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How to Find the Diameter of a Circle


We all know what circles are don’t we? We are surrounded by circular objects, from wheels to aerosol cans. But if you needed to find the diameter of a circle would you be able to? In this post we provide a reminder of basic school mathematics and look at why its useful to know how to figure out circle dimensions.

Online Shopping Fails

Most of us will have read some amusing stories of people who have made online purchases which, when they arrived, turned out to be entirely the wrong size. The COVID-19 pandemic has fuelled an explosion in online shopping with many people using e-commerce for the very first time. But even experienced online shoppers can be caught out by not checking sizes.

In some cases, advertisements have purposely mislead shoppers by omitting dimensions. For example, Fleur Webb paid £60 for what appeared to be a bargain patio furniture set, on sale from what she thought was a trusted Amazon vendor. Images showed the furniture in a garden setting, surrounded by candles, with champagne on the table. But when the patio set arrived it turned out to be miniature furniture for a doll’s house. In this case the advertisement was clearly an intentional con but Fleur acknowledged that she should have spotted the dubious absence of dimensions.

There are many circumstances in which customers must determine and confirm the dimensions they need before placing their product orders. Making mistakes in measuring and calculating what’s needed can be expensive for both the customer and supplier and will inevitably lead to delays and disappointment.

What Does Diameter of a Circle Mean?

The Anti Climb Spiked Collars for Poles and Pipes, available from our online store, are an example of products for which its necessary to provide the correct dimensions when placing an order.

For example, our Spiked Anti Climb Collars for Round Poles are specified for poles of various diameters from 76mm (3 inches) up to 272mm (10.75 inches.) When placing orders for these products its therefore important to get the size right.

circle diameter circumference
Circles - Circumference - Diameter - Radius

 If you cast your mind back to school days hopefully you will recall how the diameter of a circle is the distance from side to side, via the centre point. Half the distance is called the radius and the distance around the circle is the circumference.

How to Calculate the Diameter of a Circle

Its often possible to measure the diameter of a circle simply by using a tape measure or ruler. But this requires the measuring tape to be laid across the circle, via the centre point, which isn’t possible for in-situ poles.

A caliper tool can potentially be used to get an accurate diameter measurement. But what most people need to know is is how to calculate the diameter of a circle from the circumference.

Measuring the circumference of a pole, with a measuring tape, is easy. Armed with this single dimension, along knowledge of the ratio between a circle’s diameter and its circumference, is all you need to calculate the pole diameter.

circle formula
Formulae for Calculating Circumference from Radius or Diameter

As shown, the circumference of a circle can be calculated by multiplying the diameter (d) by the constant, pi (π). Pi is generally approximated to 3.14 or 3.142, which is adequately accurate for most practical applications. Therefore, if the circumference of a circle is known then the diameter can be simply calculated by dividing this measurement by pi.

The following table lists the round pole diameters for which various off-the-shelf anti climb collar products are available from our store alongside their corresponding pole circumferences.

Circumference mm  Circumference inches  Diameter mm  Diameter inches 
  239  9.4  76  3
 280  11  89  3.5
 315  12.6  100  4
 359  14.2  114  4.5
 437  17.3  139  5.5
 528  20.4  168  6.5
 607  23.6  193  7.5
 689  26.7  219  8.5
 858  33.8  273  10.75

Note: all circumference dimensions have been rounded.

For example, if you have measured the circumference of a pole on which you need to install anti-climb protection and the measurement is 345mm, dividing this by pi (3.14) provides a diameter of 109.9mm. Therefore the anti climb pole collar suitable for poles with diameters up to 114mm (4.5 inches) would fit as required.

anti climb pole collar
Anti Climb Pole Collars from Insight Security


Products Requiring Circle Dimensions

As noted, our anti climb spiked pole collars are just one example of products from our online store for which accurate pole dimensions are needed when placing orders. Other examples include:

The old saying “measure twice and cut once”, generally applied to woodworking, can potentially be adapted for online shopping to: “measure twice and order once”, highlighting how getting the required dimensions right-first-time will prevent disappointment and the often costly need to return goods and re-order the right size.

If you have any questions about the dimensions of products you need or if you have any special requirements remember we are here to help. Give us a call on 01273 475500 and we’ll provide you with free, expert advice.

This message was added on Thursday 20th January 2022


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