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\u2019t have to do the math. We also have a team of folks who can help if you\u2019re not sure\u2014just shoot us a message with the specs you have, and we\u2019ll tell you exactly what size you need, whether you should stick to your system, or if a close convert will work for your project.<\/p>\n

Specialty Bolts<\/a> At the end of the day, converting between metric and imperial hex bolt sizes doesn\u2019t 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\u2019s when you end up with bolts that don\u2019t fit, or worse, aren\u2019t strong enough for the job. If you\u2019re working on a project, double-check every size, know the difference between diameter, length, thread pitch, and head size, and don\u2019t 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\u2019ve got you covered, and we\u2019re happy to answer any questions about sizing or conversions. Don\u2019t stress about the numbers\u2014we\u2019re here to make sure your bolt order goes right the first time.<\/p>\n

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Handan Dongchao Hardware Products Co., Ltd.<\/a>
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\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

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