Bolt Calculator is a free suite of fastener engineering calculators for the three questions every bolted joint asks: how hard to tighten it, what the hardware weighs, and whether it survives the side load. We built it around one belief: a torque value is only trustworthy when you can see where it came from — the K constant, the lubrication factor, the bolt grade, and the scatter band around the answer.
Every joint starts with a numberPhoto: Unsplash3
Calculators, with more in development
M3 – M48
Metric sizes, plus #10 – 1½" UNC/UNF
2
Unit systems, converting in place
0
Accounts, uploads, or saved inputs
The idea for this site came from a familiar frustration. A torque value is rarely written where you need it: it lives in a manufacturer’s catalog page that assumes one friction condition, or in a forum thread that assumes another, or in a spreadsheet a former colleague left behind — and every generic online calculator returns an answer with no visible method behind it. Chasing the number is easy; knowing whether to believe it is the hard part.
The strange thing is that the arithmetic is old, public, and short. The torque–preload relationship has been in machine-design textbooks for generations: torque equals the nut factor times the clamp force times the nominal diameter, with a lubrication correction where the threads are oiled. What is scarce is not the math — it is the presentation. Calculators that show the formula, let you pick the K factor that matches your coating and lubrication, and refuse to pretend that a single number is the answer.
So we built the calculator we wanted to use. The flagship tool takes the bolt material, diameter, thread series, and lubrication as explicit selections rather than hidden defaults, and answers with a range — the torque band that friction scatter and tool accuracy actually produce — alongside a grade-aware strength check that tells you how much of the bolt’s yield the preload consumes. The weight and shear tools follow the same pattern: printed method, real standards, honest limits.
And the site stays the way it started: free, without accounts, with every calculation running locally in your own browser. We have no interest in your bolt data — architecturally, we never receive it. The advertising that keeps the lights on sits clearly apart from the tools, and nothing about the answer is gated behind a subscription.
The method, visiblePhoto: UnsplashThe one-sentence version
We put the textbook formula, the real standards data, and the friction assumptions on the same screen — so the number you walk away with is one you can defend.
A bolted joint raises four practical questions — how hard to tighten it, how much scatter to expect, what the hardware weighs, and whether it survives the side load. Each calculator takes one of those and answers it completely, and the answers chain: the torque tool sizes the preload, the strength check verifies it, the shear tool checks the side load, and the weight tool totals the hardware.
The flagship bolt torque calculator converts wrench torque into joint clamping force — and back — using the classic T = K·F·d·(1 − l/100) model, with bolt-material K constants, lubrication factors, and a worked example you can check by hand. Enter a target clamp load to get the wrench setting, or enter a torque to see the preload it should deliver.
Real joints never see one torque. The tool widens the answer into a min/max band from friction scatter and wrench accuracy, then verifies the bolt grade: tensile stress area, preload target as a percentage of yield, and a utilization ratio with a pass/fail verdict. Coarse and fine thread series are handled separately, because a fine thread changes the stress area and therefore the strength verdict.
The weight calculator prices the whole kit, not just the bolt: hex bolt plus hex nut, heavy hex nut, and washer options with individual quantities, per-piece and total weights in kilograms and pounds, computed from nominal ASME head and nut geometry at steel density. It is the number a freight quote actually needs.
The shear calculator checks the side-load failure mode with interactive sliders: bolt count, diameter, single or double shear, grade-based allowable stress following the 0.62·Fu / 0.48·Fu convention, a safety-factor control, and a live pass/fail diagram. Move a slider and the margin updates on the same keystroke.
The same arithmetic serves very different jobs. These are the people we had in mind while building it — if you recognize yourself in one of them, start with the calculator that answers your question.
The daily question is practical: what do I set the wrench to, for this bolt, in this condition? Pick the coating and lubrication you actually have, get a torque plus the scatter band around it, and know which lever moves the number when the spec and the shop floor disagree.
Preload targets, stress areas including fine-pitch corrections, and a utilization check against the grade's yield — enough to cross-check a drawing note or a supplier's installation spec in seconds, before the review meeting rather than during it.
Every formula is printed on the page, every default is a teaching moment, and every worked example can be re-derived by hand. The tools double as course material: change the K factor in class and watch the scatter band widen — friction stops being an abstraction.
Automotive work, trailers, engine hoists, structural attachments to frames — the calculator gives honest arithmetic for the joint you are actually building, and is blunt about which checks belong at the bench and which belong to a qualified engineer.
Received a torque table from a supplier? Cross-check it against the grade and the friction condition it assumes. Most discrepancies between two 'correct' tables live in the assumptions, not the arithmetic — and the calculator makes those assumptions visible.
A bolted connection is never one bolt. The weight calculator totals the complete set — bolt, nut, washer, multiplied by the pattern — in kilograms and pounds, so the number on the purchase order matches the number on the truck scale.
Six commitments that shape every page on the site — the same commitments we would want from a tool we used on the shop floor.
Every number the tools produce can be re-derived on paper. The torque formula, the stress-area equations for metric and unified threads, and the geometric weight model are all written out on the pages where they are used — a calculator that hides its method is a black box, and black boxes do not belong in engineering work.
Metric bolt grades follow ISO 898-1 minimum strengths; imperial grades follow SAE J429 and ASTM A307/A325/A490. Thread geometry uses ISO 68-1 and ASME B1.1 tensile stress areas, head and nut proportions follow ASME B18.2.1 and B18.2.2, and the shear allowables follow the AISC 0.62·Fu / 0.48·Fu convention.
There is no account, no server round-trip, and no saved history. Every calculation runs locally on each keystroke, which is also why results appear instantly and why the tools work offline once the page has loaded. Close the tab and the inputs are gone.
Torque-controlled tightening carries ±20–30% scatter in an uncontrolled joint even with a perfect K value. Rather than presenting a single number as truth, the tools show the band — friction scatter, tool accuracy class, safety factor — so the uncertainty is part of the answer instead of a footnote.
There is no pro tier, no paywall in front of the real answer, and no sign-up step between you and the calculation. The site is funded by clearly separated advertising; the calculators themselves are not the product being sold, and the output is never gated behind an upsell.
Defaults that match common practice, unit parsing that accepts 100' 6" as happily as 100.5, thread-series hints where a fine pitch changes the answer, and worked examples beside the method — the tools are written to be understood on first read, not decoded.
Every constant and table the calculators read from traces to a published standard. These are the sources, what each one governs, and where it shows up in the tools.
Minimum yield and tensile strengths behind the strength check — the first digit of a class times 100 is nominal tensile strength in MPa, the second times 10 gives yield as a percentage of it.
Proof, yield, and tensile minimums for the unified sizes, including the structural grades used in steel construction.
Tensile stress areas for metric coarse and fine series and for UNC/UNF threads — the area the preload target and utilization check actually act on.
The width-across-flats and height proportions that drive the volume — and therefore the weight — of every part of the fastener set.
0.62·Fu where the shear plane passes through the unthreaded shank, 0.48·Fu where threads cross the plane — the basis of the shear calculator's PASS/FAIL margin.
Bolt Calculator is not affiliated with, endorsed by, or sponsored by any standards body or fastener manufacturer. Standards are referenced for identification and educational purposes — for work governed by a specification, consult the current edition of the standard and the manufacturer’s published data.
Three honest comparisons — against the tools that hide their method, against the manufacturer tables every shop relies on, and against the folklore that fills the gap between them.
Plenty of online calculators return a number and nothing else. Here, the formula sits beside the inputs, the constants are in a visible table, and the worked example can be followed line by line. When you can see the method, you can audit it — and an auditable answer is the only kind worth trusting on a real joint.
A fastener manufacturer publishes torque values for the friction conditions and safety margins they assume — and two manufacturers' tables for the same bolt can differ by twenty percent while both are correct. The calculator's job is different: show you which assumption moved the number. When a table disagrees with a calculation, compare the K values and utilization first; the arithmetic is almost never the culprit.
Forum advice — 'a bit tighter never hurt', 'grade 8 for everything' — ignores that friction, not strength, dominates tightening: roughly half of the wrench torque is consumed under the head, another large share in the threads, and only about ten percent becomes clamp load. The same bolt can need roughly 30% more torque dry than lightly oiled. Numbers with their assumptions attached beat anecdotes every time.
The toolset grows from what users ask for at the bench. Three directions are active right now.
Long-form guides on torque vs. preload, the K factor deep dive, and the ISO vs. SAE grade comparison are on the workbench — each one written to end at a number you can enter somewhere.
Thread pitch lookup, bolt length estimation for the grip stack, and flange bolt-pattern spacing are the requests that come in most often from the bench.
Torque control is only one way to tighten a bolt. Turn-of-nut practice and yield-controlled methods are on the roadmap, explained with the same printed-formula approach.
Have a request? The fastest way to see a tool built is to describe the problem you were trying to solve — the contact page routes requests straight to the work queue.
A note on responsibility
The calculators are engineering aids for planning, learning, and cross-checking — not a substitute for a published specification or a qualified engineer’s judgment. For safety-critical assemblies, verify against the governing standard and the fastener manufacturer’s data. See our full disclaimer for details.
Questions about the site, the data, or the method? Write to us — every message is read.