Where is 3/16 on a tape measure

Look further down the tape and you will see it repeated every 19+ inches, always at some imprecise measurement, not really a multiple of 3/16 of an inch. Finally when you get to 8 feet it is dead on.

In fact if you divide 96 inches by 5 you will get 19.2 inches, a decimal measurement on a fractions tape.

These marks are for laying out 5 engineered flooring trusses per 8 feet, like those wooden I beams.

Studs and traditional floor joists are usually either 16 or 24 inches on centre, and every 16 inches on the tape is marked with a triangle.  Three times 16 is 48 -- so you get an even three spans every 4 feet.  With 24 inch spacing you get two spans in 4 feet.  Both work for nicely fitting 4 foot wide panels, like drywall or plywood. 

Floors are usually laid out on the 8 foot run -- 24 inch centers will give you 4 spans in 8 feet -- but engineered floor truses are lighter and less expensive than solid wood, but require one more support in that 8 foot span, giving 5 spans in 8 feet.  That means that you have to install floor trusses on 19.2" centers -- which is exactly what those diamond marks are for. 

They are referred to as a 'truss marks'.  Thanks Stef for a 2017 correction to my original entry made way back in 2003.

And in 2018, Tom gives us a lesson in history showing that people were using 5 spans in 8 feet a long time ago:

"I've been noticing across the internet that the common answer to the "black diamond framing" question is that it is for engineered trusses. In fact, the markings pre-date engineered lumber by probably over 100 years. It's really common to find diamond spaced framing on the floor joists of homes build around the turn of the century (1900). Thinking about it, using diamond framing now would be problematic because there is no insulation designed to fit it."  Thanks Tom.   You are right it might be problematic for wall framing, and really not necessary from a structural point of view, but floors are not often insulated and do need more strength. 

Whilst many of you will think you know how to use a tape measure, do you actually know how to use all of it? Do you know what the odd selection of markings refer to that are usually found within the first 20cm of a tape’s length?

The UK Government’s Measuring Instruments (Material Measures of Length) Regulations implemented the European Measuring Instruments Directive (MID) in 2006. In order to comply with these regulations it became a legal requirement for manufacturers (with a few exceptions) to include a number of markings along the length of their blades. This article should help you understand what they are and how they should affect which tape measure you choose to buy.

If you want to learn more about tape measures and, in particular, how to read them properly, why not check out some of our other articles?

Other Articles

• How to Read a Tape Measure
• Tape Measure Tips - Getting The Most Out of Your Tape
• Taking Care of Your Tape Measure
• How to Choose the Right Tape Measure

Metric and Imperial Markings

Whilst some countries such as the USA still focus primarily on Imperial or ‘English’ markings (feet, inches, yards etc.), most countries have moved onto the metric system. The UK, however, still uses a combination of the two – the mandatory metric system as enforced by the European Union and the more traditional Imperial system still in use by many trades.

Roman Numerals for Accuracy

Pull the blade out of the case and you may notice a Roman numeral within either a circle or a rectangle, generally printed in red or black. This refers to the accuracy of that particular tape.

There are four generally accepted levels of tape measure accuracy. Class 1 (denoted by the Roman numeral ‘I’) is the most accurate. Class 2 (denoted by ‘II’) the second most accurate with Class 3 (III) logically being the third. Unclassified tapes will not feature any marking on their blade. By ‘unclassified’ we mean that they have not been tested and guaranteed to conform to a particular level of accuracy.

Class 1 tapes are becoming more and more popular, especially amongst those who demand absolute precision in their work. That said, most everyday and professional tape measures on the market today (including everything from Stanley, Irwin, Draper, Bahco and more) are EC Class II accurate. When used properly Class II tapes are more than accurate enough for the vast majority of users.

You can learn more about the difference between Class 1 and Class 2 tape measures by reading our article – The Advantages of a Class 1 Tape Measure.

Length

Whilst the length of most tapes is printed on the case, it is a legal obligation for manufacturers to print it on the blade as well. The length must be printed on each scale – one for metric and one for imperial.

Black Diamonds

By far the most common question asked by our customers is “what do the little black diamonds mean?” Even though they’re not commonly used in the UK, these marks are still worth explaining.

The black diamonds, as they’re commonly called (sometimes shown as black circles instead), are actually stud or joist marks. Whilst not featuring on all tapes, stud marks frequently appear at 16” intervals and joist marks at 19.2” intervals.

The centre of one stud is generally positioned 16 inches from the centre of the stud before it – hence the 16” stud mark. Joists, on the other hand, are generally spaced 19.2” inches from the centre of the joist before it. Whilst this may seem to be a pretty odd measurement, many American building materials (such as boarding etc.) typically come in 8ft lengths. 19.2” divides perfectly (5 times) into 8ft.

Year of Manufacture

On many tapes (but by no means all) you will find a small red rectangle with the letter M printed inside followed by a number. The number simply denotes the year that the blade was stamped as conforming to the Measuring Instruments Directive. On the whole this will also be the year in which that tape was manufactured. M09 generally signifies a tape measure that came off the production line in 2009 whilst M13 (as seen in the picture below) shows a tape most likely manufactured in 2013.

CE Mark

The CE mark indicates that the tape measure conforms to all European Union regulations that apply to it. In the case of tape measures that means the European Measuring Instruments Directive (MID). This doesn’t mean that it was manufactured in the EU but is, to a certain extent, an indicator of quality.

Please note that the lack of a CE mark in no way indicates a tape measure of poor quality. It could simply mean, for example, that the blade is of a slightly older design which in no way affects its accuracy or quality.

Temperature & Tension

Many tape measures feature the marks ‘20°C’ and ‘50N’ near to the tip of the blade. This relates to the tape measure’s quoted level of accuracy. EC levels of accuracy (Class 1 or Class 2) state that a tape’s blade is accurate to that particular standard at a temperature of 20°C at a pulling force of 50N (Newtons). 20°C being the temperature of the laboratory where the accuracy of the blade is tested.

If the tape measure is being used in temperatures other than 20°C or at a pulling force of more/less than 50 Newtons then the accuracy of that tape cannot be guaranteed. Whilst that doesn’t necessarily mean that the tape’s accuracy has been affected, the user will have to satisfy themselves of this and may have to manually compensate for that change in temperature and/or force. Information on how to do this can be found on Wikipedia by searching for ‘tape correction’.

Testing Body

The four digit number (generally 0126 for tape measures) relates to the agency responsible for issuing the certification. 0126 is the designation for the National Weights & Measures Laboratory in Middlesex.

Other Markings

A number of other symbols can be printed in the first 20cm of the blade depending upon the individual manufacturer. Some tape measures will include the calibration number provided by UK Weights and Measures and some will feature a batch number to further help the manufacturer identify a product’s build date.

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