The Tool Desk
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Measure the fastener head across two opposite, parallel flat faces. That across-flats (AF) width is the dimension an adjustable wrench must grip. Choose a wrench whose maximum jaw opening is at least the AF measurement, then close the movable jaw until both jaws sit tightly on the flats. This measurement tells you how wide the head is for wrench fit; it does not identify the thread diameter or pitch.
Measure across the flats, not across the corners
A standard nut or bolt head is hexagonal. It has six flat faces and six corners. Across-flats is the straight-line distance between two opposite, parallel flat faces. It is the same basic dimension used for wrench openings and socket sizes.
Place your measuring tool so it spans the two flats directly. Do not measure from one corner to the opposite corner: that larger, diagonal distance is not the size used to select the wrench. A wrench set to a corner-to-corner measurement will be too loose on the flats and can damage them.
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Across-flats is sometimes written as AF, A/F or simply the jaw-opening size. Metric fasteners are normally described in millimetres; inch-based fasteners are commonly described in fractions or decimal inches. Record the unit with the number, such as 13 mm AF or 1/2 inch AF.
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What you need to measure the head
- Ruler or steel rule: adequate when the flats are clean, accessible and not close in size to another wrench.
- Caliper: useful for a small, awkward, recessed or worn fastener. It is optional, not a requirement for an ordinary exposed nut or bolt.
- Light and cleaning material: remove dirt, paint, rust scale or burrs that can make a flat appear wider or narrower than it is.
- Paper or a notes app: write down the number and unit before converting or shopping.
Do not measure a damaged corner as though it were a flat. If the head is badly rounded, take several readings on the remaining usable flats and treat the largest reliable reading as a warning that a normal adjustable wrench may not be the safest solution.
Step-by-step: measure a nut or bolt before buying the wrench
- Expose and clean the head. Brush away debris and remove anything that prevents the measuring tool from touching the actual metal flats.
- Identify opposite flats. On a hex head, select two faces directly opposite one another. They should run parallel to each other.
- Position the rule or caliper. Span the two faces at a right angle to the faces. Keep the tool straight rather than angled across the head. With a caliper, close the jaws lightly on the flats; do not force them into the corners.
- Read the across-flats width. For a ruler, read the graduations where the two flats meet the scale. For a caliper, read the displayed value after confirming that both contacts are on the flats.
- Take a second check. Move the tool to another pair of opposite flats. Matching readings indicate that you found the correct surfaces; a different reading often means the tool was tilted, the head is damaged, or contamination is present.
- Write down the value and unit. Keep “mm” or “in” attached to the number. A bare value such as “13” is ambiguous when selecting between metric and inch tools.
- Compare with the wrench specification. The wrench’s published maximum jaw opening must meet or exceed the measured AF width. A scale printed on the wrench is a setup aid, not a substitute for checking the actual fit.
- Close the jaw on the fastener. Open the jaws slightly wider than the head, place them squarely on the flats, and turn the knurl until there is very little side-to-side play.
Converting millimetres and inches without creating a bad fit
If the fastener and wrench use different units, convert the measured AF value before comparing them. One inch equals 25.4 millimetres, so millimetres can be divided by 25.4 to obtain decimal inches, and decimal inches can be multiplied by 25.4 to obtain millimetres.
Conversion is for comparison; it does not make nearby nominal sizes interchangeable. A converted value that is close to a familiar size may still leave enough clearance to round the flats. After any conversion, put the wrench on the fastener and verify that the jaws seat fully with minimal play.
| Measurement you recorded | What to compare | What the result means |
|---|---|---|
| Metric AF, for example 19 mm | Metric maximum jaw opening in millimetres | The opening must be at least 19 mm, followed by a physical fit check. |
| Inch AF, for example 3/4 in | Inch maximum jaw opening in inches | The opening must be at least 3/4 inch, followed by a physical fit check. |
| Converted value | The wrench’s stated unit and actual jaw position | Use the conversion to select a candidate; do not assume a visually close nominal size is equivalent. |
Choose the adjustable wrench by jaw capacity
Look for a specification called maximum jaw opening, maximum opening or maximum width across flats. It must be no smaller than the AF measurement you recorded. Capacity is not the same as handle length: a 12-inch wrench is identified by its nominal length, not by a guaranteed 12-inch jaw opening.
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| Selection check | Why it matters | How to verify it |
|---|---|---|
| Maximum jaw opening | Determines whether the wrench can reach both flats. | Read the manufacturer’s capacity specification and compare it with AF. |
| Metric and/or SAE scale | Helps position the jaw approximately before fitting. | Check whether the tool has the unit markings you use; markings do not replace a fit check. |
| Jaw clearance and condition | Excess clearance lets the wrench rock and can round the head. | Inspect the jaws closed and open; reject a visibly bent, chipped or excessively loose jaw. |
| Head thickness and access | A large head may not enter a confined space even when its capacity is sufficient. | Compare the wrench head and handle dimensions with the space around the fastener. |
| Handle length | Changes reach and leverage, but does not establish jaw capacity. | Choose a length that fits the work and allows controlled force; do not infer capacity from length. |
As concrete category examples, Würth article 071522108 lists a metric scale and a maximum across-flats opening of 30 mm. That is the capacity of that particular model, not a universal adjustable-wrench limit. Stanley model 87-473 is described as a 12-inch adjustable wrench with a knurl adjustment and laser-etched SAE and metric jaw scale; the 12-inch designation is its nominal length, not its jaw opening. Neither example establishes that it is the right choice for every fastener.
Set the jaws for a safe, usable grip
Seat both jaws on the flats
Hold the wrench so the jaws are parallel with the two opposite faces. The fixed jaw should contact one flat and the movable jaw the other over as much of the face as the head allows. A diagonal or corner contact concentrates force on a small area.
Remove play without forcing the adjustment
Turn the knurl until the movable jaw just closes against the opposite flat. The wrench should not rattle or visibly rock, but the adjustment should not be so tight that the jaws distort the head before you apply turning force. Recheck the fit after the first small movement, especially if the fastener is rusty or partly rounded.
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Apply force so the fixed jaw carries the main load rather than trying to spread the movable jaw. Keep the wrench aligned with the fastener axis. If the wrench slips, stop and correct the fit instead of increasing force.
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Do not use a loose or damaged adjustable wrench
A worn jaw, bent handle or excessive adjustment clearance can damage the corners even when the nominal opening is large enough. The ERIC-hosted U.S. Department of Education training material specifically describes the damage risk of an oversized wrench. The Bharat Skills/Directorate General of Training trade text (2022) likewise illustrates selecting the jaw opening to match the fastener.
Head width is not thread size
The AF measurement answers one question: which opening will grip the head? It does not tell you the thread’s outside diameter, pitch or whether the thread is metric, Unified National Coarse, Unified National Fine or another system. Different fasteners can have similar head widths, and the same thread diameter can appear with different head dimensions depending on the fastener type and standard.
If you are replacing a bolt or nut, measure the threaded portion separately:
- Thread diameter: measure the outside of the male threads, ideally with a caliper, without measuring over an accidental burr.
- Pitch: measure the distance from one thread crest to the next, or use a thread-pitch gauge. Metric pitch is normally expressed in millimetres per thread; inch threads are commonly expressed as threads per inch.
- Thread form and length: confirm the required thread system, bolt length, shoulder or shank details, and nut type from the application or the original fastener.
A head measurement alone therefore cannot identify a replacement fastener. Use AF to choose the wrench and separate thread measurements to identify the fastener itself.
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When a ruler is enough and when to use a caliper
For a clean, accessible head with clearly separated sizes, a rigid rule is usually sufficient to determine whether the opening is approximately 10, 13, 17 or 19 mm, for example. A caliper is worthwhile when the head is small, recessed, painted, worn, or close to a size boundary. Zero the caliper, keep its jaws square to the flats, and use light contact; squeezing a soft or corroded head can produce a misleadingly small reading.
Regardless of the instrument, the final test is the wrench on the actual fastener. Measurement selects a candidate; the fitted jaws reveal whether contamination, wear or an unusual head shape changes the practical result.
Worked examples
Example: a metric head measured at 19 mm
- Clean the head and measure opposite flats with a rule or caliper.
- Record “19 mm AF.”
- Choose an adjustable wrench whose published maximum opening is at least 19 mm.
- Open the jaws, seat them squarely on the 19 mm flats, and close the knurl until play is minimal.
- If the jaws reach 19 mm only at their extreme opening and wobble, use a larger-capacity or better-condition wrench rather than forcing the tool.
Example: an inch head measured at 3/4 inch
- Measure across opposite flats and record “3/4 in AF,” not the corner-to-corner diagonal.
- Compare that value with the wrench’s inch capacity. If the tool publishes only metric capacity, convert 3/4 inch to 19.05 mm for comparison.
- Fit the jaws and check for rocking. A nearby nominal metric size is not automatically interchangeable; the actual fit decides.
Example: the wrench fits, but the replacement nut is unknown
Keep the AF value for tool selection, then measure the replacement nut’s thread diameter and pitch separately. Do not order a replacement solely because its head width matches.
Troubleshooting a poor fit
The wrench will not open wide enough
Confirm that you compared the fastener’s AF measurement with the wrench’s maximum opening, not with handle length or a scale marking. Select a model with a larger stated capacity. The 30 mm maximum listed for Würth article 071522108 is one product example, not a general benchmark.
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The wrench fits but slips under load
Remove dirt or paint, reset the jaws on the flats, and close the knurl further. Inspect for a rounded head or worn jaw. If either is damaged, stop using the adjustable wrench; a tool with a more suitable fixed opening or another approved method may be safer for that task.
The measured value changes from one reading to another
Check that the rule or caliper is perpendicular to the flats and that you are not touching corners. Clean the head and repeat on another pair of opposite flats. Inconsistent readings can indicate corrosion, burrs or a deformed head rather than a different nominal size.
The head fits, but the tool cannot reach the fastener
Capacity alone does not solve access. Compare the wrench head thickness, jaw depth and handle sweep with the surrounding parts. A shorter or slimmer tool may fit the space, while a longer handle may be unsuitable even if it offers more leverage.
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You need to know whether the fastener is metric or inch
AF alone may not resolve the system. Measure the thread diameter and pitch, inspect markings or documentation, and test candidate nuts or gauges where appropriate. Treat a close jaw fit as evidence of head width only, not proof of thread identity.
What the adjustable-wrench standard covers
BS ISO 6787:2018 specifies dimensions for adjustable wrenches, admissible adjustable-jaw clearance and conditions for performance tests. The standard is useful context for why jaw clearance and tool condition matter: a nominal opening is not the whole performance specification. It does not turn any particular commercial model into a universal recommendation.
Manufacturer markings and capacity figures should therefore be read together with the physical inspection. A clear scale can speed setup, but the jaws still need to be square, sound and snug on the measured flats.
Quick checklist before you buy or turn the wrench
- Have you measured two opposite flat faces rather than the corners?
- Did you record the value with its unit?
- Is the wrench’s published maximum jaw opening at least that value?
- Are the jaws clean, parallel and free of damaging wear?
- Does the head physically fit the available space?
- After fitting, is there little play without excessive force on the knurl?
- If replacing the fastener, have you measured thread diameter and pitch separately?
Following those checks gives you the dimension that matters for wrench fit, prevents the most common corner-damaging mistake, and keeps head measurement separate from thread identification.
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