Bolt and Wrench Size Chart: SAE, Metric, Torque & Thread Pitch

Why the Numbers Don’t Match

When someone says “bolt size” they could mean three different measurements, and confusing them is why people arrive at the tool box with the wrong socket.

• Nominal diameter — the thread size. A 1/2" bolt has a 1/2" threaded shank. An M10 has a 10 mm shank.
• Width across flats — the straight-line distance between two parallel sides of the hex head. This is the wrench size.
• Width across corners — point to point across the hex. Only matters for clearance in tight spaces.

The wrench number stamped on the tool describes the fastener, not the tool. A 3/4" wrench has an opening slightly over 3/4" so it will actually go on. ASME B18.2.2 sets those tolerances — for a bolt with 1.5" flats, the wrench opening should measure between 1.508" and 1.520".

SAE Bolt and Wrench Size Chart

Inch-series hex bolts and hex cap screws, per ASME B18.2.1. Standard hex heads — see the heavy hex section below if you are working with structural bolts.

Width across flats per ASME B18.2.1, maximum dimension. Thread counts per ASME B1.1. The wrench size IS the width across flats — it is not the bolt diameter.

Metric Bolt and Wrench Size Chart

Metric is where most of the confusion happens, because the same bolt can take three different wrenches depending on which standard it was made to.

Width across flats per ISO 4014 / 4017. The DIN column is shown only where the withdrawn DIN 931/933 value differs. JIS values from Bolt Depot’s published metric chart.

Standard Hex vs Heavy Hex

Heavy hex bolts have a larger head for the same nominal diameter, giving more bearing area under the head. They are standard in structural steel and piping work — and they take a different wrench.

Heavy hex has a larger head for the same nominal diameter, so it takes a larger wrench. Common in structural work.

Nuts follow the same split. A heavy hex nut will not seat properly on a standard hex stud, and a standard hex nut on a heavy hex bolt leaves you short of thread engagement.

Nut Sizes

For a standard hex nut, the wrench size matches the bolt head of the same nominal diameter — a 1/2" nut takes the same 3/4" wrench as a 1/2" bolt head. That is the default, and it holds for the great majority of work.

The exceptions worth knowing:

• Flange nuts often step down one size, because the flange provides the bearing surface instead of a wide hex.
• Acorn / cap nuts vary by manufacturer.
• Jam nuts are thinner but keep the same across-flats.
• Heavy hex nuts follow the heavy hex column above.

SAE to Metric: Which Swaps Are Safe

SAE and metric wrenches are never an exact match. Several pairs are close enough to fit, and that is exactly the problem — close enough to go on, close enough to round the head off when the bolt is tight.

Gap is the difference between the two openings. Anything above roughly 0.15 mm puts the load on the corners of the hex rather than the flats, which is how heads get rounded.

The classic failure is a 13 mm socket on a 1/2" bolt. The gap is 0.30 mm, which sounds like nothing, but it means the socket contacts the six corners of the hex instead of the six flats. On a rusted or torqued fastener the corners give way and you are into extraction tools.

Use a near-swap only on something hand-tight and undamaged. Never on anything you need to break loose, and never on anything being torqued to a spec.

Socket and Wrench Sets

The only reason a near-swap tempts anyone is an incomplete set. One covering both SAE and metric removes the problem entirely, and costs less than extracting a single rounded-off bolt.

Bolt Torque Chart — and Why Published Charts Disagree

Search for a bolt torque chart and you will find numbers that contradict each other. For a 3/8"-16 Grade 5 bolt, dry, published charts give figures ranging from 23 to 40 ft-lb. That is a 74% spread on the same bolt, same grade, same condition.

They disagree because torque is not a property of the bolt. It is calculated, and the calculation contains assumptions that most charts do not state.

Every number in the tables below is calculated from that formula, at K = 0.20 dry and K = 0.15 lubricated, with clamp load at 75% of proof. The assumptions are stated so the arithmetic can be checked rather than taken on trust.

SAE torque — dry threads (ft-lb)

SAE torque — lubricated threads (ft-lb)

Grade 2 values stop at 3/4". Above that, SAE J429 reduces Grade 2 proof strength significantly and a single figure stops being meaningful.

Metric torque — dry threads (N·m)

Metric torque — lubricated threads (N·m)

Where the published charts diverge

Running the formula explains the outliers rather than just contradicting them:

• The 23 ft-lb figure some charts give for a 3/8" Grade 5 is what the formula returns at K = 0.15 — a lubricated value, presented as dry.
• The 40 ft-lb figure sits between Grade 5 (31) and Grade 8 (44), which is what happens when a Grade 8 table gets labelled ambiguously.
• 31 ft-lb is what you get at K = 0.20 and 75% proof, and it is what the charts that publish their assumptions agree on.

Adjustments that actually matter

• Lubricant, anti-seize or thread locker — use the lubricated column. Applying a dry value to oiled threads over-tightens the bolt, and that is how threads get pulled and bolts get snapped during installation.
• Stainless steel — reduce by 20–25% and use anti-seize. Stainless galls readily, where the threads effectively cold-weld together during tightening.
• Aluminium threads — often 50–75% of the steel value. The threads in the softer material fail long before the bolt does.
• Zinc, cadmium and other platings reduce friction. Treat a plated fastener as closer to lubricated than dry.
• Head bolts and rod bolts are frequently torque-to-yield and single-use. Reusing them is a failure waiting to happen.
• Multi-bolt joints need a tightening sequence. Working around a flange in order pulls it crooked.

For concrete anchors specifically, installation torque is set by the anchor manufacturer rather than derived from bolt grade — see our concrete anchor torque chart for those figures.

Torque Wrenches

A torque spec is only useful if you can hit it. Match the wrench range to the fasteners you actually work on — the tables above run from 5 ft-lb for a 1/4" Grade 2 bolt to over 900 ft-lb for 1" Grade 8, and no single wrench covers that span.

None of these reaches the 376–909 ft-lb the tables show for 3/4" and 1" Grade 8 fasteners; that range needs a torque multiplier or hydraulic tensioning. The Craftsman starts at 50 ft-lb, so it cannot do small fasteners.

How to Identify an Unknown Bolt

Four measurements tell you everything you need, and you can take three of them with a caliper and a pitch gauge.

1. Measure across the flats of the head. That is your wrench size directly, and it is the only number you need if all you want is the right socket.
2. Measure the shank diameter — across the unthreaded body if there is one, or across the thread crests if not. That gives you the nominal size. If it comes out at a neat fraction it is SAE; if it lands on a whole millimetre it is metric.
3. Check the thread pitch. A pitch gauge is quickest. Failing that, count threads across one inch for SAE (that is your TPI) or measure crest to crest in millimetres for metric.
4. Read the head markings for grade. See the table below.

Bolt length is measured from under the head to the tip — except on countersunk and flat-head fasteners, where it includes the head, because the head sits flush with the surface.

Measuring Tools

Two inexpensive tools remove the guesswork. A thread checker board gives you diameter and pitch by trial fit in seconds; a caliper that displays fractions saves converting 0.750" to 3/4" in your head every time.

Grade and Class Markings

SAE grades per SAE J429, metric classes per ISO 898-1. Metric equivalents are approximate — they are close in strength, not interchangeable in specification.

Grade matters more than most people assume. Substituting a Grade 2 bolt where a Grade 8 was specified cuts proof strength by more than half, and the failure will not announce itself until the joint is loaded. For the full comparison including stainless, see our bolt grades guide.

Frequently Asked Questions

What size wrench do I need for a 1/2 inch bolt?

A 3/4" wrench. The 1/2" refers to the thread diameter, not the head. Across the flats of a standard 1/2" hex head measures 0.750", so the wrench is 3/4". A 1/2" heavy hex bolt takes 7/8" instead.

Why is the wrench size bigger than the bolt size?

Because they measure different parts of the fastener. The bolt size is the threaded shank; the wrench size is the width across the flats of the head, which has to be wide enough to give a wrench something to grip. For SAE hex bolts it works out at roughly 1.5× the diameter.

What wrench fits an M10 bolt?

16 mm under current ISO 4014, but 17 mm under the older DIN 931 standard and 14 mm under JIS. All three exist in the field. If a socket almost fits, that is usually the reason — measure across the flats rather than assuming from the thread size.

Can I use a 13 mm wrench on a 1/2 inch bolt?

Only on something hand-tight. A 1/2" head measures 12.70 mm, so a 13 mm wrench leaves a 0.30 mm gap. That gap puts the load on the six corners of the hex instead of the flats, and on a tight or rusted bolt it rounds the head off.

Is socket size the same as wrench size?

Yes. Both are the width across the flats of the fastener, so a bolt needing a 3/4" wrench needs a 3/4" socket. The difference between the tools is access and grip, not sizing.

How much torque for a 1/2 inch Grade 5 bolt?

About 75 ft-lb on dry threads, or 57 ft-lb lubricated — calculated at K = 0.20 and 0.15 respectively, with clamp load at 75% of proof. Treat it as a starting point: real preload scatter with a torque wrench is around ±25–30%.

Why do bolt torque charts give different numbers?

Because they assume different friction factors and rarely say so. The same 3/8" Grade 5 bolt appears at 23, 31 and 40 ft-lb across published charts. 31 is what the formula returns at K = 0.20 dry; 23 is the lubricated value; 40 is closer to Grade 8. A chart that does not state its assumptions cannot be checked.

Do I reduce torque for stainless bolts?

Yes — by 20–25%, and use anti-seize. Stainless galls easily, where the threads effectively cold-weld during tightening and the fastener seizes before it is properly torqued.

How do I measure bolt length?

From under the head to the tip. The exception is countersunk and flat-head fasteners, where length includes the head because it finishes flush with the surface.

What is the difference between UNC and UNF?

Thread count. UNC (coarse) is the default for general fastening — faster to run down, more tolerant of damage and dirt. UNF (fine) packs more threads into the same length, giving slightly more tensile area and better vibration resistance, at the cost of being fussier to start and easier to cross-thread.

Related Guides

• Bolt Grades Explained: Grade 5 vs Grade 8 vs Stainless
• Screw Size Chart: Every Type, Gauge & Pilot Hole
• Lag Bolts vs Carriage Bolts: Which One Do You Need?
• Structural Screws vs Lag Bolts: Strength, Code & Cost
• Concrete Anchor Torque Chart & Torque Wrench Guide
• Pilot Hole Size Chart for Screws
• Nail Size Chart: Penny Sizing Explained