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What Size Spanner Do You Need?

Spanner Set
27 June 2023 491 view(s)

When you have nuts and bolts within your home or workplace, you’ll need spanners to loosen or tighten them. Knowing which types of spanner you need is crucial when doing this. However, there is other vital information you need to know.

You need to recognise what spanner size you’ll require for your DIY project so that you can safely and accurately complete it. Whilst you can find the size of most spanners on the shaft, this does not establish which one you should use on a specific nut or bolt.

The size of a fastener refers to the measurement across the bolt flats. The number assigned to this nut or bolt will most likely differ from the spanner size you need.

To determine what size spanner is appropriate for your task, you must also ask yourself what thread the fastener has. The thread will help you establish the size of the fastener.

If you don’t know how to establish which spanner you need, we have a guide to help you with the different threads and spanner sizes you can use. Heamar also have various tools available if you need to purchase a spanner for your project.

What Thread Types Are There?

Copper tapered threads

Though there are several threads a nut can have, you can categorise them into two types: parallel threads and tapered threads. Identifying the thread type will help determine the size spanner you need.

You also need to know if the threads are male or female. Male threads are located on the outside, whereas female threads are on the inside. This information is vital for later identifying which thread type you have with a pitch gauge.

Parallel Threads

This type of thread maintains the same diameter throughout. The thread walls remain at a constant distance from each other no matter how long those walls are.

Parallel threads include the following:

  • BSPP (British Standard Pipe Parallel)
  • Metric Parallel
  • UNF/UN (Unified threads)

To form a seal with a parallel thread, you need to use a bonded washer, seal or an o-ring.

Tapered Threads

Unlike their parallel counterparts, tapered threads are narrower at one end than the other. As the metal from the thread wedges against the surrounding metal, it creates a seal.

Tapered threads include the following:

  • BSPT (British Standard Pipe Tapered)
  • Metric Tapered
  • NPT/NPTF (American pipe threads)

Identifying Which Thread Type You Have

The thread type is sometimes noticeable by sight alone, although this is not always true. If you cannot identify the type of thread visually, you can use tools to help you, including the calliper and the pitch guide.

1) Calliper

Vernier caliper

If you want to measure the thread’s diameter, you use a calliper

Male threads require you to measure the external diameter, whereas you measure the internal diameter for a female thread.

Measure the diameter of the first, fourth, and last full threads. Parallel threads would have all these figures the same. Tapered threads would be different sizes, each one decreasing. If you are measuring metric threads, use millimetres to establish the diameter.

Digital callipers are also available.

2) Pitch Gauge

Pitch Gauge tool

This tool helps establish the pitch size of your thread. If you are working with imperial nuts, the pitch size refers to how many threads are within an inch. For metric nuts, you measure the distance between the threads.

Nuts and bolts can have very similar pitch sizes that aren’t the same, so you must establish the correct pitch size for your fastener. To do this, use a pitch gauge.

Most metric pitch sizes are 1mm, 1.5mm or 2mm, although other sizes exist.

Type of Thread Pitch Size
BSPP (British Standard Pipe Parallel) 11
Metric Parallel 1.0
BSPT (British Standard Pipe Tapered) 11
Metric Tapered 1.0

Spanner Size and Nut Size Conversion Chart

Range of spanner sizes

Some spanner sizes can be very similar, so finding one that fits close enough can be tempting. However, doing this can cause issues.
If you use a spanner slightly too big for your fastener, the spanner can slip, which can round off the hexagon’s edges. Instead, find the correct size spanner for your nut or bolt to prevent this from happening. For example, you can use metric spanners with metric fasteners and imperial spanners with imperial nuts or bolts.
You should be able to find the spanner sizes listed below on the tool shaft.

Metric Nuts and Spanner Sizes

Metric nuts have a different measuring system than metric spanners, which can make it harder to decide which one to use. Spanners are measured in millimetres (mm) and specify the tool’s head size. The fastener size refers to the thread size and is preceded by M, which means ‘metric’.

Spanner sizes Nut or bolt sizes
5.5mm M3
7mm M4
8mm M5
10mm M6
13mm M8
17mm M10
19mm M12
22mm M14
24mm M16
27mm M18
30mm M20
36mm M24
46mm M30
55mm M36

Imperial Nuts and Spanner Sizes

In contrast to their metric counterparts, an imperial nut measurement refers to the distance between two parallel sides of the fastener across flats (AF). The below measurements use the AF measurements.

Spanner sizes Nut or bolt sizes
7/16" 1/4"
1/2" 5/16"
9/16" 3/8"
5/8" 7/16"
3/4" 1/2"
7/8" 9/16"
15/16" 5/8"
1 1/8" 3/4"
1 1/2" 1"


After reading this blog, you now understand how to establish the correct spanner size for your project. But what about the other tools you may need?

Heamar have a range of hand tools, perfect for your DIY needs. Whether you’re a DIY master or just starting your career as a mechanic, these tools are the ideal companions for your toolbox and can support you in your future projects.

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